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Effect of dietary crude protein level on lactation performance, feed efficiency, and nitrogen use efficiency of dairy cows at different stages of lactation.

P. Letelier

Events

06-24-2020

Abstract:

350
Effect of dietary crude protein level on lactation performance, feed efficiency, and nitrogen use efficiency of dairy cows at different stages of lactation.
P. Letelier*1, G. I. Zanton2, M. A. Wattiaux1. 1University of Wisconsin-Madison Madison, WI, 2USDA-ARS, U.S. Dairy Forage Research Center Madison, WI.

Responses of dairy cows to level of dietary crude protein (CP) may depend on stage of lactation. In this study, we used a randomized complete block design to determine the effects of 4 levels of dietary CP on dry matter intake (DMI), fat- and protein-corrected milk (FPCM), feed efficiency (FE: FPCM/DMI) and nitrogen use efficiency (NUE: 100 x milk protein N/N intake) of 64 multiparous Holstein cows blocked by days in milk (DIM; initial average � SD 71 � 14.2 (E), 105 � 9.6 (ME), 153 � 21.6 (ML), and 225 � 10.9 (L)). Diets were formulated for 13.5, 15.0, 16.5, and 18.0% CP (dry matter basis) by exchanging solvent and expeller soybean meal for soyhulls and dried ground corn. For 14 d cows were individually fed a covariate diet followed by 56 d of treatment diets. With advancing DIM, both DMI and FPCM decreased linearly (from 28.8 to 26.9 kg/d for DMI and from 44.6 to 31.7 for FPCM for E and L cows, respectively). In contrast, the responses to decreasing dietary CP were both linear and quadratic with the greatest values observed when cows were fed the 16.5% CP diet (29.2 and 41.8 kg/d for DMI and FPCM, respectively) and the lowest values observed when cows were fed the 13.5% CP diet (26.6 and 37.1 kg/d for DMI and FPCM, respectively). However, both DMI and FPCM responses were influenced by an interaction between dietary CP and DIM. In E, ME, and ML, FPCM was not different when cows were fed the 16.5 compared with the 18% CP diets (averaging 43.7 kg/d). Conversely, in L, FPCM decreased by 7.0 kg/d from 35.1 to 28.1 kg/d when CP increased from 16.5 to 18.0%. In addition, when cows were fed the 13.5% CP diet, E cows produced 9.0 kg more FPCM compared with L cows (40.8 vs. 31.7 kg/d), whereas cows fed the 18% CP diet produced 19.3 kg more FPCM in E compared with L (47.4 vs. 28.1 kg/d). Feed efficiency (1.4 � 0.1) was not influenced by dietary CP level, but declined linearly from 1.6 to 1.2 for E to L cows. Finally, NUE declined linearly from 29.0 to 25.8% for E to L cows, and from 31.7 to 24.2% for 13.5 to 18.0% dietary CP.

Keywords: crude protein, stage of lactation, nitrogen use efficiency.