Centre for Nutrition Modelling, Department of Animal and Poultry Science, University of Guelph, Guelph, ON, Canada N1G 2W1.
J Dairy Sci. 2011 Jun;94(6):3105-18. doi: 10.3168/jds.2010-4059.
High-sugar grass varieties have received considerable attention for their potential to reduce nitrogen (N) excretion and increase milk yield in cattle. However, considerable variation exists in the magnitude of response in published results. The purpose of this study is to explain the variation in response using a dynamic mechanistic model to predict observed N and milk yield results from the literature, and from simulated data. Examined effects were (1) water-soluble carbohydrate [WSC; g/kg of dry matter (DM)] increase; (2) change in crude protein (CP) and neutral detergent fiber (NDF) content of the plant with WSC increase; and (3) the level of N fertilization. The database for evaluation of model N and milk yield predictions consisted of 4 published studies with 28 treatment means for which high-sugar grasses were being evaluated. Water-soluble carbohydrate content of the diets ranged from 95 to 248 g/kg of DM, CP content ranged from 115 to 263 g/kg of DM, and the NDF content ranged from 400 to 568 g/kg of DM. Urine N, milk N, and total N excretion were predicted well by the model and followed the directional pattern of observed values within each study. Simulation results showed that the N utilization ratio increased as the WSC content of the diet increased, but to varying degrees depending on the grass scenario examined. The greatest benefit in terms of N utilization ratio and urine N levels were seen when the WSC content of grass increased at the expense of CP, followed by a 50:50 CP and NDF mix, followed by a trade for NDF. Simulated milk yield decreased slightly when WSC increased at the expense of CP, increased slightly when it increased at the expense of a CP and NDF mix, and increased most when WSC increased at the expense of NDF. Results were amplified slightly under conditions of low-N fertilization and in the absence of grain feeding. Overall, modeling is useful as an explanatory tool. The variation from results in the literature with high-WSC grass feeding may be, at least in part, the result of the level of WSC (g/kg of DM) increase, concurrent changes occurring within the CP and NDF fractions of the plant, and the plane of nutrition of the diet (grain feeding and N fertilization levels).
高糖牧草因其能降低奶牛氮排泄量并提高产奶量而备受关注。然而，已发表的研究结果中存在很大的响应差异。本研究的目的是使用动态机制模型来解释这种响应变化，以便根据文献和模拟数据预测观察到的氮和产奶量结果。研究的影响因素有：（1）水溶性碳水化合物（WSC；以干物质[DM]计，g/kg）增加；（2）随着 WSC 增加，植物的粗蛋白（CP）和中性洗涤纤维（NDF）含量的变化；（3）施氮水平。评估模型氮和产奶量预测的数据库包括 4 项已发表的研究，其中有 28 个处理平均值正在评估高糖牧草。试验日粮的 WSC 含量范围为 95-248 g/kg DM，CP 含量范围为 115-263 g/kg DM，NDF 含量范围为 400-568 g/kg DM。模型对尿氮、乳氮和总氮排泄的预测效果较好，且符合每个研究中观察值的方向模式。模拟结果表明，随着日粮 WSC 含量的增加，氮利用率增加，但不同的草种情况增加程度不同。当 WSC 增加以牺牲 CP 为代价时，或增加 CP 和 NDF 混合物时，或增加 NDF 时，氮利用率和尿氮水平的获益最大。当 WSC 增加以牺牲 CP 为代价时，模拟产奶量略有下降；当 WSC 增加以牺牲 CP 和 NDF 混合物为代价时，略有增加；当 WSC 增加以牺牲 NDF 为代价时，增加最多。在低氮施肥和不喂谷物的情况下，结果略有放大。总的来说，模型作为一种解释工具是有用的。用高 WSC 牧草喂养时与文献结果的差异可能至少部分是由于 WSC（以 DM 计，g/kg）增加的水平、植物的 CP 和 NDF 成分中同时发生的变化以及日粮的营养水平（谷物喂养和氮施肥水平）。
