Interaction between cow slurry and ¹⁵N‐labelled calcium nitrate as fertilisers for ryegrass production

A field‐plot experiment with ryegrass was carried out to study the interaction between 15 N‐labelled calcium nitrate at 0, 25 and 50 kg N ha −1 and cow slurry supplying 0, 33 and 68 kg NH ‐N ha −1 . Treatments were applied on a single occasion to a site where first‐cut silage had just been removed. The effects of the treatments on dry matter yield, nitrogen offtake and % utilisation of calcium nitrate were measured in two cuts of herbage taken after 5 and 17 weeks. Increases in dry matter yield due to slurry were smaller at the 50 kg N ha −1 rate of calcium nitrate than at the 0 and 25 kg N ha −1 rates, possibly due to non‐linearity of the yield response curve. Nitrogen offtake data indicated that the two sources of nitrogen were taken up additively by the ryegrass. Rate of calcium nitrate application or slurry treatment had no significant effect on the % utilisation of calcium nitrate in either cut. The average values for % utilisation of calcium nitrate over all treatments were 50 and 7% at cuts 1 and 2, respectively. There was no evidence for enhanced rates of denitrification in the presence of slurry. Recovery values of nitrogen (estimated by difference) were lower for NH ‐N in slurry than for calcium nitrate. Volatilisation of ammonia from surface‐spread slurry is the most likely reason for the low efficiency of NH ‐N in slurry.