Role of Nitrogen in Grassland Productivity

The quantity of fertiliser N necessary to obtain a given yield from grassland depends on the contribution from non-fertiliser N sources, that is, soil, clover and animal returns.

On grass swards, splitting fertiliser N into several dressings over the season rather than applying one large initial dressing gives bigger annual yields. Responses to fertiliser N are greater in the first half of the season, but too much N in this period will depress even further the lower yields obtained in the second half of the season. Fertiliser N should be applied within one week after defoliation if responses to N are to be kept at an optimum level.

On grass/clover swards, the grass xlover balance is elastic to variations in time of N input. Although omission of spring N dressing leads to a higher annual yield of clover than where spring N is applied, it also lowers grass yield, leading to little difference in total yield.

As soil and clover N are not released evenly over the season, the regular use of a constant fertiliser N rate over the year does not give a level total N supply to the sward.