Department of Animal and Veterinary Sciences, University of Vermont, Burlington 05405.
University of Vermont Extension, University of Vermont, St. Albans 05478.
J Dairy Sci. 2022 Feb;105(2):1099-1114. doi: 10.3168/jds.2021-20355. Epub 2021 Nov 17.
Variability of protein and energy supply from pasture during the grazing season is a primary factor that can influence milk production of grazing organic dairy herds in the Northeast United States. This study evaluated the effects of altering the crude protein (CP) content of dietary supplements included in dairy rations fed to grazing organic dairy herds, on milk production and composition. Six commercial organic farms participated in a 6-wk trial, consisting of a 2-wk baseline period and 4-wk experimental period. Farms were paired by their summer 2017 milk urea nitrogen profile, and farms within each pair were assigned by restricted randomization to (1) continuation of their regular supplements (n = 3, control group, CON), or (2) supplement with altered CP as percentage of dry matter, formulated using an organic barley and roasted soybean mix (n = 3, treatment group, TRT). Throughout the 6-wk trial, individual milk samples were collected at 2 consecutive milkings weekly, while pasture and supplement samples, pasture measurements, and management information were collected twice weekly per farm. Data were statistically analyzed using the MIXED procedure of SAS (version 9.4, SAS Institute Inc.) for all parameters, and effects of treatment, week, and their interaction (treatment × week) were determined. The supplement CP (percentage of dry matter) during the baseline period was 13.5% for CON and 15.3% for TRT and 14.8% for CON and 19.3% for TRT during the experimental period. Milk production was 21% higher during the experimental period for TRT compared with CON (24.1 vs. 19.9 kg of milk per day, respectively). Milk production decreased for CON from wk 1 to wk 6 (23.6 vs. 20.4 kg of milk per day), whereas TRT maintained milked production from wk 1 to wk 6 (22.8 vs. 22.7 kg of milk per day). Milk composition was different between groups, with CON having higher fat percent (4.21 vs. 3.73%, respectively) and protein percent (3.15 vs. 3.05%, respectively) compared with TRT for the 6 wk. The milk urea nitrogen concentrations were similar between TRT and CON for the baseline period (11.9 vs. 12.1 mg/dL) and the final week of the experimental period (14.5 vs. 14.2 mg/dL). Although the effects of different diet CP fractions, particularly rumen undegradable protein and soluble protein, must be further delineated, these results indicate that altering the CP content of dietary supplements fed to grazing organic dairy cattle during the summer period in the Northeast US could be a useful mechanism to maintain milk production.
放牧季节牧草中蛋白质和能量供应的可变性是影响美国东北部放牧有机奶牛群产奶量的主要因素。本研究评估了改变放牧有机奶牛日粮中补充饲料粗蛋白（CP）含量对产奶量和组成的影响。六家商业有机农场参加了为期 6 周的试验，包括 2 周的基线期和 4 周的试验期。根据 2017 年夏季牛奶尿素氮谱对农场进行配对，并通过限制随机分配将每个配对中的农场分配到（1）继续使用常规补充剂（对照组，CON），或（2）使用经过调整的 CP 作为干物质的补充剂，使用有机大麦和烤大豆混合物配制（处理组，TRT）。在整个 6 周的试验期间，每周两次在两次连续挤奶时收集个体牛奶样本，同时每周两次收集牧场和补充剂样本、牧场测量值和管理信息。使用 SAS（版本 9.4，SAS Institute Inc.）的 MIXED 过程对所有参数进行统计分析，并确定处理、周和它们的交互（处理×周）的影响。在基线期，CON 的补充 CP（干物质的百分比）为 13.5%，TRT 为 15.3%，而在试验期，CON 为 14.8%，TRT 为 19.3%。与 CON 相比，TRT 在试验期的产奶量提高了 21%（分别为 24.1 和 19.9 千克/天）。CON 的产奶量从第 1 周到第 6 周下降（分别为 23.6 和 20.4 千克/天），而 TRT 的产奶量从第 1 周到第 6 周保持不变（分别为 22.8 和 22.7 千克/天）。两组之间的牛奶成分不同，CON 的脂肪百分比（分别为 4.21%和 3.73%）和蛋白质百分比（分别为 3.15%和 3.05%）均高于 TRT，持续 6 周。在基线期（11.9 与 12.1 毫克/分升）和试验期最后一周（14.5 与 14.2 毫克/分升），TRT 和 CON 的牛奶尿素氮浓度相似。尽管不同饲料 CP 成分（特别是瘤胃未降解蛋白和可溶性蛋白）的影响还需要进一步阐明，但这些结果表明，在东北美国的夏季期间，改变放牧有机奶牛日粮中补充饲料的 CP 含量可能是维持产奶量的一种有用机制。
