Feeding low crude protein diets in lactating dairy cows during summer months: 1. Improvements in milk production and nitrogen utilization

Abstract Text:

Heat stress increases protein catabolism and urinary nitrogen excretion resulting in reduced nitrogen-use efficiency (NUE) in livestock.  Feeding low CP diets may improve performance of lactating dairy cows during summer.  A study was conducted to evaluate the effect of feeding low RDP and RUP levels in cows during summer.  Forty-eight mid-lactation Holstein cows were assigned to treatments using a complete randomized block design in a 2x2 factorial arrangement of treatments (n=12/treatment).  Treatments included two levels of RDP (10% and 8%) and two levels of RUP (8% and 6%).  A common diet (10% RDP-8% RUP) was fed from d 1 to 21 followed by the respective treatment diets from d 22 to 42.  Milk samples were collected from d 36 to 42.  Cows were housed in a freestall barn and exposed to the prevailing temperature and humidity of July and August with no supplemental cooling.  Main effects and their interaction were tested using the Mixed procedure of SAS and reported as least square means ±SEM.  Rectal temperatures and respiration rates were recorded a.m. and p.m. during the treatments. Compare with a.m., p.m. increased temperature and respiration rates (38.9-39.7±0.07°C; P<0.001 and 64.0-87.1±1.4 breaths/min; P<0.001).  Compared with 10% RDP, the 8% RDP treatment increased DMI and milk protein yield in the 6% RUP treatment (19.0 vs. 18.4±0.32 kg/d and 1.02 vs. 0.96±0.02 kg/d) but decreased DMI and milk protein yield in the 8% RUP treatment (19.4 vs. 20.1±0.32 kg/d; interaction P<0.01 and 1.02 vs. 1.08±0.02 kg/d; interaction P<0.01).  There was a trend (P<0.07) for an interaction such that the 8% RDP treatment increased energy-corrected milk (ECM) yield compared with 10% RDP in the 6% RUP treatment (31.7 vs. 29.4 ±0.76 kg/d), but reduced ECM yield in the 8% RUP treatment (32.5 vs. 33.0±0.76 kg/d).  The 10% RDP treatment increased (P<0.001) milk-urea nitrogen compared with the 8% RDP treatment (10.2 vs. 6.9±0.28 mg/dL).  The 8% RUP treatment increased (P<0.001) milk-urea nitrogen compared with the 6% RUP treatment (9.8 vs. 7.2±0.28 mg/dL).  The 8% RDP treatment increased (P<0.001) NUE compared with 10% RDP (35.1 vs. 31.6±0.76%).  The 6% RUP treatment increased (P<0.001) NUE compared with 8% RUP (35.1 vs. 31.6±0.76%).  Therefore, lower RDP diets can be fed with 6% RUP diets without compromising milk production, whereas the combination of low RDP with 8% RUP depressed productivity.  Lower RDP and RUP diets increase NUE in heat-stressed cows.

Keywords: crude protein, nitrogen-use efficiency, heat stress