EFFECTS OF SEASON AND LEVEL OF GRAZING ON THE UTILIZATION OF HEATHER BY SHEEP .2. DIET SELECTION AND INTAKE

The effect of level of removal of current season''s shoots of heather (0, 40 and 80% of the dry matter of the shoots) in the summer and autumn on the intake and diet selection by grazing Scottish Blackface wether sheep was examined in an experiment conducted over 2 yr in which all combinations of level and season of grazing were provided. The treatment plots were grazed for 5 wk at any one time and measurements made of intake and diet selection in wk 2 and 5. In both the summer and autumn grazing periods, the level of removal in wk 2 had little effect on the quality of the diet selected. In wk 5, digestibility of organic matter was higher for the 40% than the 80% level of removal in the summer (0.543 vs. 0.508) and particularly in the autumn (0.503 vs. 0.449). Digestibility of the diet selected was higher in wk 2 than in wk 5 in both the summer (0.555 vs. 0.525) and in the autumn (0.511 vs. 0.476). Mechanisms of diet selection are discussed. In the summer grazing period, only small differences existed in intake of organic matter between levels of removal but in the autumn intakes were higher at the 40% than the 80% level of removal. In a clipping experiment in which 0, 40 or 80% of the length of current season''s shoots was removed in May, July and Sept. in 3 consecutive years, the N content of clippings in May and July was higher and the cell wall constituent lower at the 40% level in comparison to the 80% level of removal. Higher levels of removal at the previous clipping increased the N content of the clippings in May, July and Sept. and reduced the cell wall constituent content in Sept.