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/white 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 maintain a sward surface height of 6 cm. Herbage organic matter (OM) intakes 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 treatment was not significant for either variable. There were similar levels of output of faecal N ewe(-1) but significantly more urinary N ewe(-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 stocking rates, which were 19.4, 26.6, 27.2 and 36.5 ewe ha(-1), the total faeces 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 G420, and so the intake of herbage OM ha(-1) d(-1) was related to stocking rate, i. e. the estimated herbage intake ha(-1) over the growing season for the white clover monoculture was 73% of that for N-fertilized grass. Excretal nitrogen returns to the pasture from grazed mono-cultures of clover were high, and similar to those from a grass sward receiving 420 kg fertilizer N ha(-1). Consequently potential losses of N to the environment are high under these management systems.