Does spring application of fertiliser urea reduce dairy cow performance?

Abstract

This paper reports a field experiment and a farm survey, which aimed to find out if there is evidence that elevated herbage crude protein (CP):sugar levels have negative effects on dairy cow performance. Differences in herbage CP levels were induced by using nitrogen (+N) or withholding (-N) applied as fertiliser urea for two groups of 20 dairy cows in early lactation (August to November 2003). Cows fed urea-fertilised herbage had elevated milk urea levels (-N = 5.4 mmol/l, +N = 8.3 mmol/l, P<0.001). A feed conversion efficiency (FCE) index was calculated as the ratio between theoretical daily metabolic energy (ME) requirement based on measured live weight and milksolids (MS) production data, and the observed mean daily ME intake over a 74 day observation period. Using this index, the apparent FCE was approximately 10% lower for the +N than for the -N group of cows. A survey of 16 farms indicated an association between elevated milk urea and decreased MS production per cow per day, consistent with the experimental result. It is concluded that although fertiliser urea does appear to reduce FCE, urea use can still be a viable strategy to improve the overall performance of a dairy farm system. Keywords: dairy cow performance, feed conversion efficiency, fertiliser urea, metabolisable energy, milksolids production