The effect of grazing by cattle on animal performance and floristic composition in Nardus-dominated swards

Four groups of six spring-calving beef cows and their calves were allocated to two replicated grazing treatments on a Nardus stricta-dominated hill pa sture in the UK during six consecutive summer grazing seasons, Treatments w ere applied by continuous variable stocking to maintain a sward height of e ither 4-5 cm (short) or 6-7 cm (tall) between tussocks of N. stricta. Cows on both treatments ingested a greater proportion of N. stricta than wa s present in the sward. The short treatment reduced the herbage intake of c ows proportionally by 0.35 (P < 0.001) and reduced herbage digestibility by 0.03 (P < 0.01) compared with the tall treatment. Cows lost 0.02 kg d(-1) live weight on the short treatment but gained 0.27 kg d(-1) live weight on the tall treatment (s.e.d. 0.077; P < 0.001). Calf liveweight gains were 0. 60 and 0.86 (s.e.d. 0.034; P < 0.001) kg d(-1) for the short and tall treat ments respectively. The greatest liveweight gains for both cows and calves occurred during the first 5-6 weeks of the grazing season and differences b etween treatments increased over the season. Cattle on the short treatment consistently grazed a higher proportion of N. stricta tillers more closely than those on the tall treatment, and the percentage cover of N. stricta de clined more rapidly on the short treatment. After 6 years of grazing there was a considerably greater (P < 0.001) proportion of very small tussocks, i .e. less than 140 cm(2) basal area, on the short treatment than on the tall treatment. The percentage cover of broad- and fine-leaved grasses also dec lined, in contrast to previous studies. This may have been due to lower soi l fertility on the site of the current experiment. It is concluded that high levels of utilization of N. stricta are incompati ble with high levels of individual animal performance in lactating cows, al though short periods in early summer may give acceptable performance. Lower levels of utilization such as that achieved on the 6-7 cm treatment result ed in moderate levels of animal performance and also gradually reduced the cover of N. stricta, although it took longer for the effect to be establish ed.