The implications of controlling grazed sward height for the operation and productivity of upland sheep systems in the UK. 5. The effect of stocking rate and reduced levels of nitrogen fertilizer

Abstract

The implications for UK upland sheep systems of reducing nitrogen fertilizer application to perennial ryegrass/white clover swards were studied over 3 years. Sward height (3·5–5·5 cm) was controlled for ewes with lambs until weaning using surplus pasture areas for silage; thereafter, ewes and weaned lambs were grazed on separate areas, and sward height was controlled by adjusting the size of the areas grazed and using surplus pasture areas for silage if necessary. Combinations from three stocking rates [10, 6 and 4 ewes ha−1 on the total area (grazed and ensiled)] and four nitrogen fertilizer levels (150, 100, 50 and 0 kg ha−1) provided six treatments that were replicated three times. Average white clover content was negatively correlated with level of nitrogen fertilizer. The proportion of white clover in the swards increased over the duration of the experiment. Control of sward height and the contribution from white clover resulted in similar levels of lamb liveweight gain on all treatments. All treatments provided adequate winter fodder as silage. It is concluded that the application of nitrogen fertilizer can be reduced or removed from upland sheep pastures without compromising individual animal performance provided that white clover content and sward height are maintained. Resting pastures from grazing by changing ensiled and grazed areas from year to year sustained white clover content over a 3‐year period.

The implications of controlling grazed sward height for the operation and productivity of upland sheep systems in the UK: 6. The effect of reducing stocking rate and application of fertilizer nitrogen in Wales

“…Although a substantial decline occurred in the third year of the experiment, recovery was swift and the amount of white clover, measured as stolon density or growing point density, was greater at the end than at the start of the experiment on the swards with the lower input of fertilizer nitrogen. This questions the impact of such transitory declines in white clover content on the overall production from clover‐based systems (Fothergill et al ., 1996) whilst providing support for arguments made elsewhere (Sibbald et al . 2002) that resting areas of pasture from grazing during the period when silage is accumulating gives it a competitive edge over the grass when it is growing at its most active.…”

“…There was a consistently greater amount of white clover in the N50 than in the N200 treatments. This is in agreement with other findings (Grant et al ., 1986; Laidlaw and Stewart, 1987; Dennis and Woledge, 1987; Laidlaw et al ., 1992; Harris and Clark, 1996; Sibbald et al . 2002).…”

“…Treatments N200/SR18 and N50/SR15 did not provide this level of silage (see Table 7) and would require additional feed to be purchased from outwith the system. The other treatments produced an excess of winter fodder as silage and this may or may not have a commercial value (Sibbald et al ., 2002).…”

The implications of controlling grazed sward height for the operation and productivity of upland sheep systems in the UK: 7. Sustainability of white clover in grass/clover swards with reduced levels of fertilizer nitrogen

“…In an earlier paper in this series (Sibbald et al. , 2002), it was concluded that survival of white clover in mixed grass/white clover swards with low or zero levels of applied fertilizer nitrogen was not compromised over a period of 3 years as had been reported in other studies in the UK (Laidlaw, 1984;Woledge et al.…”

“…For economic reasons, the longer‐term survival of white clover is essential in pastures in low‐input, grazed livestock systems. A continuation of part of the experiment described by Sibbald et al. (2002) allowed white clover to be studied for 5 years in order to test the hypothesis that, in an upland sheep system, closure of areas for ensiling and an annual interchange of regularly grazed and ensiled areas would result in white clover survival and that the practice would be consistent with the management of an upland sheep farming system.…”