The dry matter intake (DMI), production, and urinary nitrogen (N) excretion of dairy cows grazing forage grown under increasing N fertiliser rates were explored using a mechanistic model of a grazing ruminant. The MINDY cow model was initialised to represent a multiparous Holstein-Friesian dairy cow in late lactation (age 3.5 years, 212 days in milk). Simulated cows were offered a common allowance (30 kg DM/cow/d) of chicory, plantain, lucerne, or perennial ryegrass, fertilised at rates of 0, 100, 200, 350 and 500 kg N/ha/y (0N, 100N etc.) for 20 days in March. Forage physical and chemical composition data were obtained from a Waikato experimental field site. When N rate increased from 0–500N, predicted DMI of cows grazing plantain and ryegrass increased by 5.2 and 2.7 kg DM/cow/day (43 and 20% increase), respectively, facilitated by increased sward height and mass, and reduced herbage strength. Predicted DMI of cows grazing chicory peaked at 200N and, for cows grazing lucerne, declined with N fertilisation. Intake of highly fertilised (>200N) chicory and lucerne diets were potentially affected by high herbage soluble N content resulting in high predicted rumen ammonia concentrations (>18 mmol/L), reducing incentive for grazing. While predicted milk production from cows grazing chicory, plantain and ryegrass increased, the efficiency of dietary N used for milk production declined, with increasing N applied. Increasing N rate also increased predicted urine N concentration, and urinary N excretion from simulated cows fed chicory, plantain, and ryegrass. This study suggests that N fertiliser rates of 200N for chicory, plantain and ryegrass provides a balance between cow production and N excretion.

EMK Minnee, RH Bryant, DF Chapman, and P Gregorini

Proceedings of the New Zealand Society of Animal Production, Volume 78, Rotorua, 141-145, 2018
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