The development of response to fertilizer nitrogen in the first regrowth of perennial ryegrass

The experiment was undertaken to examine the interactions between yield responses to secondary applications of fertilizer N and length of regrowth interval following three levels of primary N. A sward of S24 perennial ryegrass received all combinations of three fertilizer treatments, nil, 50, and 100 kg ha^-1 N for primary growth harvested on 21 May and four treatments, nil, 33, 66 and 100 kg ha^-1 N for the second growth harvested 2, 3, 4, 5, 6 or 7 weeks later.
Cubic equations of growth were derived from the yields of dry matter and both the direct response to secondary N and the residual responses to primary N increased as regrowths developed. A positive residual effect to increasing primary N was obtained at the three lower levels of secondary N for weeks 4–7 but a negative response was obtained with the highest level of secondary N. The implications are discussed in terms of target responses and yields and the growth interval required to attain these targets.