Theoretical and Applied Genetics 45, 2 0 5 - - 2 t 0 (t974) | b y Springer-Verlag t 974
Intergenotypic Competition Studies in Corn (Zea mays L.) I. A m o n g E x p e r i m e n t a l H y b r i d s 1 A. JURADO-TOVAR and W. A. COMPTON U n i v e r s i t y of N e b r a s k a , L i n c o l n , N e b r a s k a (USA)
Summary. Four experimental single-cross hybrids were evaluated for intergenotypic competition in a split-plot design with 7 replicates and a stand density of 51,700 plants/hectar in 1970, 1971 and 1972 at Lincoln, Nebraska. The arrangement of rows used in this study allowed the measurement of effects of different levels of competition on the traits grain yield, plant height and a selection index. There were some definite inter-genotypic competitive effects for all three traits among the pairs of hybrids studied. Variation in types of intergenotypie interaction was found. A two-step process was suggested to take advantage of favorable competitive interactions for increasing grain yield. Failure of mixtures in corn to take advantage of favorable competitive situations was discussed.
Introduction and Literature R e v i e w P l a n t research a i m e d at m e a s u r i n g c o m p e t i t i o n effects is p r o v i d i n g e v i d e n c e t h a t the p e r f o r m a n c e of a g e n o t y p e is o f t e n affected b y o t h e r g e n o t y p e s g r o w i n g n e a r - b y . T h u s we are faced w i t h t h e fact t h a t selective values of g e n o t y p e s in pure s t a n d are n o t always the same as t h e v a l u e s t h a t occur u n d e r i n t e r g e n o t y p i c c o m p e t i t i o n . T h i s s i t u a t i o n is an i m p o r t a n t c o n s i d e r a t i o n in p l a n t b r e e d i n g , b u t to date, c o m p e t i t i o n i n f o r m a t i o n has n o t b e e n w i d e l y u t i l i z e d i n b r e e d i n g programs. T h e c h a r a c t e r i z a t i o n of c o m p e t i t i v e responses b y use of a hillplot a r r a n g e m e n t of t r e a t m e n t s , as origin a l l y suggested b y S c h u t z a n d B r i m (t967) i n soybeans, has beeu m a d e i n o t h e r s e l f - p o l l i n a t e d crops such as b a r l e y , w h e a t a n d oats (Allard 1969, S m i t h et al. 1970). I n f o r m a t i o n on c o m p e t i t i v e effects a m o n g corn h y b r i d s is v e r y m e a g e r (Stringfield t959, E b e r h a r t et al. 1964 a n d F u n k a n d A n d e r s o n 1964), a n d no results h a v e b e e n r e p o r t e d w h e n more t h a n 2 levels (dosages) of c o m p e t i t i o n are o p e r a t i n g . I t also appears t h a t critical c o m p e t i t i o n i n f o r m a t i o n i n b r o a d - b a s e d m i x t u r e s or r a n d o m - m a t i n g p o p u l a t i o n s is Virtually n o n - e x i s t e n t . T h e m a j o r o b j e c t i v e of this s t u d y was to e v a l u a t e a n d characterize responses u n d e r 3 levels of compet i t i o n of e x p e r i m e n t a l h y b r i d s .
Materials and Methods Pedigrees and reference codes of the corn hybrids used in this study are : Pedigree N128 (Nebr. B synthetic) • B67 (Iowa SSS). N20 (Nebr. sel. of Stiff Stalk Synthetic) • A257 (Sel. f r o m A 7 3 X Os420) N t 32 (Nebr. 13 synthetic) • N 138 (Sel. from []3t4 • Oh43] BI4) N 20 • N2 (Nebr. B synthetic)
Reference Code A ]3 C D
The experiment was planted in 1970, 1971 and 1972. I t was analyzed as a split-plot arrangement of treatments with a hybrid pair as the 8-row m a i n plot and combinations of the hybrids in sequence (as shown below) as subplots. Randomization was not done for the hybrid combinations. The total sequence (whether one starts with X or Y each time) was randomized. Seven replicates were used with 3 levels of competition that could be evaluated. The arrangement of an 8-row main plot for comparing two hybrids, one represented by X and the other by Y can be demonstrated in the following way: X
X
X
Y
X
Y
Y
X X X X X X X X
X X X X X X X X
X X X X X X X X
Y Y Y Y Y Y Y Y
X X X X X X X X
Y Y Y Y Y Y Y Y
Y Y Y Y Y Y Y Y
X
X
X
Y
X
Y
Y
Y
-- discarded hills
T Y
-- discarded hills
Rows : t 2 3 4 5 6 7 8 X levels : Lo L1 L~ Y levels: L2 L 1 Lo discarded row discarded row X and Y d@signate one hill of an eight-hill row of a hybrid and its competitor hybrid, respectively. At harvest time the two rows on either end were discarded. Referring to the diagram above, rows 2, 3, and 5 provide information on level 0 (L0), level t (L1) and level 2 (L2) of competition for hybrid X, and rows 7, 6, and 4 provide the same information on Y. Level 0 designates the variety in pure stand since the competitor rows on either side contain the same hybrid. Level I is used to designate t h a t the hybrid was bordered by I competitor row and t row of the same hybrid, and level 2 means that the hybrid was bordered by two competitor rows. Level 2 also specified the m a x i m u m inter-row competition level. The assumption was made that, for instance AAB and 1 Published as Paper Number 3752, Journal Series, Nebraska Agricultural Experimental Station.
206
A. Jurado-Tovar and W. A. Compton: Intergenotypic Competition Studies in Corn I.
13AA Were equivalent with respect to the yield of the test genotype A in relation to the competitor or border genotype B. The reference code for the hybrid pairs involved is as follows : A and B ~ hybrid pair I (HP1) A and C ~ hybrid pair 2 (HP2) A and D ~ hybrid pair 3 (HPa) B and C = hybrid pair 4 (HP4) B and D = hybrid pair 5 (HPs) C and D ~ hybrid pair 6 (HP~) Four seeds per hill were planted and seedlings were thinned to three plants per hill for a final stand density of 51,700 plants/hectare. I n 1970, hills with one or more plants missing were eliminated along with the surrounding hills. I n t 971 and 1972, hills with 2 plants were utilized along with three plant hills and only those with two or three plants missing were removed along with the surrounding hills. The experiment was irrigated when needed. Nitrogen was applied at the rate of 168 kg/ha in 1970 and 1971 and 280 kg/ha in 1972. Measured agronomic variables were the following: 1) Grain weight in quintals per hectare (q/ha) at 15.5% moisture content, 2) Percent moisture, 3) Average plant height of 10 competitive plants, 4) Percent broken stalks, and 5) Percent dropped ears. The selection index (S. I.) was calculated as (S. I.) = X 1 (100 -- X~) (100 -- Xa) 'lO0
100
where Xl, Xfl and X a are yield, percent lodging and percent dropped ears, respectively. This index has been used in the Nebraska Corn Project since 1969 and can be used as an estimate of machine-harvestable yield from hand-harvested data. I t is very easy to apply and no >arameter estimation is involved in its construction Subandi, Compton and Empig, 1973),
Experimental Results There were some definite i n t e r g e n o t y p i c compet i t i v e effects f o u n d in the h y b r i d pairs studied. On the whole, these effects were l i m i t e d to a m a x i m u m of a b o u t t 0 q u i n t a l s / h a (about t 0 - - 1 3 % ) in this set of m a t e r i a l . E v i d e n c e of c o m p e t i t i v e effects is c o n t a i n e d i n the analyses of v a r i a n c e shown in T a b l e t. There is a c o n s i s t e n t increase in the m e a n squares w i t h i n each h y b r i d pair (HP,) as one e x a m i n e s first the " w i t h i n l e v e l 0" source, t h e n the " w i t h i n level 1 ", a n d finally the " w i t h i n level 2" source. The o n l y real v a r i a n t from the p a t t e r n is in the c o m p a r i s o n for HPo. Similar conclusions can be d r a w n from Fig. 1, where the m e a n s referred to in the above p a r a g r a p h are p r e s e n t e d graphically. Graphical displays are sometimes more easily u n d e r s t o o d t h a n s t a t i s t i c a l jargon. Again note the general t r e n d t o w a r d divergence of the m e a n s with i n c r e a s i n g levels of competitioI~. Since the largest differences were f o u n d at comp e t i t i v e level 2, o n l y those m e a n s are shown in T a b l e 2 for c o m p a r i s o n with pure s t a n d values. Note t h a t there were d e v i a n t s t h a t were h i g h l y significant. T a b l e 3 c o n t a i n s analyses of v a r i a n c e m e a n squares for the other two traits. The p l a n t height m e a n squares for " w i t h i n levels 0, t, a n d 2" have a divergence t r e n d opposite to t h a t shown above for g r a i n yield. I n other words, i n c r e a s i n g levels of compe-
Table 1. A nalyses of variance of mean grain weight in quintals~hectare of hybrid pairs z through 6 combined over years Analyses of variance Mean squares Source of variation
Degrees of freedom
HP 1
HP=
HP 3
HP 4
HP 5
Entries/HPi 1 Level 0 vs. levels 1 and 2/HPi Level t vs. level 2/HPi Within level 0/HPi W i t h i n level t / H P i Within level 2/HPi Pooled Error
5 1 1 1 1 1 60
2125"* 56 46 2292"* 2297** 5936** 147
1435"* 4 0 t 33 1533'* 5505** 147
977** 128 54 55 608* 4041"* 147
294 83 t56 29 639* 562 147
284 89 107 28 106 t2 2 320 2t7 52 987* 34 t47 147
HP 6
1 i subscript used to identify hyiJrid pairs 1, 2, --6. * significant at the .05 level of probability. ** significant at the .01 level of probability. 100
H8
q/ha
H~ ;.,c"
HP5
H~ _z2.
7fl 0
1
2'0
1
;tO
1
20
Levels of competition within each hybrid pair
1
20
1
2
Fig. t. Diagramatic illustrations of grain weight with increasing levels of competition for each hybrid within each hybrid pair
Theoret. Appl. Genetics, Vol. 45, No. 5
A. Jurado-Tovar and W. A. Compton: I n t e r g e n o t y p i c Competition Studies in c o r n I. Table 2. Mean competitive and pure stand grain yields (q/ha) for each individual hybrid and level 2 of competition, averaged over 3 years Grain yield (q/ha) Test hybrid
Competitor hybrid
A ]3 A C A D 13 C B D C D
13 A C A D A C A D 13 D C
With Pure competition stand
Difference+
98.62 74.84 93.82 70.92 85.73 66.12 83.27 90.59 8O.55 90.24 8t.t9 82.98
2.36 --6.65 i0.t0"* --9.25* 7.32 --t0.00"* --2.48 3.t8 --3.79 5.46 --4.28 3.04
96.26 81.49 83.72 80.17 78.41 76.12 85.75 87.41 84.34 84.78 85.47 79.94
+ Differences (column 3 -- column 4). * Significant at the 0.05 level of probability. ** Significant at the 0.0t level of probability.
207
N o s i g n i f i c a n t differences were f o u n d a m o n g levels for a n y of t h e t r a i t s . I n o t h e r words, w h e n one h y b r i d i n c r e a s e d in v a l u e , t h e o t h e r t e n d e d to d e crease p r o p o r t i o n a t e l y . T h i s i m p l i e s t h a t t h e effects are l a r g e l y of a c o m p l e m e n t a r y n a t u r e . Discussion
T h e r e s u l t s of t h i s e x p e r i m e n t i n d i c a t e c l e a r l y t h a t i n t e r g e n o t y p i c c o m p e t i t i o n as o b s e r v e d b e t w e e n t h i r t y - i n c h rows can h a v e a n effect u p o n y i e l d . I t is also s h o w n t h a t t h e r e s p o n s e s of a m a i z e g e n o t y p e t o t h e s h a r i n g of t h e e n v i r o n m e n t w i t h a d i f f e r e n t g e n o t y p e c a n n o t be d e s c r i b e d e n t i r e l y in t e r m s of c o m p l e m e n t a r y effects. T a b l e 4 p r e s e n t s a s u m m a r y of t h e i n t e r - g e n o t y p i c r e l a t i o n s h i p s for g r a i n y i e l d a t t h e d i f f e l e n t levels of c o m p e t i t i o n . V a l u e s g r e a t e r or less t h a t 50/0 of t h e p u r e s t a n d v a l u e s were u s e d to c h a r a c t e r i z e o v e r c o m p e n s a t o r y a n d u n d e r c o m p e n s a t o r y effects, res p e c t i v e l y . C o m p l e m e n t a r y effects a c c o u n t e d for 50% of t h e c o m p e t i t i v e i n t e r a c t i o n s . N e u t r a l , u n d e r c o m -
Table 3. Means squares for plant height and the selection index from the analyses of variance combined over years Mean squares Plant height (cms.) Source of variation Entries/HPi I Level 0 vs level t & 2/HPi Level t vs level 2/HPI With-in level 0/ttPt W i t h i n level t / H P t W i t h i n level 2/HPi Pooled error
d.f. 5 1 1 t l 1 60
HP 1 366t** 75 15 7809** 5964** 4441"* t49
HP 2 333 65 22 653* 617" 307 149
HP 3 2346** 420 43 3696** 5522** 2053** 149
HP, t905"* t49 118 4248** 3741"* t268"* 149
HP 1 2119"* 267 J 60 1160" 2010"* 7046** 1t7
HP~ 374 1 57 338 t41 1330" 117
HP3 HP4 566* 595 1 0 295 t74 550 271 33 .1077" t950"* 1455"* 117 t17
HP5 32 5 62 74 3 16 149
HPs 1 586 19 19 3060** 3020** 1810"* 149
HP~ 603* 33 t21 130 397 2332** t17
HP s 339 19 38 1089" 435 t13 1t7
Mean squares Selection index (q/ha) Source of variation Entries/HPi 1 Level 0 vs level I & 2/HPi Level t vs level 2/HPi W i t h i n level 0/HPi W i t h i n level t / H P i W i t h i n level 2/HPi Pooled error
1 i subscript used to identify hybrid pairs t, 2, --6. * sigfiificant at .05 level of probability. * * significant at .0t level of probability. t i t i o n reduce differences in p l a n t h e i g h t b e t w e e n t w o competing hybrids. T h e selection i n d e x values, as one m i g h t e x p e c t , a r e m u c h less c o n s i s t e n t t h a n a r e t h o s e for g r a i n yield. T h e r e is a slight t r e n d for a n i n c r e a s i n g difference w i t h i n c r e a s i n g levels of c o m p e t i t i o n , b u t p r o b a b l y n o t e n o u g h to w a r r a n t f u r t h e r p u r s u i t . Theoret. Appl. Genetics, Vol. 45, No. 5
p e n s a t o r y a n d o v e r c o m p e n s a t o r y effects a c c o u n t e d for t h e o t h e r 50% of t h e c o m p e t i t i v e i n t e r a c t i o n s in p r o p o r t i o n a l a m o u n t s (16.6%). T h e s e p e r c e n t a g e s reflect c o n s i d e r a t i o n of b o t h levels of c o m p e t i t i o n . H o w e v e r , w h e n t h e t e s t g e n o t y p e is s u r r o u n d e d on b o t h sides b y t h e c o m p e t i t o r g e n o t y p e (level 2), t h e n o n l y c o m p l e m e n t a r y effects 6 7 % ) , n e u t r a l effects
A. Jurado-Tovar and W. A. Compton: Intergenotypic Competition Studies in Corn I.
208
Table 4. Characterization of intergenotypic relationships for grain yield of 4 hybrids grown under 2 levels of competition. 3 years data +
Hybrids
Hybrids
A
LI L2
B
Lt L2
C
L2 L1
B
C
D
N UC
C C
UC C
OC N
OC C C C
L1 -- Level I of competition L2 = Level 2 of competition N = Neutral C = Complementary OC-- Overcompensatory UC = Undercompensatory Based on Schutz et al., 1968. (17%) and u n d e r c o m p e n s a t o r y effects (t7%) were observed. W e should be reminded t h a t these specific characterizations of competitive interactions apply only to the restricted sets of hybrids studied. The experimental hybrids were selected without any morphological or physiological considerations. It appears t h a t the grain yield response of different corn hybrids under different levels of competition has not been reported previously. However, for purposes of comparison, we can relate our findings to other experiments in corn where intergenotypic competition effects of some sort were measured. Stringfield (1959), using mixtures involving equal numbers of seed from two contributing members, reported no advantages in grain yield of the nfixtures over the average of the contributing hybrids grown separately. Since the arrangement of 4 seeds per hill of the component hybrids was at random, it appears that intra- and intergenotypic competition within a row was involved. Neither distance between rows nor plant density was reported. Funk and Anderson (1964) reported that the blending of two corn hybrids in alternate rows did not appear to increase grain yield over the mean of the component hybrids grown separately. Inter-row competitive effects were considered with only one border row as a competitor genotype, which in our s t u d y was designated as level I of competition. The distance between rows was 36 inches. The results of their experiment are in agreement with our s t u d y when only one level of competition is considered. E b e r h a r t et al. (t964) measured intra-plot competition among two sets of maize single crosses. The blending of two hybrids was done either in the hill or in alternate hills every t3 inches. Only intergenotypic competition within a row was involved since only one plant represented a particular hybrid. The competition effects were
of a comparative type (complementary) similar to the results of Stringfield. Caution should be exercised in making comparisons of within-row competition and between-row competition. We will elaborate on this point later in the discussion. T h a t certain hybrids show a constancy of intergenotypic competitive effects in response to different genotypes is shown b y the behavior of hybrid D under level 2 of competition. The complementary effects of hybrid D are displayed regardless of the competitor present and the increases or decreases in grain yield of hybrid D are specific to the competitor involved. Also, it appears that for some hybrid pair combinations, the similarity in response to intergenotypic competition m a y not be affected b y the levels of competition involved. The competitive situation of most interest is the one where cooperation is involved (overcompensation). These favorable interactions have been reported mainly in selfpollinated crops (Schutz and Brim 1967, Jensen and Federer 1964). In the search for increased productivity, new imaginative approaches will have to be taken b y breeders and ecologists. If the environments and the genotypes can be specified whereby favorable interactive relationships are established, then potential yield levels can be raised over the yield levels of the pure stands of the genotypes themselves. In this connection, the following statement b y Haldane is suggestive. "While the most obvious symbiosis to look for and to exploit, if discovered, to increase agricultural production, is between different species, particularly cereals and legumes; nevertheless, if such symbiotic relations are common, they should be looked for between different genotypes of the same species". Use of favorable competitive interactions has been suggested b y Jensen and Federer (1964) in wheat and by Schutz and Brim (t967) in soybeans. In inter-row competitive situations only two levels of competition are well defined. They are determined b y the n u m b e r of adjacent rows used as competing genotypes. When the genotype is in pure stand, then intergenotypic competition is not operating. If we want to take advantage of favorable interactions for increasing grain yield a two-step process is required: 1. Determination of the appropriate competition level for a pair of hybrids such t h a t a net increase over the sum of both genotypes in pure stand is shown. 2. Duplication of the experimental combinations on large scale using the favorable interacting genotypes and the defined competitive environment. If we proceed to step 2 with the hybrids B and C, from our study, using level I of competition for hybrid C and level 2 for hybrid B, we could expect to have a net increase of 4.1% over the pure stand value of the component genotypes, or 3.1% over C Theoret. Appl. Genetics, Vol. 45, No. 5
A. Jurado-Tovar and W. A. Compton: Intergenotypie Competition Studies in Corn I. in pure stand (for values see Appendix Table At). The highest yielding genotype (C) constitutes 2/3 of the component genotypes, but some increase f r o m having B as a competitor gives the combination an advantage. For obtaining level t for C and level 2 for I3, using a six-row ptanter, we would proceed in the following w a y : C B C CB C C B C C B C
11111l]1 t
2 3 4
5 6 6
IIII 5 4
3 2
4
t In the first pass the C 13 C C B and C are planted and in the following pass the inverted sequence. The question t h a t arises is then " W h y can we not take advantage of the favorable competitive situtions in mixtures in corn ?" The results of experiments utilizing mixtures in corn are very meager. The experiments reported b y Stringfield (1959), F u n k and Anderson (t964) and Kannenberg and H u n t e r (t972) show t h a t no consideration was given to the intergenotypie relationships for the establishm e n t of mixtures. The evidence available in other crops t h a t within-row competition is different from between-row competition (Jensen and Federer 1954, Schutz and B r i m 1967 and Smith et al. t970) could also be true in corn. If optimization of proportions in mixtures is required, additional characterization of intergenotypic competition effects as observed in hill-plot arrangement m a y be helpful. Also, competition effects between drilled rows might also be observed and utilized. The diversity of genotypes and environments involved m a k e a difficult t a s k of ascertaining the factor or factors involved in competitive situations. Plant height undoubtedly plays a role in plant competition. The best competitor in our s t u d y turned out to be the tallest genotype in relation to the other genotypes. Also, it was observed t h a t genotypes virtually identical in plant height showed a yield response due to competition. The basic function of the corn canopy is the interception of light. One can speculate t h a t tall genotypes adjacent to shorter genotypes in border rows should intercept more light while the short ones m a y be expected to be at a disadvantage for available light. Is it possible to m a k e a general s t a t e m e n t about the plant height and grain yield relationship under competitive conditions ? The answer is yes, but with some reservations. Earlier we mentioned the consistent increase and decrease in the size of the mean squares for grain yield and plant height, respectively. I n fact the tallest genotype (hybrid A), which also was the stronger competitor with respect to grain yield, decreased in plant height in all the combinations analyzed under competitive conditions. The opposite effect was found for the shortest genotype (with two exceptions). F r o m the above considerations, it appears t h a t the shorter plants (shaded) t e n d to elongate more rapidly t h a n Theoret. Appl. Genetics, Vol. 45, No. 5
209
the taller plants (unshaded). Hozmni et al. (t955) also using corn, noted t h a t under closely spaced conditions, the shorter plants had a higher elongation rate t h a n the taller ones t h a t were shading them. We have to keep in mind t h a t in t h a t group of species which has a low compensating point, of which corn is one, light saturation is not reached even at full sunlight intensities (Hesketh and Moss t963). The resistance to carbon dioxide diffusion in the leaf is low, and the photosynthetic rate correspondingly high (E1-Sharkawy and Hesketh t965). On the other hand, in the high compensating group, such as soybeans, there is little response to light under a fixed level of carbon dioxide (Brun and Cooper 1967). In response to the question of relating plant height to grain yield under competitive conditions, perhaps one should expect a significant response for grain
Appendix Table At. Over-all mean of grain yield (q/ha), plant height (cms), and selection index (q/ha) for 6 entries within each hybrid pair, averaged over 5970, 1971 and J972 Hybrid
Entries
Level
yield
height
Plant
Selection index
A A h B B B
Lo L1 L~ L, L1 L~
(q/ha) 96 96 99 81 81 75
(cms) 3o2 299 296 270 270 271
(q/h a)
I
2
C C C A A A A A A D D D
L0 L1 L~ L0 L1 L2 Lo L1 L2
80 76 7t 84 88 94 78 78 86 76 7t 66
285 285 285 295 293 292 294 293 287 271 266 270
73 67 65 67 71 76 58 59 71 65 61 57
13 B 13 C C C B B 13 D D D
Lo L1 L~ Lo
86 86 83 87 94 91 84 85 81 85 90 90
266 265 266 289 287 279 265 a67 266 268 267 264
77 76 72 82 86 84 74 7t 64 70 77 79
85 80 81 80 82 83
287 289 286 268 269 27t
79 75 75 68 75 72
pair
3
4
5
6
C c C D D D
Lo
L1 L~
L~ L2
L0 L~ L~ L0 L1 L2 Lo L, L~ L0 L~ L2
Grain
83 82 87 72 69 61
2t0
A. Jurado-Tovar and W. A. Compton: Intergenotypic Competition Studies in Corn I.
yield under competitive conditions when height differences are i n v o l v e d in t h e low c o m p e n s a t i n g g r o u p . L i g h t i n t e r c e p t i o n m e a s u r e m e n t s were n o t i n c l u d e d in t h i s s t u d y b u t t h e y s h o u l d be t r i e d in f u t u r e s t u d i e s . Likewise, m e a s u r e m e n t s of r o o t sys t e m s m i g h t h e l p to f u r t h e r u n d e r s t a n d b e l o w - g r o u n d factors in explaining competitive interactions among different genotypes. T h e i n f l u e n c e of t i l l e r i n g a b i l i t y on c o m p e t i t i v e i n t e r a c t i o n s was n o t e v a l u a t e d since v i s u a l observ a t i o n s d e t e c t e d no t i l l e r i n g a m o n g t h e d i f f e r e n t g e n o t y p e s . Also, m a t u r i t y differences as d e t e r m i n e d b y m o i s t u r e c o n t e n t of t h e g r a i n were n o t d e t e c t e d . D a t a on n u m b e r of ears p e r p l a n t (not i n c l u d e d in this p a p e r ) s u g g e s t e d t h e absence of prolificacy. Literature
Allard, R. W. : Populations studies in predominantly selfpollinating species. X l I I . Intergenotypic and population structure in barley and wheat. Amer. Natur. lo3, 6 2 t - - 6 4 5 (1969). Brun, W. A., Cooper, R. L. : Effect of light intensity and carbon dioxide concentration on photosynthetic rate of soybean. Crop Sci. 7, 451--454 (1967). Eberhart, S. A., Penny, L. H., Sprague, G. F. : I n t r a p l o t competition among maize single crosses. Crop Sci. 4, 467--471 (1964).
Received April t6, t974 Communicated b y R. Riley
E1-Sharkawy, M., Hesketh, J.: Photosynthesis among species in relation to characteristics of leaf a n a t o m y and COs diffusion resistances. Crop Sci. 5, 517--521 (1965). Funk, C. R., Anderson, J. C.: Performance of mixtures of field of Corn (Zea mays L.) hybrids. Crop Sci. 4, 353--356 (1964). Hesketh, J. D., Moss, D. N. : Variation in the response of photosynthesis to light. Crop Sci. 3, I 07 -- 110 (1963). Hozumi, K., Koyami, Hirosi, Kira, Tatuo: Interspecific competition among higher plants. IV. A preliminary account of the interaction between adjacent individuals. J. Inst. Polytechnics, Osaka City Univ. Series D. Vol. 6 (~955). Jensen, N. F., Federer, W. T. : Adjacent row competition in wheat. Crop Sci. 4, 641--645 (1964). Kannenberg, L. W., Hunter, R. ]3. : Yielding ability and competitive influence in hybrids mixtures of maize. Crop Sci. 12, 274--277 (1972). Schutz, W. M., Brim, C. A. : Inter-genotypic competition in soybeans. I. Evaluation of effects and proposed field plot design. Crop Sci. 7, 37t --376 (1967). Smith, D. D., Kleese, R. A., Stutham, D. D. : Competition among oat varieties grown in hill plots. Crop Sci. lo, 381--384 (1970). Stringfield, G. H. : Performance of corn hybrids in mixtures. Agron. J. 5 1 , 4 7 2 - 4 7 3 (t959). Subandi, Compton, W. A., Empig, L. T.: Comparison of the efficiencies of selection indices for three traits in two variety crosses of corn. Crop Sci. t3, 184--186 (1973).
A. Jurado-Tovar Assistant Professor Universidad Nacional Tecnia de Piura Piura (Peru) W. A. Compton Associate Professor University of Nebraska Lincoln, Nebraska 68 508 (USA)
Theoret. Appl. Genetics, Vol. 45, No. 5
Intergenotypic Competition Studies in Corn (Zea mays L.) I. A m o n g E x p e r i m e n t a l H y b r i d s 1 A. JURADO-TOVAR and W. A. COMPTON U n i v e r s i t y of N e b r a s k a , L i n c o l n , N e b r a s k a (USA)
Summary. Four experimental single-cross hybrids were evaluated for intergenotypic competition in a split-plot design with 7 replicates and a stand density of 51,700 plants/hectar in 1970, 1971 and 1972 at Lincoln, Nebraska. The arrangement of rows used in this study allowed the measurement of effects of different levels of competition on the traits grain yield, plant height and a selection index. There were some definite inter-genotypic competitive effects for all three traits among the pairs of hybrids studied. Variation in types of intergenotypie interaction was found. A two-step process was suggested to take advantage of favorable competitive interactions for increasing grain yield. Failure of mixtures in corn to take advantage of favorable competitive situations was discussed.
Introduction and Literature R e v i e w P l a n t research a i m e d at m e a s u r i n g c o m p e t i t i o n effects is p r o v i d i n g e v i d e n c e t h a t the p e r f o r m a n c e of a g e n o t y p e is o f t e n affected b y o t h e r g e n o t y p e s g r o w i n g n e a r - b y . T h u s we are faced w i t h t h e fact t h a t selective values of g e n o t y p e s in pure s t a n d are n o t always the same as t h e v a l u e s t h a t occur u n d e r i n t e r g e n o t y p i c c o m p e t i t i o n . T h i s s i t u a t i o n is an i m p o r t a n t c o n s i d e r a t i o n in p l a n t b r e e d i n g , b u t to date, c o m p e t i t i o n i n f o r m a t i o n has n o t b e e n w i d e l y u t i l i z e d i n b r e e d i n g programs. T h e c h a r a c t e r i z a t i o n of c o m p e t i t i v e responses b y use of a hillplot a r r a n g e m e n t of t r e a t m e n t s , as origin a l l y suggested b y S c h u t z a n d B r i m (t967) i n soybeans, has beeu m a d e i n o t h e r s e l f - p o l l i n a t e d crops such as b a r l e y , w h e a t a n d oats (Allard 1969, S m i t h et al. 1970). I n f o r m a t i o n on c o m p e t i t i v e effects a m o n g corn h y b r i d s is v e r y m e a g e r (Stringfield t959, E b e r h a r t et al. 1964 a n d F u n k a n d A n d e r s o n 1964), a n d no results h a v e b e e n r e p o r t e d w h e n more t h a n 2 levels (dosages) of c o m p e t i t i o n are o p e r a t i n g . I t also appears t h a t critical c o m p e t i t i o n i n f o r m a t i o n i n b r o a d - b a s e d m i x t u r e s or r a n d o m - m a t i n g p o p u l a t i o n s is Virtually n o n - e x i s t e n t . T h e m a j o r o b j e c t i v e of this s t u d y was to e v a l u a t e a n d characterize responses u n d e r 3 levels of compet i t i o n of e x p e r i m e n t a l h y b r i d s .
Materials and Methods Pedigrees and reference codes of the corn hybrids used in this study are : Pedigree N128 (Nebr. B synthetic) • B67 (Iowa SSS). N20 (Nebr. sel. of Stiff Stalk Synthetic) • A257 (Sel. f r o m A 7 3 X Os420) N t 32 (Nebr. 13 synthetic) • N 138 (Sel. from []3t4 • Oh43] BI4) N 20 • N2 (Nebr. B synthetic)
Reference Code A ]3 C D
The experiment was planted in 1970, 1971 and 1972. I t was analyzed as a split-plot arrangement of treatments with a hybrid pair as the 8-row m a i n plot and combinations of the hybrids in sequence (as shown below) as subplots. Randomization was not done for the hybrid combinations. The total sequence (whether one starts with X or Y each time) was randomized. Seven replicates were used with 3 levels of competition that could be evaluated. The arrangement of an 8-row main plot for comparing two hybrids, one represented by X and the other by Y can be demonstrated in the following way: X
X
X
Y
X
Y
Y
X X X X X X X X
X X X X X X X X
X X X X X X X X
Y Y Y Y Y Y Y Y
X X X X X X X X
Y Y Y Y Y Y Y Y
Y Y Y Y Y Y Y Y
X
X
X
Y
X
Y
Y
Y
-- discarded hills
T Y
-- discarded hills
Rows : t 2 3 4 5 6 7 8 X levels : Lo L1 L~ Y levels: L2 L 1 Lo discarded row discarded row X and Y d@signate one hill of an eight-hill row of a hybrid and its competitor hybrid, respectively. At harvest time the two rows on either end were discarded. Referring to the diagram above, rows 2, 3, and 5 provide information on level 0 (L0), level t (L1) and level 2 (L2) of competition for hybrid X, and rows 7, 6, and 4 provide the same information on Y. Level 0 designates the variety in pure stand since the competitor rows on either side contain the same hybrid. Level I is used to designate t h a t the hybrid was bordered by I competitor row and t row of the same hybrid, and level 2 means that the hybrid was bordered by two competitor rows. Level 2 also specified the m a x i m u m inter-row competition level. The assumption was made that, for instance AAB and 1 Published as Paper Number 3752, Journal Series, Nebraska Agricultural Experimental Station.
206
A. Jurado-Tovar and W. A. Compton: Intergenotypic Competition Studies in Corn I.
13AA Were equivalent with respect to the yield of the test genotype A in relation to the competitor or border genotype B. The reference code for the hybrid pairs involved is as follows : A and B ~ hybrid pair I (HP1) A and C ~ hybrid pair 2 (HP2) A and D ~ hybrid pair 3 (HPa) B and C = hybrid pair 4 (HP4) B and D = hybrid pair 5 (HPs) C and D ~ hybrid pair 6 (HP~) Four seeds per hill were planted and seedlings were thinned to three plants per hill for a final stand density of 51,700 plants/hectare. I n 1970, hills with one or more plants missing were eliminated along with the surrounding hills. I n t 971 and 1972, hills with 2 plants were utilized along with three plant hills and only those with two or three plants missing were removed along with the surrounding hills. The experiment was irrigated when needed. Nitrogen was applied at the rate of 168 kg/ha in 1970 and 1971 and 280 kg/ha in 1972. Measured agronomic variables were the following: 1) Grain weight in quintals per hectare (q/ha) at 15.5% moisture content, 2) Percent moisture, 3) Average plant height of 10 competitive plants, 4) Percent broken stalks, and 5) Percent dropped ears. The selection index (S. I.) was calculated as (S. I.) = X 1 (100 -- X~) (100 -- Xa) 'lO0
100
where Xl, Xfl and X a are yield, percent lodging and percent dropped ears, respectively. This index has been used in the Nebraska Corn Project since 1969 and can be used as an estimate of machine-harvestable yield from hand-harvested data. I t is very easy to apply and no >arameter estimation is involved in its construction Subandi, Compton and Empig, 1973),
Experimental Results There were some definite i n t e r g e n o t y p i c compet i t i v e effects f o u n d in the h y b r i d pairs studied. On the whole, these effects were l i m i t e d to a m a x i m u m of a b o u t t 0 q u i n t a l s / h a (about t 0 - - 1 3 % ) in this set of m a t e r i a l . E v i d e n c e of c o m p e t i t i v e effects is c o n t a i n e d i n the analyses of v a r i a n c e shown in T a b l e t. There is a c o n s i s t e n t increase in the m e a n squares w i t h i n each h y b r i d pair (HP,) as one e x a m i n e s first the " w i t h i n l e v e l 0" source, t h e n the " w i t h i n level 1 ", a n d finally the " w i t h i n level 2" source. The o n l y real v a r i a n t from the p a t t e r n is in the c o m p a r i s o n for HPo. Similar conclusions can be d r a w n from Fig. 1, where the m e a n s referred to in the above p a r a g r a p h are p r e s e n t e d graphically. Graphical displays are sometimes more easily u n d e r s t o o d t h a n s t a t i s t i c a l jargon. Again note the general t r e n d t o w a r d divergence of the m e a n s with i n c r e a s i n g levels of competitioI~. Since the largest differences were f o u n d at comp e t i t i v e level 2, o n l y those m e a n s are shown in T a b l e 2 for c o m p a r i s o n with pure s t a n d values. Note t h a t there were d e v i a n t s t h a t were h i g h l y significant. T a b l e 3 c o n t a i n s analyses of v a r i a n c e m e a n squares for the other two traits. The p l a n t height m e a n squares for " w i t h i n levels 0, t, a n d 2" have a divergence t r e n d opposite to t h a t shown above for g r a i n yield. I n other words, i n c r e a s i n g levels of compe-
Table 1. A nalyses of variance of mean grain weight in quintals~hectare of hybrid pairs z through 6 combined over years Analyses of variance Mean squares Source of variation
Degrees of freedom
HP 1
HP=
HP 3
HP 4
HP 5
Entries/HPi 1 Level 0 vs. levels 1 and 2/HPi Level t vs. level 2/HPi Within level 0/HPi W i t h i n level t / H P i Within level 2/HPi Pooled Error
5 1 1 1 1 1 60
2125"* 56 46 2292"* 2297** 5936** 147
1435"* 4 0 t 33 1533'* 5505** 147
977** 128 54 55 608* 4041"* 147
294 83 t56 29 639* 562 147
284 89 107 28 106 t2 2 320 2t7 52 987* 34 t47 147
HP 6
1 i subscript used to identify hyiJrid pairs 1, 2, --6. * significant at the .05 level of probability. ** significant at the .01 level of probability. 100
H8
q/ha
H~ ;.,c"
HP5
H~ _z2.
7fl 0
1
2'0
1
;tO
1
20
Levels of competition within each hybrid pair
1
20
1
2
Fig. t. Diagramatic illustrations of grain weight with increasing levels of competition for each hybrid within each hybrid pair
Theoret. Appl. Genetics, Vol. 45, No. 5
A. Jurado-Tovar and W. A. Compton: I n t e r g e n o t y p i c Competition Studies in c o r n I. Table 2. Mean competitive and pure stand grain yields (q/ha) for each individual hybrid and level 2 of competition, averaged over 3 years Grain yield (q/ha) Test hybrid
Competitor hybrid
A ]3 A C A D 13 C B D C D
13 A C A D A C A D 13 D C
With Pure competition stand
Difference+
98.62 74.84 93.82 70.92 85.73 66.12 83.27 90.59 8O.55 90.24 8t.t9 82.98
2.36 --6.65 i0.t0"* --9.25* 7.32 --t0.00"* --2.48 3.t8 --3.79 5.46 --4.28 3.04
96.26 81.49 83.72 80.17 78.41 76.12 85.75 87.41 84.34 84.78 85.47 79.94
+ Differences (column 3 -- column 4). * Significant at the 0.05 level of probability. ** Significant at the 0.0t level of probability.
207
N o s i g n i f i c a n t differences were f o u n d a m o n g levels for a n y of t h e t r a i t s . I n o t h e r words, w h e n one h y b r i d i n c r e a s e d in v a l u e , t h e o t h e r t e n d e d to d e crease p r o p o r t i o n a t e l y . T h i s i m p l i e s t h a t t h e effects are l a r g e l y of a c o m p l e m e n t a r y n a t u r e . Discussion
T h e r e s u l t s of t h i s e x p e r i m e n t i n d i c a t e c l e a r l y t h a t i n t e r g e n o t y p i c c o m p e t i t i o n as o b s e r v e d b e t w e e n t h i r t y - i n c h rows can h a v e a n effect u p o n y i e l d . I t is also s h o w n t h a t t h e r e s p o n s e s of a m a i z e g e n o t y p e t o t h e s h a r i n g of t h e e n v i r o n m e n t w i t h a d i f f e r e n t g e n o t y p e c a n n o t be d e s c r i b e d e n t i r e l y in t e r m s of c o m p l e m e n t a r y effects. T a b l e 4 p r e s e n t s a s u m m a r y of t h e i n t e r - g e n o t y p i c r e l a t i o n s h i p s for g r a i n y i e l d a t t h e d i f f e l e n t levels of c o m p e t i t i o n . V a l u e s g r e a t e r or less t h a t 50/0 of t h e p u r e s t a n d v a l u e s were u s e d to c h a r a c t e r i z e o v e r c o m p e n s a t o r y a n d u n d e r c o m p e n s a t o r y effects, res p e c t i v e l y . C o m p l e m e n t a r y effects a c c o u n t e d for 50% of t h e c o m p e t i t i v e i n t e r a c t i o n s . N e u t r a l , u n d e r c o m -
Table 3. Means squares for plant height and the selection index from the analyses of variance combined over years Mean squares Plant height (cms.) Source of variation Entries/HPi I Level 0 vs level t & 2/HPi Level t vs level 2/HPI With-in level 0/ttPt W i t h i n level t / H P t W i t h i n level 2/HPi Pooled error
d.f. 5 1 1 t l 1 60
HP 1 366t** 75 15 7809** 5964** 4441"* t49
HP 2 333 65 22 653* 617" 307 149
HP 3 2346** 420 43 3696** 5522** 2053** 149
HP, t905"* t49 118 4248** 3741"* t268"* 149
HP 1 2119"* 267 J 60 1160" 2010"* 7046** 1t7
HP~ 374 1 57 338 t41 1330" 117
HP3 HP4 566* 595 1 0 295 t74 550 271 33 .1077" t950"* 1455"* 117 t17
HP5 32 5 62 74 3 16 149
HPs 1 586 19 19 3060** 3020** 1810"* 149
HP~ 603* 33 t21 130 397 2332** t17
HP s 339 19 38 1089" 435 t13 1t7
Mean squares Selection index (q/ha) Source of variation Entries/HPi 1 Level 0 vs level I & 2/HPi Level t vs level 2/HPi W i t h i n level 0/HPi W i t h i n level t / H P i W i t h i n level 2/HPi Pooled error
1 i subscript used to identify hybrid pairs t, 2, --6. * sigfiificant at .05 level of probability. * * significant at .0t level of probability. t i t i o n reduce differences in p l a n t h e i g h t b e t w e e n t w o competing hybrids. T h e selection i n d e x values, as one m i g h t e x p e c t , a r e m u c h less c o n s i s t e n t t h a n a r e t h o s e for g r a i n yield. T h e r e is a slight t r e n d for a n i n c r e a s i n g difference w i t h i n c r e a s i n g levels of c o m p e t i t i o n , b u t p r o b a b l y n o t e n o u g h to w a r r a n t f u r t h e r p u r s u i t . Theoret. Appl. Genetics, Vol. 45, No. 5
p e n s a t o r y a n d o v e r c o m p e n s a t o r y effects a c c o u n t e d for t h e o t h e r 50% of t h e c o m p e t i t i v e i n t e r a c t i o n s in p r o p o r t i o n a l a m o u n t s (16.6%). T h e s e p e r c e n t a g e s reflect c o n s i d e r a t i o n of b o t h levels of c o m p e t i t i o n . H o w e v e r , w h e n t h e t e s t g e n o t y p e is s u r r o u n d e d on b o t h sides b y t h e c o m p e t i t o r g e n o t y p e (level 2), t h e n o n l y c o m p l e m e n t a r y effects 6 7 % ) , n e u t r a l effects
A. Jurado-Tovar and W. A. Compton: Intergenotypic Competition Studies in Corn I.
208
Table 4. Characterization of intergenotypic relationships for grain yield of 4 hybrids grown under 2 levels of competition. 3 years data +
Hybrids
Hybrids
A
LI L2
B
Lt L2
C
L2 L1
B
C
D
N UC
C C
UC C
OC N
OC C C C
L1 -- Level I of competition L2 = Level 2 of competition N = Neutral C = Complementary OC-- Overcompensatory UC = Undercompensatory Based on Schutz et al., 1968. (17%) and u n d e r c o m p e n s a t o r y effects (t7%) were observed. W e should be reminded t h a t these specific characterizations of competitive interactions apply only to the restricted sets of hybrids studied. The experimental hybrids were selected without any morphological or physiological considerations. It appears t h a t the grain yield response of different corn hybrids under different levels of competition has not been reported previously. However, for purposes of comparison, we can relate our findings to other experiments in corn where intergenotypic competition effects of some sort were measured. Stringfield (1959), using mixtures involving equal numbers of seed from two contributing members, reported no advantages in grain yield of the nfixtures over the average of the contributing hybrids grown separately. Since the arrangement of 4 seeds per hill of the component hybrids was at random, it appears that intra- and intergenotypic competition within a row was involved. Neither distance between rows nor plant density was reported. Funk and Anderson (1964) reported that the blending of two corn hybrids in alternate rows did not appear to increase grain yield over the mean of the component hybrids grown separately. Inter-row competitive effects were considered with only one border row as a competitor genotype, which in our s t u d y was designated as level I of competition. The distance between rows was 36 inches. The results of their experiment are in agreement with our s t u d y when only one level of competition is considered. E b e r h a r t et al. (t964) measured intra-plot competition among two sets of maize single crosses. The blending of two hybrids was done either in the hill or in alternate hills every t3 inches. Only intergenotypic competition within a row was involved since only one plant represented a particular hybrid. The competition effects were
of a comparative type (complementary) similar to the results of Stringfield. Caution should be exercised in making comparisons of within-row competition and between-row competition. We will elaborate on this point later in the discussion. T h a t certain hybrids show a constancy of intergenotypic competitive effects in response to different genotypes is shown b y the behavior of hybrid D under level 2 of competition. The complementary effects of hybrid D are displayed regardless of the competitor present and the increases or decreases in grain yield of hybrid D are specific to the competitor involved. Also, it appears that for some hybrid pair combinations, the similarity in response to intergenotypic competition m a y not be affected b y the levels of competition involved. The competitive situation of most interest is the one where cooperation is involved (overcompensation). These favorable interactions have been reported mainly in selfpollinated crops (Schutz and Brim 1967, Jensen and Federer 1964). In the search for increased productivity, new imaginative approaches will have to be taken b y breeders and ecologists. If the environments and the genotypes can be specified whereby favorable interactive relationships are established, then potential yield levels can be raised over the yield levels of the pure stands of the genotypes themselves. In this connection, the following statement b y Haldane is suggestive. "While the most obvious symbiosis to look for and to exploit, if discovered, to increase agricultural production, is between different species, particularly cereals and legumes; nevertheless, if such symbiotic relations are common, they should be looked for between different genotypes of the same species". Use of favorable competitive interactions has been suggested b y Jensen and Federer (1964) in wheat and by Schutz and Brim (t967) in soybeans. In inter-row competitive situations only two levels of competition are well defined. They are determined b y the n u m b e r of adjacent rows used as competing genotypes. When the genotype is in pure stand, then intergenotypic competition is not operating. If we want to take advantage of favorable interactions for increasing grain yield a two-step process is required: 1. Determination of the appropriate competition level for a pair of hybrids such t h a t a net increase over the sum of both genotypes in pure stand is shown. 2. Duplication of the experimental combinations on large scale using the favorable interacting genotypes and the defined competitive environment. If we proceed to step 2 with the hybrids B and C, from our study, using level I of competition for hybrid C and level 2 for hybrid B, we could expect to have a net increase of 4.1% over the pure stand value of the component genotypes, or 3.1% over C Theoret. Appl. Genetics, Vol. 45, No. 5
A. Jurado-Tovar and W. A. Compton: Intergenotypie Competition Studies in Corn I. in pure stand (for values see Appendix Table At). The highest yielding genotype (C) constitutes 2/3 of the component genotypes, but some increase f r o m having B as a competitor gives the combination an advantage. For obtaining level t for C and level 2 for I3, using a six-row ptanter, we would proceed in the following w a y : C B C CB C C B C C B C
11111l]1 t
2 3 4
5 6 6
IIII 5 4
3 2
4
t In the first pass the C 13 C C B and C are planted and in the following pass the inverted sequence. The question t h a t arises is then " W h y can we not take advantage of the favorable competitive situtions in mixtures in corn ?" The results of experiments utilizing mixtures in corn are very meager. The experiments reported b y Stringfield (1959), F u n k and Anderson (t964) and Kannenberg and H u n t e r (t972) show t h a t no consideration was given to the intergenotypie relationships for the establishm e n t of mixtures. The evidence available in other crops t h a t within-row competition is different from between-row competition (Jensen and Federer 1954, Schutz and B r i m 1967 and Smith et al. t970) could also be true in corn. If optimization of proportions in mixtures is required, additional characterization of intergenotypic competition effects as observed in hill-plot arrangement m a y be helpful. Also, competition effects between drilled rows might also be observed and utilized. The diversity of genotypes and environments involved m a k e a difficult t a s k of ascertaining the factor or factors involved in competitive situations. Plant height undoubtedly plays a role in plant competition. The best competitor in our s t u d y turned out to be the tallest genotype in relation to the other genotypes. Also, it was observed t h a t genotypes virtually identical in plant height showed a yield response due to competition. The basic function of the corn canopy is the interception of light. One can speculate t h a t tall genotypes adjacent to shorter genotypes in border rows should intercept more light while the short ones m a y be expected to be at a disadvantage for available light. Is it possible to m a k e a general s t a t e m e n t about the plant height and grain yield relationship under competitive conditions ? The answer is yes, but with some reservations. Earlier we mentioned the consistent increase and decrease in the size of the mean squares for grain yield and plant height, respectively. I n fact the tallest genotype (hybrid A), which also was the stronger competitor with respect to grain yield, decreased in plant height in all the combinations analyzed under competitive conditions. The opposite effect was found for the shortest genotype (with two exceptions). F r o m the above considerations, it appears t h a t the shorter plants (shaded) t e n d to elongate more rapidly t h a n Theoret. Appl. Genetics, Vol. 45, No. 5
209
the taller plants (unshaded). Hozmni et al. (t955) also using corn, noted t h a t under closely spaced conditions, the shorter plants had a higher elongation rate t h a n the taller ones t h a t were shading them. We have to keep in mind t h a t in t h a t group of species which has a low compensating point, of which corn is one, light saturation is not reached even at full sunlight intensities (Hesketh and Moss t963). The resistance to carbon dioxide diffusion in the leaf is low, and the photosynthetic rate correspondingly high (E1-Sharkawy and Hesketh t965). On the other hand, in the high compensating group, such as soybeans, there is little response to light under a fixed level of carbon dioxide (Brun and Cooper 1967). In response to the question of relating plant height to grain yield under competitive conditions, perhaps one should expect a significant response for grain
Appendix Table At. Over-all mean of grain yield (q/ha), plant height (cms), and selection index (q/ha) for 6 entries within each hybrid pair, averaged over 5970, 1971 and J972 Hybrid
Entries
Level
yield
height
Plant
Selection index
A A h B B B
Lo L1 L~ L, L1 L~
(q/ha) 96 96 99 81 81 75
(cms) 3o2 299 296 270 270 271
(q/h a)
I
2
C C C A A A A A A D D D
L0 L1 L~ L0 L1 L2 Lo L1 L2
80 76 7t 84 88 94 78 78 86 76 7t 66
285 285 285 295 293 292 294 293 287 271 266 270
73 67 65 67 71 76 58 59 71 65 61 57
13 B 13 C C C B B 13 D D D
Lo L1 L~ Lo
86 86 83 87 94 91 84 85 81 85 90 90
266 265 266 289 287 279 265 a67 266 268 267 264
77 76 72 82 86 84 74 7t 64 70 77 79
85 80 81 80 82 83
287 289 286 268 269 27t
79 75 75 68 75 72
pair
3
4
5
6
C c C D D D
Lo
L1 L~
L~ L2
L0 L~ L~ L0 L1 L2 Lo L, L~ L0 L~ L2
Grain
83 82 87 72 69 61
2t0
A. Jurado-Tovar and W. A. Compton: Intergenotypic Competition Studies in Corn I.
yield under competitive conditions when height differences are i n v o l v e d in t h e low c o m p e n s a t i n g g r o u p . L i g h t i n t e r c e p t i o n m e a s u r e m e n t s were n o t i n c l u d e d in t h i s s t u d y b u t t h e y s h o u l d be t r i e d in f u t u r e s t u d i e s . Likewise, m e a s u r e m e n t s of r o o t sys t e m s m i g h t h e l p to f u r t h e r u n d e r s t a n d b e l o w - g r o u n d factors in explaining competitive interactions among different genotypes. T h e i n f l u e n c e of t i l l e r i n g a b i l i t y on c o m p e t i t i v e i n t e r a c t i o n s was n o t e v a l u a t e d since v i s u a l observ a t i o n s d e t e c t e d no t i l l e r i n g a m o n g t h e d i f f e r e n t g e n o t y p e s . Also, m a t u r i t y differences as d e t e r m i n e d b y m o i s t u r e c o n t e n t of t h e g r a i n were n o t d e t e c t e d . D a t a on n u m b e r of ears p e r p l a n t (not i n c l u d e d in this p a p e r ) s u g g e s t e d t h e absence of prolificacy. Literature
Allard, R. W. : Populations studies in predominantly selfpollinating species. X l I I . Intergenotypic and population structure in barley and wheat. Amer. Natur. lo3, 6 2 t - - 6 4 5 (1969). Brun, W. A., Cooper, R. L. : Effect of light intensity and carbon dioxide concentration on photosynthetic rate of soybean. Crop Sci. 7, 451--454 (1967). Eberhart, S. A., Penny, L. H., Sprague, G. F. : I n t r a p l o t competition among maize single crosses. Crop Sci. 4, 467--471 (1964).
Received April t6, t974 Communicated b y R. Riley
E1-Sharkawy, M., Hesketh, J.: Photosynthesis among species in relation to characteristics of leaf a n a t o m y and COs diffusion resistances. Crop Sci. 5, 517--521 (1965). Funk, C. R., Anderson, J. C.: Performance of mixtures of field of Corn (Zea mays L.) hybrids. Crop Sci. 4, 353--356 (1964). Hesketh, J. D., Moss, D. N. : Variation in the response of photosynthesis to light. Crop Sci. 3, I 07 -- 110 (1963). Hozumi, K., Koyami, Hirosi, Kira, Tatuo: Interspecific competition among higher plants. IV. A preliminary account of the interaction between adjacent individuals. J. Inst. Polytechnics, Osaka City Univ. Series D. Vol. 6 (~955). Jensen, N. F., Federer, W. T. : Adjacent row competition in wheat. Crop Sci. 4, 641--645 (1964). Kannenberg, L. W., Hunter, R. ]3. : Yielding ability and competitive influence in hybrids mixtures of maize. Crop Sci. 12, 274--277 (1972). Schutz, W. M., Brim, C. A. : Inter-genotypic competition in soybeans. I. Evaluation of effects and proposed field plot design. Crop Sci. 7, 37t --376 (1967). Smith, D. D., Kleese, R. A., Stutham, D. D. : Competition among oat varieties grown in hill plots. Crop Sci. lo, 381--384 (1970). Stringfield, G. H. : Performance of corn hybrids in mixtures. Agron. J. 5 1 , 4 7 2 - 4 7 3 (t959). Subandi, Compton, W. A., Empig, L. T.: Comparison of the efficiencies of selection indices for three traits in two variety crosses of corn. Crop Sci. t3, 184--186 (1973).
A. Jurado-Tovar Assistant Professor Universidad Nacional Tecnia de Piura Piura (Peru) W. A. Compton Associate Professor University of Nebraska Lincoln, Nebraska 68 508 (USA)
Theoret. Appl. Genetics, Vol. 45, No. 5
