Colmenero J J Olmos, Broderick G A
Department of Dairy Science, University of Wisconsin, Madison 53706, USA.
J Dairy Sci. 2006 May;89(5):1704-12. doi: 10.3168/jds.S0022-0302(06)72238-X.
Forty lactating Holstein cows, including 10 with ruminal cannulas, were blocked by days in milk into 8 groups and then randomly assigned to 1 of 8 incomplete 5 x 5 Latin squares to assess the effects of 5 levels of dietary crude protein (CP) on milk production and N use. Diets contained 25% alfalfa silage, 25% corn silage, and 50% concentrate, on a dry matter (DM) basis. Rolled high-moisture shelled corn was replaced with solvent-extracted soybean meal to increase CP from 13.5 to 15.0, 16.5, 17.9, and 19.4% of DM. Each of the 4 experimental periods lasted 28 d, with 14 d for adaptation and 14 d for data collection. Spot sampling of ruminal digesta, blood, urine, and feces was conducted on d 21 of each period. Intake of DM was not affected by diet but milk fat content as well as ruminal acetate, NH3, and branched-chain volatile fatty acids, urinary allantoin, and blood and milk urea all increased linearly with increasing CP. Milk and protein yield showed trends for quadratic responses to dietary CP and were, respectively, 38.3 and 1.18 kg/d at 16.5% CP. As a proportion of N intake, urinary N excretion increased from 23.8 to 36.2%, whereas N secreted in milk decreased from 36.5 to 25.4%, as dietary protein increased from 13.5 to 19.4%. Under the conditions of this study, yield of milk and protein were not increased by feeding more than 16.5% CP. The linear increase in urinary N excretion resulted from a sharp decline in N efficiency as dietary CP content increased.
40头泌乳期荷斯坦奶牛（其中10头安装有瘤胃瘘管），按泌乳天数分为8组，然后随机分配到8个不完全5×5拉丁方设计中，以评估5种日粮粗蛋白（CP）水平对产奶量和氮利用的影响。日粮以干物质（DM）计，包含25%苜蓿青贮、25%玉米青贮和50%精料。用溶剂萃取豆粕替代碾压高水分带壳玉米，使日粮CP水平从占DM的13.5%提高到15.0%、16.5%、17.9%和19.4%。4个试验期各持续28天，其中14天用于适应期，14天用于数据收集期。在每个试验期的第21天对瘤胃消化物、血液、尿液和粪便进行现场采样。日粮对干物质采食量没有影响，但随着CP水平升高，乳脂率以及瘤胃乙酸、氨、支链挥发性脂肪酸、尿囊素、血液和乳中尿素均呈线性增加。产奶量和蛋白质产量对日粮CP呈二次反应趋势，CP水平为16.5%时，产奶量和蛋白质产量分别为38.3和1.18千克/天。随着日粮蛋白质水平从13.5%提高到19.4%，尿氮排泄量占氮摄入量的比例从23.8%增加到36.2%，而乳中氮分泌量从36.5%下降到25.4%。在本研究条件下，日粮CP水平超过16.5%时，产奶量和蛋白质产量并未增加。随着日粮CP含量增加，氮效率急剧下降，导致尿氮排泄量线性增加。
