Research has shown that incidence of flystrike is reduced in sheep grazing low endophyte ryegrass. This experiment was designed to test the hypothesis that where lambs are bred specifically for resistance to ryegrass staggers (RGS), their susceptibility to flystrike on the body should be correspondingly low, even when challenged with high levels of endophyte. Ewe and ram lambs were sourced from two lines, one selected for resistance (n = 85) and the other for susceptibility (n = 143) to ryegrass staggers. A further group of lambs (n = 63) produced by intercrossing the two lines was included. The lambs were run in 2 mobs, on high and low endophyte pasture, and monitored daily for signs of flystrike on the body. During the course of the experiment, susceptible (n = 34) and resistant (n = 1) lambs suffering from clinical ryegrass staggers had to be removed from the high endophyte pasture. These were excluded from the analysis. Ram lambs made up the majority of animals removed. Only ram lambs were flystruck on the body, particularly around the scrotal area. Significantly more resistant ram lambs (7 of 49) were flystruck than susceptible (3 of 85) or intercross ram lambs (2 of 27), regardless of pasture endophyte level (P < 0.1). More lambs (P < 0.1) were flystruck grazing ryegrass with high endophyte (n = 9) than low endophyte (n = 3). The only lambs flystruck while grazing non-endophyte pasture were RGS resistant rams. In contrast to the hypothesis, this experiment indicates that selection of animals for resistance to RGS may exacerbate their susceptibility to flystrike. The evidence presented here supports the literature that sheep grazing high endophyte pasture are more susceptible to flystrike. The question for farmers is whether their selection priority should be for pasture with resistance to insect pests, or for stock with resistance to RGS, or both.
Proceedings of the New Zealand Society of Animal Production, Volume 64, Hamilton, 297-299, 2004
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