Early spring and late autumn response to applied nitrogen in four grasses

Abstract

SummaryThe regrowth of Aberystwyth S. 23 perennial ryegrass, S. 24 perennial ryegrass, S. 59 red fescue and S. 170 tall fescue was studied in field swards, comparing four levels of applied nitrogen, for 8 weeks following a clearing cut. The clearing cuts were in mid-October, mid-February and mid-March in each of 3 years, different plots being used on each occasion.Grass yield was more closely related to solar radiation receipt than to temperature. When radiation during the 3 or 4 weeks before a period of study was added to radiation during the 8-week period, to incorporate some allowance for temperature, the amount of additional yield per unit of additional radiation comparing the first spring period with the autumn period was about the same as that comparing the second with the first spring period. Where no N was applied, there was little or no increase in yield above ground level or above 4 cm from week 1 to about week 6 in the autumn and first spring period, whereas where N was applied yield increased steadily, though rather slowly, during those 5-week periods. Response to N, measured as kg dry matter above 4 cm/kg N applied, was about twice as great in the second as in the first spring period; on the other hand response to N measured as number of days saved in reaching a given yield was greater in the first than the second spring period. The positive effect of applied N on relative growth rate occurred mainly in the rather early stages of regrowth, which seems to be typical of any time of year.Applied N increased the N and nitrate-N content of herbage and reduced the watersoluble carbohydrate content. Applied N increased the number of tillers and the proportion of yield above 4 cm. Yield below 4 cm was not much affected by N or by stage of regrowth. The ‘earliness’ in spring of S. 24, S. 59 and S. 170 compared with S. 23 was associated with greater height and a higher proportion of yield above 4 cm and not with higher above-ground yield. The ryegrasses responded more to N than the fescues in respect of yield above 4 cm and number of tillers. In 2 years in which the number of tillers was relatively low at the beginning of the spring periods of study, the number increased during those periods and there was a large positive effect of applied N, whereas, in a year in which the number was high initially, there was no increase during the periods of study and little response to N.