Beef production from the grazing ley as influenced by the application of nitrogen fertiliser

Abstract

Nitrogen is essential to crop production and, moreover, crop yields all over the world are probably limited more often by shortage of nitrogen than any other nutrient. The nitrogen made available to crops and grass by the soil comes from the continual breakdown of organic matter, and this may maintain high yields for a considerable time when there are large quantities of soil organic matter, such as occurred in the prairie soils of the American mid -west. Cropping such soils however, eventually reduces the organic nitrogen supply to a level in equilibrium with the particular type of farming undertaken. The level reached under crop production conditions is insufficient to maintain high yields, and for non- leguminous crops, nitrogen must be supplied in the form of fertilizer if high yields are to be attained.

In the case of the pure grass sward nitrogen is no less necessary for high production, and many'have shown a linear relationship between grass yields and nitrogen supply up to high levels of nitrogen fertilizer application. In the grazing ley however, several modifying factors impinge on this simple relationship between fertilizer supply and output. 1) The sward may consist of a mixture of grasses and clover or clovers. The rhizobium bacteria living in symbiotic relationship with the clover can fix quite large quantities of atmospheric nitrogen, and so are an alternative to fertilizers as a source of nitrogen. The application of nitrogen fertilizers to a mixed grass clover sward is known to depress the amount of clover in the mixture, and also to reduce the amount of nitrogen fixed by the rhizobium bacterium present, since clovers use soil nitrogen when this is abundantly available. 2) The grazing animal returns to the sward about 90% of the nitrogen ingested in the herbage. The recycling of the nitrogen from the soil and clover nodules via the herbage and back to the sward must obviously affect the relationship between nitrogen fertilizer supply and output from the sward. 3) The production of grass from the application of nitrogen fertilizer may be affected by the way in which the sward is grazed. 4) Production of herbage is not the final criterion. Animal production is the acid test for the performance of the grazing ley, and the health and performance of the individual animal must be taken into account as well as the production per acre.

At the time this study was commenced there was very little information on the profitability of liberal application of nitrogen to the grazing ley where beef production was the livestock enterprise. Most progressive farmers producing beef or sheep agree that it is profitable to apply some nitrogen in the early spring to provide early grazing, but most rely on the clover to provide the nitrogen for the remainder of the season. This work was undertaken to investigate the profitability for beef production of liberal dressings of nitrogen fertilizer applied to the sward throughout the grazing season.