A comparison of the output from permanent swards containing clover or receiving nitrogen-fertilizer when continuously grazed by ewes and lambs

A 2-year experiment was designed to compare the output from a permanent grass sward, either containing clover (GC) or receiving 200 kg N ha-1 (GN), when continuously grazed by sheep stocked at 12 and 15 ewes ha- 1 on GC and 15 and 18 ewes ha-1 on GN. Sward surface height (SSH) was controlled within designated guidelines by adjusting the size of the grazing area with an electrified 'buffer' fence; the herbage surplus to grazing requirement was cut and conserved. A dried grass supplement was offered to the sheep during periods of low herbage availability. The experiment was carried out during 1989 and 1990. No silage was made on GC15 in either year compared with 28 and 90 kg DM ewe- 1 on GN15 in 1989 and 1990 respectively; supplementation was consistently and significantly greater on GC15 than on GN15. Clover proportion was generally higher on GC12 than GC15 (significant (P<0.01) in September 1989) and reached a maximum level of 14% on GC12 in August 1990. Herbage organic matter digestibility was little affected by the inclusion of clover in the sward. Differences in lamb growth rate were not significant and, at the common stocking rate, there was no difference in lamb output between GC and GN. In 1990, GC12 and GN15 treatments proved to be successful after weaning in balancing the increasing nutritional requirements of the ewes during the period prior to mating, the requirements of a declining lamb population as the lambs satisfied the criteria for slaughter and were sold, and a continuing need to conserve surplus herbage. The metabolizable energy requirements of the ewes and lambs over 2 years were 75-8 and 74.7 GJ ha-1 for GC15 and GN15 respectively, and the differences in total utilized metabolizable energy output of the two systems were due to the amounts of herbage conserved and supplement consumed. The 2-year mean total UME output on GC 1 5 was 80% of that on GN 1 5 (67-8 and 84.4 GJ ha- 1 for GC 1 5 and GN 1 5 respectively) and the experiment provided further evidence that grass/white clover swards with no fertilizer N applied are capable of producing about 80% of the total output of grass swards receiving 200 kg N ha-1.