Response to Recombinant Bovine Somatotropin in Dairy COws with Different Genetic Merit for Milk Production A. J. NYTES,1 D. K. COMBS, G. E. SHOOK, and R. D. SHAVER Department of Dairy Science University of Wisconsin Madison 53706 R. M. CLEALE American Cyanamid Company Princeton, NJ 08540 ABSTRACT

Thirty-nine multiparous cows obtained from two genetic lines were utilized to determine the effect of genetic merit on lactation response to long-term administration of recombinant bST. Cow index ranged from -70 to 456 (X = 183) and -494 to -88 (X -288) kg milk for high and low genetic groups, respectively. Cows were blocked by calving date and randomly assigned to treatment within genetic group. Treatments were 0, 10.3, 20.6, and 30.9 mg somatotropin injected daily from wk 14 through 44 postpartum. Cows were fed one of two total mixed rations. Diet I (NEI = 1.65 Mcal/kg, CP = 18%, and ADF = 22%) was fed from start of lactation to at least 4 wk after initiation of treatment. Cows were switched to diet 2 (NE( = 1.56 Mcal/kg, CP = 16%, and ADF = 27%) when milk output fell below 25 kg/d. Forty-four week lactation yields were 9800 and 9447 kg milk; 364 and 354 kg fat; and 322 and 309 kg protein for high and low genetic groups, respectively. Milk, milk fat, or protein yield due to somatotropin did not differ between genetic groups. Increasing dosage of bST increased milk, 4% FCM, fat, and protein yields in a linear fashion. Percentages of fat and protein of milk were similar for all treatment groups. Body weight changes were not significantly different among treatments, but condition score changes decreased lin-

early with increasing dose of bST. Longterm treatment with recombinant bST had no apparent effect on incidence of health problems or reproduction. (Key words: somatotropin, cow index, milk)

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Received May 3, 1989. Accepted October 4, 1989. 1Present address: Purina Mills, SI. Cloud. MN 5630 I.

1990 J Dairy Sci 73:784-791

INTRODUC110N

Pituitary extracts containing somatotropin (ST) were shown to increase milk production in dairy cattle more than 50 yr ago (2). Short-term administration of ST caused no change or small reductions in feed intake. This provided large increases in the conversion of feed to milk without major changes in maintenance requirements or digestive physiology (18, 25). Several recent experiments have shown increases in milk yield due to long-term administration of recombinant bST (rbST) ranging from 900 to over 2700 kg per lactation (1, 4, 5, 6, 8, 9, 12, 23, 24). Many of these studies have demonstrated increases in feed intake (I, 4, 5, 6, 8, 23). Bauman et al. (5) reported energy intake increased gradually, and by wk 10 of treatment all rbST treatment groups were in positive energy balance. If appears cows treated with rbST for extended periods adjust their feed intake upwards in an attempt to meet nutritional demands. Few experiments have attempted to evaluate the response to rbST among cows with different genetic merit for milk yield. Hart et al. (11) noted that endogenous ST concentration of plasma was high in high producing Friesians in early lactation and declined as lactation advanced. Lower producing Friesian x Hereford and Frieisian x Sussex cattle had low concentrations of ST in plasma throughout lactation. Johnson and Vanjonack (13) noted higher con784

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GENETIC MERIT AND BOVINE SOMATOTROPIN

centrations of circulating somatotropin in high than in low yielding Holstein cows. It is not clear if differences in endogenous secretion of ST are due to genetic merit for milk yield or to changes in energy status due to level of production. Leitch et al. (14) found that cows with lower predicted production had a greater response, in terms of kilograms of milk produced due to rbST, than did cows of high predicted production potential. However, they also found that a cow's estimated transmitting ability did not explain a significant amount of variation in response of milk production when cows were administered rbST. McDaniel and Hayes (15) could find no evidence that high yielding cows or daughters of bulls with high PD for milk yield would respond any differently to rbST than cows of lower genetic merit. Objectives of this trial were to evaluate the effect of dosage level of rbST on lactational performance and the production response of rbST administration to dairy cows with different genetic production potentials. MATERIALS AND METHODS

Cows were from a 2Q-yr selection experiment designed to measure correlated responses to selection for high milk production. One-half the herd was bred continuously to progeny tested bulls with high PTA for milk yield and the other half was bred to bulls with PTA approximately 450 kg below the high bulls. Forty multiparous Holstein cows (20 from each genetic group) were blocked by calving date and randomly assigned to treatment within genetic group. One cow in the low genetic group was removed from the trial 10 d after the experimental period began due to chronic bloat throughout the pretreatment period. Therefore, only 39 cows were used in the analysis. Treatments began during wk 14 of lactation and continued until 70 d prior to next expected freshening date if the cow conceived prior to 200 d in milk. If a cow did not conceive prior to 200 d in milk, treatment continued until 400 d in milk or until milk output fell below 9 kg/d. Treatments were 0 (control), 10.3, 20.6, or 30.9 mg rbST/d administered in 2 ml of buffered saline and I, 2, or 3 ml of rbST solution. Somatotropin was injected at 1100 h daily in one of two alternating sites (soft tissue between the tailhead and pins) using a disposable sterile

TABLE I. Nutrient composition of total mixed rations on a dry matter basis. Item

Diet I

Diet 2

Dry matter, % Organic malter. % Acid detergent fiber. % Neutral detergent fiber, % Crude protein, % Ether extract, % NEIoMcal/kgl Calcium, % Phosphorus, % Potassium, %

65.51

51.36

91.87

91.33

22.10 31.42

26.54 36.0 15.73

17.73 2.77 1.65

.89 .46 1.90

3.32 1.56 1.03 .43 2.02

lCaiculated from data of NRC (17).

.86 X 12.7-mm needle fitted in a 3-ml disposable sterile syringe. Somatotropin was reconstituted with saline and stored at 5°C until just prior to injection. Cows were milked twice daily, and individual milk yields were measured by weigh jar. Milk was sampled from p.m. and a.m. milkings 1 dlwk and analyzed for fat, protein, and SCC by near infrared spectroscopy (NIR, Wisconsin DHIA Center, Appleton). Milk components from p.m. and a.m. milk samples were calculated from percentage of total milk output from each milking. Cows were fed ad libitum com silage and alfalfa silage (50:50, DM basis) and 2.7 kg/d ground shelled corn with trace mineral salt prior to calving. Three to 7 d postpartum, cows were adapted to a high energy density ration (diet I, Table 1). Cows remained on diet 1 until at least 18 wk after calving. Thereafter, cows were switched to a lower density ration (diet 2) when average daily milk output for 5 d fell below 25 kg/d. Diets were fed as total mixed rations (TMR) and offered twice daily for ad libitum intake. TMR ingredients were alfalfa silage, corn silage, ground shelled com, soybean meal, dicalcium phosphate, trace-mineral salt, and vitamins A, D, and E. Amount of ration offered was adjusted daily to obtain a minimum of 5% feed refusal. Feed refusals were weighed and recorded daily prior to a.m. feeding. Forage and concentrate was sampled 1 d/wk prior to a.m. feeding and analyzed by NIR (Soil and Plant Analysis Laboratory, Cooperative Extension Service, University of Wisconsin-Extension, Madison) for OM, CP, ADF, Ca, Journal of Dairy Science Vol. 73,

No.3, 1990

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NYTES ET AL.

and P. These analyses were used to rebalance rations weekly. A second forage and concentrate sample was collected weekly and pooled on a monthly basis. Forage composites were analyzed for OM by toluene distillation (3). The OM of concentrates was determined by drying at 100°C in a forced air ov~n for 24 h. Forage and concentrate ADF (10) and NDF (10) were determined. Ether extract was measured by side-arm extractor (3), and CP was determined by the Kjeldahl assay (3). Calcium, P, and K were determined after wet ashing by inductively coupled plasma emission spectroscopy (Soil and Plant Analysis Laboratory, Cooperative Extension Service, University of Wisconsin-Extension, Madison). Body weights were obtained during 3 consecutive d the 1st wk after calving and monthly until wk 14 of lactation. Body weights were then recorded on 3 consecutive d just prior to first injection and biweekly thereafter. Body condition scores were assessed monthly. Condition scores were the average of five individuals scoring each cow on a five-point scale (26), where 1 = thin and 5 = fat. Health and reproductive records were maintained for each cow for the entire lactation and subsequent dry period. Data obtained during the experimental period (wk 15 to 44 of lactation) were used for statistical analysis. Six cows were dried off before wk 44 of lactation because they were expected to calve within 70 d. Missing values were calculated by extrapolation of individual cow's lactation curves. Missing values were not calculated for two cows that exhibited symptoms similar to those for milk fever near the end of the experimental period. These cows were considered to have completed their lactation, because milk production did not return after recovery. Statistical analysis was by general linear model procedure of SAS (21). The randomized block model used for analysis of variance was y = Il + Bj + ~ + Tk + GTk + ejjkl, where Il = overall mean; Bi = block ellect i; OJ = genetic group effect j; Tk = treatment effect k; GTjk genetic group x treatment interaction; and eijkl = residual. Covariate adjustments for pretreatment yield were inappropriate because the genetic group effect was inherent in both pretreatment and treatment periods. Linear and quadratic effect of rbST dosage level were tested by orthogonal contrasts.

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Journal of Dairy Science Vol. 73.

No.3, 1990

RE5UL15 AND DISCUSSION

Diet 1 contained adequate CP and minerals to support 40 kg of milk (17) and was estimated to contain 1.65 Mcal NEllkg DM (17). Diet 2 contained adequate CP, minerals, and energy to maintain at least 25 kg of milk according to NRC (17). Cows were switched to diet 2 after average milk output for 5 d fell below 25 kg/d. Five cows receiving rbST remained on diet 1 their entire lactation. Control cows were switched to diet 2 sooner than cows treated with rbST since milk yield of control cows declined more rapidly as the lactation progressed. Lactation yields for high and low genetic groups were 9800 and 9447 kg milk, respectively. There was no overlap in cow indexes (USDA, January 1986) between genetic groups (Table 2). Cow indexes contained no information from lactations in the current triaL Least squares means (LSM) for all production and intake variables except milk fat and milk protein percentages were greater for cows in the high genetic group than the low group (Table 3). However, none of these variables was significantly different between genetic groups (P>.05). Average cow index of the genetic groups differed by 470 kg milk. The LSM for milk yield during the 3Q-wk treatment period differed by 320 kg between the two genetic groups. Differences between groups was expected to be approximately 500 kg based on the difference in cow index and portion of lactation on trial. The reason for the discrepancy is unclear. The low and high genetic groups were raised in the same environment since birth and were nearly identical in age (4.6 ± 1.2 yr and 4.5 ± 1.1 yr, respectively) when rbST was first administered. No interaction was found between genetic group and dosage level of rbST for any production variable measured in this study (Table 4). Milk yield was influenced more by each of the main effects than by the interaction of genetic group by treatment. Leitch et al. (14) reported that a cow's estimated transmitting ability (ETA) did not explain a significant amount of variation in the milk production response when cows were injected with 12.5, 25.0, and 50.0 mg rbST/d. Cows in that trial were divided into three groups by their ETA. They also grouped the same cows into three groups by their pre-

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GENETIC MERIT AND BOVINE SOMATOTROPIN TABLE 2. Average 44 wk milk yields (kg) and cow indexes (kg milk) of high and low genetic groupsl Cow index

Genetic group

n

Average

Range of

44-wk Milk yield

High Low

20 19

183 -288

-70 to 456 -494 to -88

9800 9447

lThe USDA January 1986 cow indexes were used to estimate genetic groups. Cow indexes contain no infonnation from lactation performance in this trial.

dieted production level prior to initiation of injections of rbST at 5 wk postpartum; the response in kilograms of milk yield due to rbST was lower in cows of higher predicted production levels. McDaniel and Hayes (15), however, reported that the response to rbST was similar in high and low yielding cows. Daughters of bulls with high ETA for milk yield aJso did not respond any differently to rbST than daughters of bulls with lower genetic merit. Injection of rbST increased milk yield compared with that of controls within 2 wk of initiaJ injection. Peak response to rbST administration was 2 to 3 wk after initiation of treatment and was similar across treatment groups (Figure l). Milk yield during the treatment period was greater (P.05) compared with controls (Table 5). NumericaJly, however, cows injected with

10.3 or 20.6 mg rbST produced approximately ]5% more milk during the treatment period than control cows. Milk fat and protein percentages were not affected by administration of rbST. Cows treated with rbST produced more 4% FCM, fat, and protein during the treatment period than did control cows (P

Response to recombinant bovine somatotropin in dairy cows with different genetic merit for milk production.

Thirty-nine multiparous cows obtained from two genetic lines were utilized to determine the effect of genetic merit on lactation response to long-term...
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