Simulating the intake and nitrogen excretion from cows grazing forages fertilised with increasing rates of nitrogen

Abstract

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.