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The influence of oscillating dietary crude protein concentrations on milk production and nitrogen utilization in lactating dairy cows



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There is increasing public pressure on intensive dairy operations to reduce nitrogen (N) excretion into the environment, which can be achieved by adopting on-farm feeding practices that enhance the efficiency of N utilization. One such feeding strategy that has received attention is feeding diets with oscillating crude protein (CP) concentrations, and studies with finishing beef cattle and growing sheep have reported improvements in N retention when oscillating CP diets are fed compared to static CP diets. This experiment was conducted to determine: 1) the optimum frequency of oscillating dietary CP concentration (i.e., 24, 48, or 72 h); 2) the effects of feeding oscillating CP diets on feed intake, milk production and milk composition in dairy cows; and 3) the effects of feeding oscillating CP diets on ruminal fermentation characteristics (NH3, SCFA and pH), microbial protein synthesis, ruminal outflow of nutrients and N balance in dairy cows. Eight Holstein cows (714 ± 36 kg body weight; 114 ± 15 days-in-milk at the beginning of the experiment) were used in a replicated 4 × 4 Latin square design with 30-d periods (consisting of 18 d of dietary adaptation and 12 d of sample and data collection). Treatments were a diet containing 16.5% CP fed on a continuous basis (designated STATIC), and diets containing 13.5% and 19.5% CP that were fed on an oscillating regimen at 24 (OSC-24), 48 (OSC-48), or 72 (OSC-72) h. Diets were fed twice per day as total mixed rations with a 51:49 forage:concentrate ratio. The actual CP concentrations were 17.1% for the STATIC, and 14.3 and 20.3% for the oscillating CP diets, which deviated from the target CP concentrations due to large variations in forage CP content. Dry matter intake (mean = 26.6 kg/d) and milk production (mean = 36.4 kg/d) were not affected (P ≥ 0.19) by dietary treatment. Milk protein yield was greater (P = 0.03) in cows fed the STATIC diet as compared to those fed the OSC diets (1.22 vs. 1.14 kg/d). Milk urea-nitrogen tended (P = 0.10) to be greater (by 8%) in cows fed the OSC-48 diet compare to those fed the OSC-24 diet. Cows fed the OSC diets tended to have greater (P = 0.05) ruminal NH₃-N concentrations compared to those fed the STATIC diet (12.7 vs. 11.6 mg/dL). In addition, cows fed the OSC-48 diet had a greater ruminal NH3-N concentration compared to those fed the OSC-24 (P = 0.05) and the OSC-72 (P < 0.01) diets. Although N intakes, urinary, fecal and milk N outputs were similar across dietary treatments, apparent N balance was 22.5% greater (P = 0.03) in cows fed the OSC diets compared to those fed the STATIC diet. Cows fed the OSC-48 diet had a 41% greater (P < 0.01) apparent N balance compared to those fed the OSC-24 diet. Cows fed the STATIC diet tended to have greater omasal DM (P = 0.10) and OM (P = 0.09) flows compared to cows fed the OSC diets. No differences in apparent total-tract nutrient digestibilities were observed due to dietary treatment (P ≥ 0.19). Nitrogen (P = 0.06) and non-ammonia N (P = 0.07) flows at the omasal canal tended to be greater in cows fed the STATIC diet compared to those fed the OSC diets. In conclusion, these results demonstrate that feeding oscillating dietary CP diets to dairy cows on a 48-h basis improves N efficiency by enhancing N retention without compromising production when compared to feeding a static CP diet or an oscillating dietary CP on a 24-h or 72-h basis. Therefore, feeding dairy cows an oscillating dietary CP regimen could potentially decrease N excretion, thereby reducing the environmental impact that intensive dairy operations have on the environment.



Oscillating, crude protein, dairy cow



Master of Science (M.Sc.)


Agricultural and Bioresource Engineering


Animal Science


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