Grazing height effects on limpograss pasture characteristics and performance of beef heifers

Limpograss [Hemarthria altissima (Poir.) Stapf and Hubb.] is a C4 species used for pastures in Florida beef cattle production systems. Adoption by farmers has increased due to its high yield and digestibility, cool-season growth, and tolerance of poorly drained soils. Despite these advantages, performance of cattle grazing limpograss has sometimes been limited due to low herbage crude protein (CP). The objectives of this research were to evaluate effects of canopy height of continuously stocked limpograss pastures (20, 40, and 60 cm) and CP supplementation of grazing beef heifers (640 g head-1 d-1 of a 44% CP corn [Zea mays L.]-urea supplement) on (i) heifer performance, (ii) limpograss canopy characteristics, (iii) fractions and degradation characteristics of limpograss herbage CP, and (iv) competition between limpograss and the weeds vaseygrass (Paspalum urvillei Steud.) and common bermudagrass [Cynodon dactylon (L.) Pers.].;Average daily gains (644 g) were greater for unsupplemented heifers grazing 40-cm canopies than those grazing 20- (448 g) or 60-cm pastures (327 g). This was attributed in part to greater herbage mass and allowance on 40- than 20-cm pastures, but additionally the lower herbage bulk density on 40-cm pastures likely provided greater opportunity for cattle to select high CP leaf throughout the canopy than on very dense 20-cm swards. Lower performance on 60- than 40-cm canopies was likely due to lower herbage CP and digestibility and to significant trampling of grass in the tallest canopies. Supplementation increased gains only of heifers grazing 20- and 60-cm swards.;Protein fractionation of limpograss herbage indicated that the instantaneously soluble fraction was greatest in 20-cm canopies, the slowly degradable fraction greatest in 40-cm canopies, and the undegradable fraction greatest in 40- and 60-cm swards. Crude protein degradation lag times increased linearly as canopy height increased, and were much longer than those reported for temperate forages and somewhat longer than for other C4 tropical grasses.;Continuous stocking of limpograss pastures decreased vaseygrass density for all canopy heights, but the greatest decrease in density and cover was in 20-cm canopies. Concurrently, encroachment of common bermudagrass occurred on the most closely grazed pastures compromising persistence of limpograss. Continuous stocking to 40 cm reduced vaseygrass population, but unlike 20-cm pastures, major encroachment of bermudagrass did not occur.;These data support use of a 40-cm canopy height on continuously stocked limpograss pastures. This height was associated with (i) greatest daily gains of unsupplemented cattle and no response to CP supplement, suggesting the absence of CP deficiency, (ii) high live weight gain per hectare, and (iii) declining vaseygrass density with minimal invasion by bermudagrass.