Plant-animal relationships in pastoral heterogeneous environments: defoliation and selectivity patterns

The experiment was conduced to evaluate the effect of different native pasture structures, represented by four sward heights (4, 8 12 and 16 cm) on defoliation and selectivity patterns of calves and ewes, in 45-minute grazing tests. A completely randomized design with four treatments and two replicates in time and space were used. Sward structure was characterized by sward height, herbage mass, leaf blade mass, herbage bulk density, leaf blade bulk density, and number and length of fully-emerged and emerging leaves. Forty tillers were marked in each experimental unit to determine the number and length of leaf blade. Defoliation and selectivity patterns were evaluated by defoliation probability and by passive selectively (PS) and active selectively (AS) indexes. A positive correlation between sward height and herbage mass, and negative between sward height and herbage bulk density were observed. Above 4 cm, leaf blade was almost exclusive, meaning optimal conditions for herbage intake by the animals. A reduction in the defoliation probability was observed with increasing sward height due to a lower stocking rate/available forage relationship. PS and AS were negatively affected by sward height. PS did not differ between calves and ewes, and above 6.7 cm, animals consumed less emerging leaf blade in relation to fully-emerged leaf blade. However, it did not mean that animals fail to select leaf blade, since AS remained above reference value 1 for both animal species. Ewes differed from calves in terms of AS. The increase in sward height affects the opportunity of animals to find emerging leaf blade, however, they selected younger leaf blade in the attempt to obtain a better food quality. Ewes selected emerging leaf blade more effectively than calves.