The relationship between various animal and management factors and milk urea, and its association with reproductive performance of dairy cows grazing pasture

Abstract

This study was designed to determine the use of milk urea to diagnose nutrient imbalances in diets of dairy cattle grazing pasture and infertility problems associated with excess dietary nitrogen (N). A first experiment investigated the influence of the ratio of N/water soluble carbohydrate (WSC) (g/g) and N/metabolisable energy (ME) (g/MJ) (% of estimated intake) in the diet, and animal and management factors, on urea concentrations of individual cow or bulk milk samples collected at monthly intervals over a 2-year period from a herd of 108 Friesian dairy cows grazing pasture. Mean±S.E. daily milk yield was 18.4±0.11 l milk/cow with 40.3±0.25 mg urea/dl of milk. This relatively high milk urea value is in line with the high estimated N/WSC ratio, which was three times the recommended ratio for that level of production. The ratio of N/WSC in the diet alone explained 8.8% of the variation in milk urea from individual cows, and this was increased to 21.5% when genetic merit, cow age, milk yield and milk protein content were included in the model. Level of feeding and cow live-weight had no significant effect on milk urea. Milk urea levels were lowest (at 32 mg/dl) when cows were between 40 and 60 days in lactation. Cows had higher ( P<0.001) levels of urea in their milk from January to June, when they grazed kikuyu grass pasture, than for the rest of the year when they grazed ryegrass pasture (43 vs. 35 mg/dl). There was a stronger relationship between N/WSC and the urea content of bulk milk ( r 2=30%) than individual cow milk ( r 2=11%), and this relationship was curvilinear. Substituting ME for WSC explained a further 2% and 2.7% of variation in bulk and individual milk urea, respectively. In a second experiment, the relationship between milk urea and the rates of non-return to service of Friesian cows grazing pasture was investigated. Milk samples were collected from 556 cows on four farms on the day of artificial insemination. The urea content of milk was not related ( P<0.05) to the chances of not returning to service. From the results of this study, it may be concluded that the urea content of bulk, but not individual cow, milk is a useful tool for assessing the appropriateness of the N/WSC ratio in the diet of dairy cows grazing pasture. There was no evidence that high milk urea levels are associated with poor reproductive performance in dairy cows grazing pasture.