Three-year leys of cocksfoot/white clover and cocksfoot alone were dressed with 0, 35, iO5 and 210 lb. N per acre every year to provide information on the efFect of clover and N on annual production.White clover contributed 40"^, of the dry-matter yield but its presence was respotisible for IT/o of this yield where no N was used: it was responsible for less than 8%, and contributed 5%, where 210 lb. N per acre was applied annually, lt was estimated that grass alone receiving 160 lb. N per acre would yield as much dry niaiter as a mixed sward receiving none.Clover had the indirect effect of raising yield of nitrogen in the companion grass by 60 Ib. per acre per year. A maximum of lOfi Ib. was recorded in 1958.The response of the tnixed sward averaged iO Ib. of dry matter per lb. N. Prevailing economic circumstances will determine if this level is satisfactory in practice.The recovery of N by grass alone varied from 54% at ibc lowest level of N to 80°;, at the highest. On mixed swards apparent recovery was negative or low: N was to a large extent only replacing the effect of clover which was suppressed by its use.Factors influencing response and the difference between that obtained under experimental and practical conditions are discussed.The use of N to produce out-of-season grass may give good response, but may cause a reduction in clover contribution.

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Seven species or varieties of grass, and a mixture of 3 of them, were sown in pure swards and treated with 4 levels of nitrogenous fertilizer (0, 35 and 701b N per acre per cut). Each species, and the mixture, was also sown with white clover. The experiment was cut 4 or 5 times per year. The effect of fertilizer on the yield of each grass was compared with the effect of clover on the yield of the grass/clover swards. Mean annual yields showed an approximately linear response to N; there was a small but significant fall in response to the highest level of N. Response among the species ranged from 20 to 30 lb of dry matter per lb N applied for the intermediate level of fertilizer and from 14 to 23 lb for the final increment of fertilizer.S37 cocksfoot, S48 timothy, S24 ryegrass, and a mixture of these grasses, were high yielding and responded well to fertilizer N; Irish ryegrass and Agrostis tenuis were less productive and gave poorer responses to N. S215 meadow fescue and S23 ryegrass were intermediate in yield and response.There were no significant differences between the annual yields of the 8 grass/clover mixtures; the yields of the grass and clover components of each mixture were inversely related. The effect of clover on the yield of the grass/clover mixtures was estimated to be equivalent to the effect of an annual application of 205 lb N per acre to Agrostis tenuis and 120 lb N to S48 timothy. The fluctuations in annual yields were greater with grass/ clover mixtures than with grass swards receiving N.The yields of grasses when sown with clover were in similar order to their yields when sown pure; but whereas the latter tended to fall from year to year, the yields of the grass components of mixtures (except Irish ryegrass) did not.

The response of irrigated, perennial ryegrass to fertilizer nitrogen was studied in four consecutive periods of the growing season by applying 0-350 kg N/ha to a fresh sward at the start of each period, and measuring both herbage dry matter and its content of nitrogen.Responses in yield were highest in the first period, which ended at inflorescence emergence; in this period, both the percentage recovery of nitrogen and the extent of its utilization in producing dry matter were greater than in the later periods. Some damage to the sward was seen following the harvest of grass grown with the high levels of nitrogen in the first period. When the yields were 90 % of the predicted maximum the nitrate-N content of the herbage ranged from 1000 to 2000 ppm, except in the first period when it was 200 ppm.The response curves were used to calculate the nitrogen requirements of the grass which would maintain given incremental yield responses. To produce near-maximum yields, irrigated grass swards may require fertilizer nitrogen equivalent to 2 kg N/ha/day prior to inflorescence emergence, and up to 5 kg N/ha/day for the remainder of the growing season.The apparent efficiency of conversion of the radiant energy, usable for photosynthesis, into plant energy averaged 3-2 %; it did not vary greatly among the four periods.The experimental results indicate the seasonal requirements of grass for fertilizer nitrogen and some of the implications for animal husbandry are discussed.season. An exception is the work of van Burg (1960) in the Netherlands; he applied nitrogen to fresh Numerous studies of the response of grassland to areas of swards at various dates through the growfertilizer nitrogen have involved successive applica-ing season. The swards, which were of mixed grasses, tions through the growing season, often to grass/ occasionally contained white clover; they were clover swards, and the response has been measured irrigated in some experiments. The response to in total annual yield. In these studies it was not nitrogen was examined by establishing cumulative possible to measure the response of the grass at growth curves at various levels of nitrogen, and the particular periods of the year because the effect adequacy of the nitrogen supply was assessed from of the application at any one time is frequently the nitrate content of the herbage, influenced by the residual effects of previous applica-The present work was designed to measure the tions. With grass/clover swards, the obvious re-requirement for, and response to, fertilizer nitrogen sidual effect of a change in grass/clover balance by perennial ryegrass in separate periods of the following applications of nitrogen has made the growing season. It is hoped that the results will interpretation of responses difficult.provide a more logical basis for the application of Sometimes attention has been focused on the nitrogen to grass swards to support seasonal effect of fertilizer given on a single occasion to either patterns of herbage production, grass (Brockman, 1966;Hunt, 19...

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