The contribution of different feedstuffs to nitrogen reaching the duodenum was evaluated in situ. Dacron bags containing barley grain, corn grain, wheat silage, corn silage, alfalfa hay, rye grass, whole cottonseeds, or soybean meal were suspended in the rumens of three dairy cows fed roughage and concentrate diets. The effective degradability of the nitrogenous and organic matter of feedstuffs was calculated from their residues after incubation in the rumen for 3, 6, 9, 12, 24, 36, or 48 h. The duodenal nitrogen content at ruminal outflows of 2, 5, or 8%/h was calculated as the sum of undegradable dietary nitrogen and potential microbial nitrogen (assuming 32 g N/kg ruminally degradable organic matter). Comparison of the in situ estimates with previously reported in vivo measurements of duodenal nitrogen in cattle fed diets with similar ingredients to the tested feedstuffs yielded a linear relationship (r2 = .887). The dacron bag technique appears to hold promise for the prediction of nitrogen flow to the duodenum.
Elements of the metabolizable protein system in the United Kingdom were examined for their suitability as potential predictors of milk protein concentration. Models were based on data from 163 cows offered five forage mixtures for ad libitum intake plus concentrates at 3, 6, or 9 kg/d of dry matter. The models were then tested on a separate data set of 100 cows offered seven forage mixtures for ad libitum intake plus concentrates at 6 kg/d of dry matter. To minimize problems with collinearity, variables were arranged hierarchically; successive elements were components of variables at higher element levels. Variables from different element levels were not used in the same models. Models were constructed using ridge regression to minimize problems with collinearity. The fit and precision of prediction were generally poor because these models did not take into account animal variables. Models using undergradable dietary protein performed slightly better than did those using digestible undegraded protein. The use of slowly degradable protein and quickly degradable protein rather than rumen-degradable protein generally resulted in improvements in prediction. Models using neutral detergent fiber and quickly fermented carbohydrate were better than those using total carbohydrate. We concluded that there was little to be gained from using the elements of the metabolizable protein system considered here for the prediction of milk protein concentration.
The response in dairy cows fed high-concentrate diets to abomasal infusion of lysine (Lys) and methionine (Met) at two levels of dietary CP was examined. Four multiparous Israeli Holstein cows (day in milk=l 80±30, means ± SE) were utilized in a 4x4 Latin square design experiment that included a 2x2 factorial arrangement with 18-d periods. Cows were surgically prepared with abomasal cannulae and catheters implanted in the costoabdominal artery. Two diets were composed to contain high and low crude protein (CP) content (152 vs 132 g/kg dry matter). Abomasal infusion of either water or Lys (38 g/d) plus Met (14 g/d) was performed with each diet. On the last day of the experimental period the metabolism of amino acids (AA) across the mammary gland was monitored. Dry matter intakes and milk and protein yields were not affected by either dietary CP level or postruminal infusion of Lys plus Met and averaged 15.9, 21.4, and 0.694 kg/d, respectively. Milk fat content and yield were not affected by dietary CP concentration, but did increase with abomasal infusion of Lys plus Met (33.3 vs 37.2 g/kg; PO.04, and 0.703 vs 0.762 kg/d; PO.05, respectively). Arterial plasma concentration of Lys and Met increased by 2.4-and 3.5-fold, respectively, when these AA were infused abomasally.
Carbohydrates and proteins are the major dietary components supplying metabolisable protein for milk protein production. However, ether extract (EE) or fats have sometimes been related to negative effect on milk protein concentration (Spörndly, 1989; Smoler, 1996). Models for the prediction of milk protein concentration from combinations of carbohydrate and protein feed components have been constructed by Smoler (1996). In order to reduce collinearity among predictors and verify EE's negative effect on predictions of milk protein concentration, predictive models based on carbohydrate and protein dietary components were compared to those based on the same components but with the addition of EE.
Recent changes in the structure of the UK dairy industry are leading to closer specification of milk protein concentration (MPC) by customers. Accurate prediction of MPC from feed input variables is needed to allow producers to meet this demand. Previous attempts to predict MPC have been relatively unsuccessful (e.g. Rook et al., 1992). However, recent advances in feed description such as the AFRC (1993) protein system may allow further progress.
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