Nitrogen loading from urine patches of lactating dairy cows fed ryegrass and plantain as forage and silage

Abstract

The intensification of New Zealand pastoral agriculture has led to the use of irrigation, fertiliser and legumes to increase quality and yield of pastures. In order to attain these yields, nitrogen (N) content of pasture has increased beyond animal requirement. This excess to requirement N intake leads to large quantities of N excreted in urine resulting in high levels of N loading per urine patch. Plants cannot uptake all of this N therefore large quantities are lost from the system. N loading from the urine patch is a function of urine volume, urinary N concentration and urine patch area. Plantain (plantago lanceolata) has been identified as a plant species that when fed as forage reduces urinary N concentration without reducing N intake of animals and without reducing production. There is no information on the effect plantain silage has when fed to lactating dairy cows. There is little information on urine volume from cows fed plantain as forage or supplement, no data on the expected urine patch area when urine is excreted to plantain pastures, and little information on urine patch area when urine is excreted to ryegrass pastures. This trial investigated the effect plantain fed as forage and supplement had on urinary N excretion (total N excretion and urinary N concentration), urine volume and simulated the expected urine patch area of ryegrass and plantain forage at different grazing heights and different application volumes. Feeding plantain silage to cows grazing ryegrass reduced urinary N concentration by 1 gN/L compared with ryegrass silage with ryegrass pasture. N intake was 109.5 gN/cow/day lower in plantain forage treatments compared to pasture forage treatments making the ryegrass – plantain urinary N excretion comparison invalid. Diurnal variation in urine volume was evident with 1800-hour urine averaging 2.25L/urination less than 0600 hour urinations between silage treatments. Lower pasture heights increased urine patch area due to lower sward density. Lower pasture height was due to grazing severity. Plantain pasture increased urine patch area due to increased bare ground after defoliation and increased splash. Modelling of N loading per urine patch found reductions (206 kgN/ha at 0600 hours and 321 kgN/ha at 1800 hours) for cows with a primary diet of ryegrass pasture and supplemented with plantain silage compared with ryegrass silage. Cows grazing plantain pasture had N loading reductions per urine patch of 400-500 kgN/ha compared with ryegrass pasture due to increasing urine patch area and reduced urinary N concentration.