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1 Department of Dairy Science, University of Wisconsin, Madison 53706
2 Agricultural Research Service, USDA US Dairy Forage Research Center, 1925 Linden Drive West, Madison, WI 53706
Corresponding author: S. M. Reynal; e-mail: sreynal{at}wisc.edu.
Twenty-eight (8 with ruminal cannulas) lactating Holstein cows were assigned to 4 x 4 Latin squares and fed diets with different levels of rumen-degraded protein (RDP) to study the effect of RDP on production and N metabolism. Diets contained [dry matter (DM) basis] 37% corn silage, 13% alfalfa silage, and 50% concentrate. The concentrate contained solvent and lignosulfonate-treated soybean meal and urea, and was adjusted to provide RDP at: 13.2, 12.3, 11.7, and 10.6% of DM in diets A to D, respectively. Intake of DM and yield of milk, fat-corrected milk, and fat were not affected by treatments. Dietary RDP had positive linear effects on milk true protein content and microbial non-ammonia N (NAN) flow at the omasal canal, and a quadratic effect on true protein yield, with maximal protein production at 12.3% RDP. However, dietary RDP had a positive linear effect on total N excretion, with urinary N accounting for most of the increase, and a negative linear effect on environmental N efficiency (kg of milk produced per kg of N excreted). Therefore, a compromise between profitability and environmental quality was achieved at a dietary RDP level of 11.7% of DM. Observed microbial NAN flow and RDP supply were higher and RUP flow was lower than those predicted by the NRC (2001) model. The NRC (2001) model overpredicted production responses to RUP compared with the results in this study. Replacing default NRC degradation rates for protein supplements with rates measured in vivo resulted in similar observed and predicted values, suggesting that in situ degradation rates used by the NRC are slower than apparent rates in this study.
Key Words: rumen-degraded protein • dairy cow • nitrogen metabolism • microbial protein
Abbreviation key: FAB = fluid-associated bacteria, FP = omasal fluid phase, HMSC = high-moisture shelled corn, IADF = indigestible ADF, INDF = indigestible NDF, LP = omasal large particle phase, LSBM = lignosulfonate-treated soybean meal, MP = metabolizable protein, 15NB = 15N background, NANMN = nonammonia nonmicrobial N, NDIN = neutral-detergent insoluble N, OTD = omasal true digesta, PAB = particle-associated bacteria, PD = purine derivatives, RC = ruminally cannulated, SP = omasal small particle phase, SSBM = solvent soybean meal.
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