How do severity and frequency of grazing affect sward characteristics and the choices of sheep during the grazing season?

The effect of grazing frequency and severity on sward characteristics and preferences by sheep was investigated from April to September. Two levels of grazing severity were imposed by varying the numbers of ewes grazing 200 m(2) plots for 24 h: four (S, severe) or two (L, lax) ewes. Grazing frequency was either 1 d week(-1) (F, frequent) or 1 d every 2 weeks (I, infrequent). By combining frequency and severity, four treatments were obtained: SF, LF, Sl and LI. The six binary combinations (SF/LF, SF/Sl, SF/LI, LF/SI, LF/LI and SI/LI) were studied in preference tests. Treatments LF, SI and LI were characterized by a high sward surface height, biomass and amount of reproductive green tissues relative to treatment SF. Herbage quality was not different between the grazing treatments between April and July. In September, after a 6-week period of regrowth, herbage quality was significantly higher for the SF treatment than the other treatments. The sheep preferred the swards grazed at a low frequency between April and July, and then changed their preference in favour of the sward with higher quality herbage (treatment SF). The relative abundance of green laminae and the relative digestibility of the swards helped to explain the preferences observed. For a low grazing pressure at the spatio-temporal scale studied, sheep should graze swards at a relatively low frequency but at a high severity of grazing rather than the reverse.