HERBAGE INTAKE AND N EXCRETION BY SHEEP GRAZING MONOCULTURES OR A MIXTURE OF GRASS AND WHITE CLOVER

In 1988 and 1989, swards of grass (G0), white clover (C0) and grass/wh ite clover (GC0) receiving no N fertilizer, and a grass sward supplied with 420 kg N ha(-1) (G420), were grazed by non-lactating sheep to ma intain a sward surface height of 6 cm. Herbage organic matter (OM) int akes averaged between 1200 and 1700 g OM ewe(-1) d(-1). For treatments G0, C0, GC0 and G420 respectively, the ewes' liveweight gain was 102, 112, 100 and 110 g d(-1) and changes in body condition scores were +0 .28, +0.52, +0.36 and +0.44 units season(-1) However, the effect of tr eatment was not significant for either variable. There were similar le vels of output of faecal N ewe(-1) but significantly more urinary N ew e(-1) was excreted on treatments C0 and G420, where the concentrations of N in herbage laminae were also higher. For example, in 1989, total daily N excreted was 39.7, 64.4, 44.0 and 63.3 g N ewe(-1) for G0, C0 , GC0 and G420 respectively. Taking into account the mean daily stocki ng rates, which were 19.4, 26.6, 27.2 and 36.5 ewe ha(-1), the total f aeces and urine returns over the season were 161, 358, 249 and 484 kg N ha(-1) for each treatment respectively. The herbage OM intakes ewes( -1) d(-1) measured in September and October were similar for C0 and G4 20, and so the intake of herbage OM ha(-1) d(-1) was related to stocki ng rate, i. e. the estimated herbage intake ha(-1) over the growing se ason for the white clover monoculture was 73% of that for N-fertilized grass. Excretal nitrogen returns to the pasture from grazed mono-cult ures of clover were high, and similar to those from a grass sward rece iving 420 kg fertilizer N ha(-1). Consequently potential losses of N t o the environment are high under these management systems.