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Effect of a late summer diet change from pasture to brassica crop and silages on dairy cow milk production and urinary nitrogen excretion

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.