The effects of fertiliser N and cutting frequency on the growth of grass/clover and grass swards and on nitrogen fixation by white clover
The response of white clover/perennial ryegrass and pure ryegrass swards to fertiliser N were compared under two cutting frequencies, monthly and bimonthly, between 1978 and 1981. In the first year after sowing DM yield responses under only one cutting frequency (monthly) were examined. During the two subsequent years the cutting frequency difference was imposed and finally in 1981 uniform monthly cutting was again adopted. White clover species were chosen to provide a comparison between performance of a recently introduced long-petioled cultivar, Blanca, and a standard one, S100.S23 perennial ryegrass was chosen as a companion grass cultivar due to its recognised ability to respond well to fertiliser N. Fertiliser N was applied in 100 kg N/ha increments between zero and 400 kg N/ha/an on mixed swards and up to 600 kg N/ha/an on pure grass swards. Annual fertiliser N applications to swards were supplied at uniform and V3 increments before start of growth in March and thereafter following each harvest except the last on monthly and bimonthly cutting frequencies respectively. High N application rates increased DM yield more with longer growing periods than with higher cutting frequencies. N yield in herbage was positively affected by the N application rate and not or slightly by the cutting frequency. Consistent with this, appreciably higher N concentrations (max 3.2%) were found on average in the herbage at high fertiliser N rates and frequent cuttings than at lower rates and less frequent cuttings (2.2%). Blanca 127 and S100/523 mixed swards in the absence of fertiliser N yielded on average 61% of pure grass swards receiving annually 400 kg N/ha with monthly cutting. Differences between clover cultivar DM yields in response of mixed swards to fertiliser N were small. However there was some evidence that S100 (medium-small leaved cultivar) was less tolerant to small additions of fertiliser N (around 100 kg N/ha/an) than Blanca (medium-large leaved) when more frequently cut. Under less frequent cutting conditions yields of mixed swards were consistently higher under each level of fertiliser N than under more frequent cutting regimes. Fertiliser N applications of around 100 kg N/ha/an reduced DM and N yields in mixed swards during the period of maximum clover growth in mid summer to a greater degree on more than less frequently cut plots. The influence of season and fertiliser N on the level of herbage Ca, Mg, K, P and nitrate-N were discussed with some reference to K/(Ca+Mg) and Ca/P ratios. The effects of cutting frequency and fertiliser N on the seasonal variation of nitrogenase activity in Trifolium repens (cv S100) were measured by means of weekly acetylene reduction assays on clover grown in association with Lolium perenne (cv S23) over two seasons. Acetylene reduction activity (ARA) was drastically reduced after defoliation; recovery being considerably faster in May than later in the season. The seasonal pattern of ARA was significantly affected by the defoliation frequency of the sward. With only 3 cuts per year ARA of the more mature clover plants was significantly less prior to cutting than plants subjected to 6 cuts per year. The role of associated ryegrass competition for light with clover in a mixed sward was discussed. The response of ARA of white clover plants to fertiliser N varied during the two seasons. Fertiliser N generally reduced ARA but had little effect in the early months of 1978 when N treatments were first imposed. The decline in ARA associated with N applications was generally greater in the second year of fertilisation than the first. Cutting frequency of the sward significantly reduced the response of ARA to fertiliser N only under the infrequent cutting/ high fertiliser N regime suggesting that clover N fixation is affected by increased ryegrass competition in mixed swards. The molar ratio of acetylene reduced to nitrogen accumulated by clover plants during 4 week growth periods on control plots varied between 8.6 and 2.5:1. In May of 1978 and 1979 the molar ratio greatly exceeded 8.6:1. Only after July did the molar ratio approach the theoretical value of 3:1 of acetylene reduced to N fixed. It was concluded that the AR technique whilst suffering from a high degree of sampling variation and difficulties in estimating actual amounts of N fixed by clover plants can be useful in amassing quickly quantities of data that can lead to reliable comparisons between many types of imposed treatments.