Lactation Performance of Mid-Lactation Dairy Cows Fed Ruminally Degradable Protein at Concentrations Lower Than National Research Council Recommendations
2008
Cyriac, J. | Rius, A.G. | McGilliard, M.L. | Pearson, R.E. | Bequette, B.J. | Hanigan, M.D.
The aim of this study was to test whether feeding of diets containing lower proportions of ruminally degradable protein (RDP) but with a constant proportion of ruminally undegradable protein (RUP) alters feed intake, milk production and yield, and the apparent efficiency of N utilization by mid-lactation dairy cows. During the covariate period (d 1 to 28), 40 mid-lactation cows (36 Holstein and 4 Jersey x Holstein cross-breds) were fed a common diet formulated to contain 11.3% of diet dry matter (DM) as RDP. During the treatment period (d 29 to 47), cows were randomly assigned to 1 of 4 diets formulated to contain 11.3, 10.1, 8.8, or 7.6% RDP, whereas ruminally undegradable protein remained constant at 7.1% of DM. All diets contained 47.5% forage and 52.5% concentrate on a DM basis. Dry matter intake was significantly reduced for the 7.6% RDP diet. The lowest RDP content was associated with a trend for reduced milk yield. Dietary RDP had no effect on body weight or milk fat, protein, and lactose contents. Milk protein yield was not affected by RDP level; however, milk fat yield decreased linearly as dietary RDP was reduced. Concentrations of plasma essential amino acids were unaffected, whereas milk urea-N concentrations decreased linearly as dietary RDP content was reduced. The apparent efficiency of N utilization for milk N production increased from 27.7% on the 11.3% RDP diet to 38.6% on the 7.6% RDP diet. The dietary RDP requirement of cows in this study was apparently met between 15.9 and 14.7% dietary crude protein. Milk production was not significantly affected by the 8.8% RDP (15.9% crude protein) diet even though the NRC (2001) model predicted that RDP supply was 87% of that required, suggesting the current NRC recommendations for RDP may be overestimated for mid-lactation dairy cows in this study.
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