The Effect of Protein Degradability on Milk Composition and Production of Early Lactation, Somatotropin-Injected Cows M. D. WlNSRYG, M. J. ARAMBEL,1 and J. L. WALTERS

Department of AnImal Science Utah State University Logan 843224815

(Key words: protein degradability, bST, milk composition)


Twenty multiparous Holstein cows in early lactation that received 500 mg bST injected every 2 wk were assigned to one of two treatments to examine the influence of diets that varied in degradability of protein Effects of degradability were determined on milk production and components and on nutrient digestibility. Treatments consisted of a basal ration (control) containing soybean meal as its primary degradable protein source and a ration (treatment) containing com gluten and meat and bone meals as the primary undegradable protein source, representing 33% undegradable protein in CPo The undegradability of protein sources did not influence DM! and BW. Milk yield, 3.5% FCM, and production efficiency of bST-treated cows were not affected by increased undegradable protein in the diet. Milk fat and SNF were not significantly increased by treatment. Lactose was significantly higher for the control diet (5.0 vS. 4.9%) but was not biologically significant. Increasing undegradability of protein significantly increased total protein in milk and casein percentage in milk protein (3.14 vS. 2.86% and 62.11 vs. 58.24%, respectively). Total tract digestibility of nutrients was unaffected by treatment; however, CP digestibility tended to be higher as undegradability increased (67. 85 vs. 62.83%).

Abbreviation key: CGM = com gluten meal, MBM = meat and bone meal, SBM = soybean meal. UIP = undegradable intake protein. INTRODUCTION

Received August 31, 1990. Accepted December 13, 1990. 1Reprinl requests. This paper is published with the approval of the director, Utah Agricultural Expaimcnt Station, Utah State University, Logan, as 10urnal Paper Number 4032. 1991 1 Dairy Sci 74:1648--1653

High producing cows have a high requirement for protein in the form of amino acids, one that is fulfilled by intestinal absorption of amino acids from dietary, microbial, and endogenous proteins (2, 32). New systems for determining requirements for dietary proteins have emphasized the need for some protein to be resistant to rumen proteolysis (7, 28). Dietary protein sources have varying degrees of degradability with soybean meal (SBM) more susceptible (7,9) and com gluten meal (CGM) and meat and bone meal (MBM) less susceptible to proteolysis (2). Percentage degradability affects the amount that reaches the smaIl intestine. Degradable dietary protein entering the rumen is broken down to ammonia, which is necessary for microbial growth through protein synthesis (5). Microbes can provide sufficient protein for maintenance, slow growth, and early gestation (25, 32) but cannot meet the increased demands for high milk production or rapid growth in young animals (22). Therefore, the ratio of degradable to undegradable protein supplements can be altered to determine the ratio necessary to support high milk. production (30). High producing cows require even more protein when they are injected with bST, a naturally occurring protein produced by the pituitary gland of cattle that increases milk. production efficiency (35). Considerable research has been conducted on the effects of bST in lactating dairy cows (6, 10, 26, 27, 35). Administration of bST results in a dramatic




shift in partitioning of available energy toward milk yield at the expense of body tissue (19, 33). This partitioning of nutrients is mediated by the endocrine system with insulin, glucagon, and bST most likely responsible for energy and protein partitiooing (11). Because of the effect bST has on nutrient utilization, an essential area of investigation is the mechanism by which dietary protein is manipulated by the host (11, 12). The importance of utilizing bypass proteins such as MBM and COM is that they can escape degradation by rumen microorganisms and be utilized more efficiently in the intestinal tract (29). Protein that escapes rumen fermentation could improve milk production or reduce the amoWlt of conventional protein source required (16, 22, 32). Also, degradable protein is not used as efficiently as undegradable protein due to the higher loss of ammonia from the rumen; thus, less protein is utilized by the host (17, 20). Soybean meal is highly degradable and supplies microbes with adequate N to be used as ammonia However, its ease of degradation means that some ammonia is lost across the rumen wall, which is either recycled as amm0nia in saliva or excreted in urine as waste. Some studies have reported an increase in milk production when diets were formulated to have a high undegradable intake protein (UlP), (33 to 40%) (9, 21), but others have not (1, 14). Due to this variability, new systems (23) are Wlder investigation to determine the exact requirements for dietary protein in high producing dairy cows. Some investigators (28) speculate that the new requirements are too high. In vivo and in situ studies have estimated romen protein degradation (24). However, studies in which diets were formulated on estimated rumen degradability and studies including bST and rumen protein degradability are limited (11, 12, 21). The objective of this study was to test effects of feeding diets formulated to contain high or low degradable protein, in combination with 500 mg bST injected every 2 wk, on response in milk production and milk components in early lactation dairy cows.

were allotted to one of two treatments following a 2-wk adjustment period. A lo-wk lactation trial was conducted to evaluate rumen 00degradable protein sources (COM and MBM) as partial replacement for a rapidly degradable protein source (SBM) in a complete mixed ration (Table 1). The two treatments consisted of a basal ration in which the UIP fraction of the total protein was supplied by COM and MBM and represented 33% (treatment) and a basal mixed ration with SBM as the degradable protein source (control) with a UIP of approximately 25%. All animals received subcutaneous injections of 500 mg of bST every 2 wk administered in one of four alternating sites behind the shoulder at approximately 1200 h. Each cow was assigned to an individual Calan gate (American Calan Inc., Northwood. NH) and fed twice daily (0630 and 1830 h). Refusals were measured once daily, and adjustments made in daily offerings so that each cow was fed for ad libitum intake (23). Feed was sampled once weekly, composited monthly, dried (3), and ground through a Wiley mill (Arthur H. Thomas, Philadelphia, PA) using a I-mm screen. Samples were analyzed for DM (3), CP (15), ADF, NDF (37), and ADF ash (36) (Table 1). Milk yield was recorded twice daily (0500 and 1700 h) with weekly milk samples composited from the last four consecutive milkings

TABLE 1. Composition and nutrient content of experi-

mental diet Ingredient (OM basis)

Twenty multiparous Holstein cows, selected on milk production and days postpartum (60 d),



Com silage Alfalfa bay Rolled barley and com (75% barley, 25% com) Beet pulp Com gluten meal Meat and bone meal Soybean meal Dicalci1lln phosphate Limestone




ADP NDP NEr.. McaI/kg Ash

50.13 8.15

50.16 10.03


27.59 627 2.51 3.13

6.27 7.77 .37 .69 69.75 14.50 22.00 41.90 1.67 1.57

.06 .2S

71.30 14.30 20.40 4320 1.67 1.50

Journal of Dairy Science Vol. 74, No.5, 1991



for each cow and analyzed for percentages of lactose, fat, SNF, and for SCC using a (Multispec M Infrared Analyzer; Wheldrake, Yark, England). Total milk protein (casein, whey, and NPN) was detennined according to AOAC methods (3). The milk components, total protein, whey, casein, and percentage of NPN were calculated from one initial milk sample. Percentage of total protein was analyzed using .2 ml of milk followed by digestion with sulfuric acid, using the Kjeldah1 procedure (3) and titration with boric acid as an indicator. Whey was detennined by first adjusting a Io-g milk sample to pH 4.6 with .IN HCI, followed by filtering sample through Whatman (Whatman, Clifton, NJ) number 5 paper. One gram of filtrate was used for Kjeldahl digestion. The NPN was analyzed using 10 g milk plus 5 g of 6% TCA to make a 2% solution of TCA, which was filtered through Whatman number 42 paper. Filtrate (5 ml) was used for digestion. Casein was calculated by difference. Body weights were recorded every 2 wk. Nutrient Digestibility

Fecal samples were collected twice daily (0630 and 1800 h) on the last 5 consecutive d of the experiment and were composited for each cow. Following drying at

The effect of protein degradability on milk composition and production of early lactation, somatotropin-injected cows.

Twenty multiparous Holstein cows in early lactation that received 500 mg bST injected every 2 wk were assigned to one of two treatments to examine the...
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