Department of Dairy Science, University of Wisconsin-Madison, Madison 53706.
Departamento de Ciencias Pecuarias y Agricolas, Centro Universitario de Los Altos de la Universidad de Guadalajara, Tepatitlán, Jalisco, México, 47600.
J Dairy Sci. 2017 Jul;100(7):5434-5448. doi: 10.3168/jds.2016-11917. Epub 2017 May 10.
The objectives of this study were to measure performance responses and to evaluate indictors of N utilization in late-lactation cows fed diets with incremental reductions in crude protein (CP) concentration. Holstein cows (n = 128; 224 ± 54 d in milk) were stratified by parity and days pregnant (86 ± 25 d) and randomly assigned to 1 of 16 pens in a randomized complete block design. For 3 wk, all cows received a covariate diet containing 16.9% CP [dry matter (DM) basis]. For the subsequent 12 wk, pens were randomly assigned to 1 of 4 treatments that contained 16.2, 14.4, 13.1, or 11.8% CP (DM basis). Diets were offered once daily and contained 32.5% corn silage, 32.5% alfalfa silage, 13.5% high-moisture corn, and 21.5% concentrate mix. A reduction in dietary CP was achieved by replacing soybean meal with soy hulls in the concentrate mix (DM basis). Dry matter intake, milk urea N (MUN; mg/dL), and the yield of milk urea N (g/d) decreased linearly with dietary CP. Compared with a 16.2% CP diet, a 14.4% CP diet did not alter milk yield throughout the study, but the 13.1 and 11.8% CP diets reduced milk yield after 4 and 1 wk, respectively. Furthermore, milk protein percentage was reduced for all dietary CP less than 16.2%, but this negative effect was temporary and disappeared after 7 wk for the 14.4% CP diet. In contrast, MUN adjusted to a new steady state within 1 wk for all dietary treatments. Modeling quadratic responses with a plateau led to predictions of no reduction in fat- and protein-corrected milk (32.6 kg/d) and yields of fat (1.31 kg/d), lactose (1.49 kg/d), and true protein (1.12 kg/d) until dietary CP decreased below 15.5, 15.3, 15.9, and 16.2%, respectively. In this study, MUN and the yield of MUN were highly correlated with N intake, milk protein yield, and fat- and protein-corrected milk. Surprisingly, N use efficiency (milk protein N/intake N) was not correlated with any variables related to N utilization and reached an apparent upper limit of approximately 30%. Although this observation may be associated with feeding diets deficient in metabolizable protein, late-lactation cows in this study adjusted to low dietary CP concentration better than anticipated as milk production was 2.6, 3.6, 6.4, and 8.0 kg/d higher than National Research Council (2001)-predicted metabolizable protein-allowable milk for dietary CP of 16.2, 14.4, 13.1, and 11.8%, respectively.
本研究的目的是测量奶牛在采食粗蛋白（CP）浓度逐渐降低的日粮时的生产性能反应，并评估其氮（N）利用的指标。荷斯坦奶牛（n=128；泌乳期 224±54 天）根据胎次和妊娠天数（86±25 天）分层，并采用完全随机区组设计，随机分配到 16 个围栏中的 1 个。在 3 周的时间里，所有奶牛均采食含有 16.9% CP（干物质基础）的对照日粮。随后 12 周，每个围栏随机分配到 4 种处理中的 1 种，CP 浓度分别为 16.2%、14.4%、13.1%或 11.8%（干物质基础）。日粮每天提供 1 次，由 32.5%玉米青贮、32.5%苜蓿青贮、13.5%高水分玉米和 21.5%浓缩料组成。通过在浓缩料中用大豆皮代替豆粕来降低日粮 CP（干物质基础）。干物质采食量、牛奶尿素氮（MUN；mg/dL）和牛奶尿素氮产量（g/d）随日粮 CP 呈线性下降。与 16.2% CP 日粮相比，14.4% CP 日粮在整个研究期间并未改变产奶量，但 13.1%和 11.8% CP 日粮分别在第 4 周和第 1 周后降低了产奶量。此外，所有 CP 浓度低于 16.2%的日粮均降低了牛奶蛋白百分率，但这种负效应在 14.4% CP 日粮中 7 周后消失。相反，MUN 在所有日粮处理中都在 1 周内调整到新的稳定状态。用平台模型拟合二次响应表明，直到日粮 CP 降低至 15.5%、15.3%、15.9%和 16.2%以下，脂肪和蛋白校正乳（32.6 kg/d）和脂肪（1.31 kg/d）、乳糖（1.49 kg/d）和真蛋白（1.12 kg/d）的产量才不会降低。在这项研究中，MUN 和 MUN 产量与 N 摄入量、牛奶蛋白产量以及脂肪和蛋白校正乳高度相关。令人惊讶的是，氮利用效率（牛奶蛋白氮/摄入氮）与任何与 N 利用相关的变量均不相关，且达到了约 30%的明显上限。尽管这一观察结果可能与采食代谢蛋白不足的日粮有关，但本研究中的泌乳后期奶牛对低 CP 浓度的适应能力好于预期，因为产奶量分别比美国国家研究委员会（2001）预测的代谢蛋白允许的产奶量高 2.6、3.6、6.4 和 8.0 kg/d，其对应的日粮 CP 分别为 16.2%、14.4%、13.1%和 11.8%。
