The effect of fertiliser nitrogen on the production of irrigated pasture with and without clover

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Abstract
Yield responses of irrigated pasture to nitrogenous fertiliser were studied on a grass/clover sward and on a grass sward. At rates of application of up to 400 Ib nitrogen/acre/year responses on a grass/ clover sward averaged 5 to 6 Ib dry matter per pound of fertiliser nitrogen in a mowing trial. The amount of fertiliser nitrogen required on a grass sward to equal the yield of a grass/clover sward receiving no fertiliser nitrogen was 450 Ib to 500 Ib under mowing conditions (annual yield 14,000 lb dry matter per acre) and approximately 300 lb under grazing (annual yield 17,000 lb dry matter per acre). Suppression of white clover by high rates of fertiliser nitrogen was much less severe than usually found in experiments in Europe and the U.S.A. At a rate of application of about 800 Ib nitrogen/acre/year on a grass/clover sward, white clover still accounted for 15 percent of the yield, but at an application rate of 250 lb to 300 lb fertiliser nitrogen per acre, white clover apparently ceased to make a positive contribution to the nitrogen supply of associated grasses. Grown as pure swards receiving 360 lb fertiliser nitrogen/acre/ year, cocksfoot (Dactylis glomerata L.) gave a higher summer and annual yield than perennial ryegrass (Lolium perenne L.)