Effect of continuous stocking of breeding ewes at different sward surface heights during the late summer-autumn on herbage intake and productivity : a thesis presented in partial fulfilment of the requirements for the degree of Master of Agricultural Science in Animal Science at Massey University, New Zealand
Continuous stocking management is preferred by many New Zealand sheep farmers during the late summer-autumn period. At present, there are no guidelines available to farmers that define the optimum sward conditions for continuous stocking management of ewes leading up to, and during, the mating period. Three different nominal sward surface heights (SSH) (2, 4, and 6 cm) replicated twice were used for a trial with 14 mixed age breeding ewes per treatment (n=84 ewes) continuously stocked from February to April 1994. The pastures consisted of predominantly 10-year old ryegrass (Lolium perenne), white clover (Trifolium repens) and browntop (Agrostis capillaris). Sward heights were measured weekly throughout the trial. Herbage intakes by the ewes were determined indirectly from faecal output using chromic oxide controlled release capsules and in vitro digestibility of digesta samples obtained from oesophageal-fistulated sheep run with the ewes. The average actual sward surface heights for the 2, 4, and 6 cm SSH treatments were 2.7 vs 4.3 vs 6.1 cm (± 0.05 cm (SEM), P<0.001). The pasture characteristics in terms of herbage mass, dead matter content and organic matter digestibility (OMD) for the 2,4, and 6 cm SSH treatments were: 2723 vs 3880 vs 4337 (± 204 kg DM/ha, P<0.05); 69.74 vs 64.62 vs 51.37 (± 2.78%, P<0.05); 66.52 ± 0.85 vs 60.29 ± 0.90 vs 69.56 ± 0.84% (P<0.01). The daily liveweight gain, condition score, wool growth rate and mean fibre diameter for ewes grazing the 2, 4, and 6 cm SSH treatments were: 103 vs 122 vs 195 (± 15 g/day, P<0.05); 2.89 vs 3.05 vs 3.23 (± 0.06 condition score units, P<0.1); 1.30 vs 1.26 vs 1.41 (± 0.03 mg/cm2/day, P<0.1); 43.01 vs 44.07 vs 44.48 (± 0.35 microns, P>0.1). The results suggest that swards of at least at 6 cm height are required to support adequate liveweight gain and condition score of breeding ewes in the period prior to and during mating. The accumulation of weed and dead material appear to be the major problems limiting intake and ewe performance.