Fertilizer-nitrogen: Effects on dairy cow health and performance

Abstract

The effects on dairy cow health and performance of applying very high rates of inorganic fertilizer nitrogen to grassland have been studied. Two comparable areas of grassland provided the grazing and silage requirements for two separate herds of Friesian dairy cows. These two areas received 250 and 750kg fertilizer nitrogen ha⁻¹ yr⁻¹. The higher rate was chosen to represent a rate well in excess of the requirements for maximum grass production. The performance and health of the two herds were monitored over five years by measuring milk yield and milk and blood composition for each animal every three weeks.

The only difference in milk yield and composition between the two herds was a consistently higher concentration of NPN in the milk from the N750 herd, but the values recorded were still within the ranges quoted in the literature for dairy cows in commercial herds.

There was some evidence of slightly higher incidences of milk fever and infertility problems in the N750 herd, although in both herds, the incidences were so low that any differences were of little practical significance. Both herds showed an increase in general reproductive disorders as the trial progressed and this may have been associated with a decline in the copper status of both herds.

The most consistent significant differences between the herds in blood composition was that in serum urea-N values. These were higher in the N750 herd but no consequential clinical effects were observed.

There was some evidence to suggest that cows in the N750 herd were able to adapt to a high intake of nitrate by increasing the amount of functional haemoglobin circulating in the blood.

The results of this trial demonstrate that grazing cattle are able to tolerate much higher levels of nitrate-nitrogen in herbage than those commonly quoted as causing acute toxicity; levels of up to 0.52% nitrate-nitrogen were measured in the dry-matter of the N750 herbage. Furthermore, they demonstrate that the potential effects of increased herbage nitrate levels on the health and performance of dairy cows do not impose constraints on the use of nitrogen fertilizer rates considerably in excess of the current economic optimum. Dairy farmers are in a position therefore to apply the rate of nitrogen fertilizer to grassland that will produce the optimum financial reward, without consideration of the possibility of adverse effects on herd health and performance.