Estrutura da pastagem e padrões de desfolhação em capim-mombaça em diferentes alturas do dossel forrageiro

To evaluate the pattern of defoliation in a Mombaca (Panicum maximum) grass pasture, experimental paddocks were grazed by Holstein heifers to maintain canopy height of: 60, 80, 100, 120 and 140 cm height. The experimental design was completely randomized, with two replications. The assessed sward structures were: canopy height; herbage and leaf lamina mass; canopy, sheath plus stems and leaves bulk density; tiller density and number and length of fully-expanded and emerging leaves. Linear decrease in plant defoliation probability ( = 0.369606 + 0.001555 x, R 2 = 0.5560, P = 0.008) was observed as pasture height increased, as a reflection of increasing canopy herbage mass. Ingestion of fully expanded leaves increased with canopy height, mainly for the 85 cm canopy, in spite of the animal preference for emerging leaves. The intensity of defoliation of fully expanded leaves decreased linearly ( = 80.866791 - 0.370979 x, R 2 = 0.6076, P = 0.004), reflecting their increased length ( = -104.311106 + 2.877570 x - 0.012035 x 2 , R 2 = 0.9271, P = 0.002) and their greater resistance to apprehension by the animal, as well. The increase in canopy heights changed defoliation pattern of the animals, as the animals began to graze around the plants, due to the restricted access to the younger leaves, of greater nutritive value.