ABSTRACT When pasture supply becomes limiting during the summer dry period in the southern North Island of New Zealand, dairy cows often receive silage and forage crop dietary supplements. The present study investigated the effect of dietary change in late summer-autumn on cow’s urinary N excretion. Fifty-four dairy cows balanced for milk production, age, liveweight, body condition score and days in milk were allocated into three feeding treatments, high crude protein (HCP, grazing pasture only), medium crude protein (MCP, leafy turnips and silages during the day and grazing of pasture at night) and low crude protein (LCP, leafy turnips and silages). Urinary N concentrations were greatest for the HCP group, with no significant difference between the MCP and LCP groups. The percentage of urinary N as urea was 84, 61 and 52% for the HCP, MCP and LCP treatments respectively. Urine urea was more responsive to dietary changes and management than total urinary N. Urinary N:creatinine ratios were strongly related to dietary N surplus. Feeding a mix of silage and turnips in addition to grazing pasture is a viable option to reduce excretion of N to the environment without compromising productivity of lactating cows during summer and autumn.

中文翻译：

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夏末饮食从牧草改为芸苔属作物和青贮对奶牛产奶量和尿氮排泄的影响

摘要 当新西兰北岛南部夏季干旱时期的牧场供应变得有限时，奶牛通常会接受青贮饲料和饲料作物膳食补充剂。本研究调查了夏末秋季饮食变化对奶牛尿氮排泄的影响。将 54 头奶牛的产奶量、日龄、活重、体况评分和产奶天数平衡分配到三个饲喂处理，高粗蛋白（HCP，仅放牧牧场）、中等粗蛋白（MCP、叶萝卜和青贮饲料）白天和夜间放牧）和低粗蛋白（LCP、多叶萝卜和青贮饲料）。HCP 组的尿 N 浓度最高，MCP 组和 LCP 组之间没有显着差异。对于 HCP，尿 N 作为尿素的百分比分别为 84%、61% 和 52%，分别是 MCP 和 LCP 处理。尿尿素比总尿 N 对饮食变化和管理更敏感。尿 N：肌酐比率与饮食 N 过剩密切相关。除了放牧外，还混合饲喂青贮饲料和萝卜是一种可行的选择，可以减少氮向环境的排泄，同时又不影响夏秋两季泌乳奶牛的生产力。