Defoliation intensity of marandu palisade grass-pintoi peanut mixed pasture affects canopy structure and forage intake

Abstract

This dissertation aims to evaluate the effectiveness of clipping samples at stubble height to estimate the nutritive value of diets composed by marandu palisade grass and pintoi peanut grazed at three stubble heights; and to determine a defoliation intensity that would favor forage intake with more quality, increasing the apparent efficiency of nitrogen utilization. The trial was conducted at the Universidade Federal de Lavras, MG, Brazil for two consecutive years. The area was divided into 12 experimental units (paddocks) with three defoliation intensities (severe, moderate and light). The experimental area was constituted by marandu palisade grass (Urochloa brizantha cv. Marandu) and pintoi peanut (Arachis pintoi cv. Mandobi). In order to compare the nutritive value of samples from hand-plucking and clipping at stubble height, two hypotheses were tested: H0: β0 = 0 and H0: β1 = 1. Pearson’s correlation analysis was performed to test if chemical composition of each methodology were correlated. The hypothesis H0: β0 = 0 was accepted for crude protein (CP), ash and protein-free neutral detergent fiber (NDF), and organic matter (OM) concentrations for marandu palisade grass and pintoi peanut. Hypothesis H0: β1 = 1 was rejected for CP, NDF, and OM concentrations for marandu palisade grass and for CP and OM of pintoi peanut (P<0.01), but not for NDF concentration of pintoi peanut (P = 0.24). The averages for CP, NDF, and OM for marandu palisade grass by hand-plucking were 7.9, 74.5, and 91.4 g/100 g DM and by clipping were 7.7, 68.7, and 90.7 g/100 g DM. For pintoi peanut the averages for CP, NDF, and OM by hand-plucking were 15.6, 63.5, and 92.0 g/100 g DM and by clipping were 15.9, 49.3, and 90.8 g/100 g DM. The Pearson’s correlation coefficient was not significant for any chemical compound evaluated regardless the species; thus, the methodologies were not correlated (P>0.05). Defoliation intensity (DI), season (S) and the interaction between the two (DI×S) were considered fixed effects, year and replicate were considered random effects. Light defoliation showed greater forage allowance and lower stocking rate than the other intensities (6.3 kg DM/kg LW and 1.3 AU/ha). Grazing time did not differ in the first (390 min; P=0.15) or the last occupation day (440 min; P=0.20). During the last day, light defoliation showed 70% greater intake rate (33.2 g DM/min) than the average of the other treatments, related to greater biting rate (38.0 bites/min) and bite weight (0.9 g DM/bite). Grass and total dry matter intake (DMI; %LW) were greater for moderate and light defoliations, but legume intake was lower for severe and moderate defoliation. The efficiency of microbial synthesis (EMS) was 100 g of microbial protein/kg DOM greater during summer compared to spring but did not vary according to DI (P=0.80). Clipping mixed pasture at stubble height is not an efficient methodology to evaluate the nutritive value of the diet consumed because it is not capable of incorporating grazing selection, spatial and morphological heterogeneity. Moderate defoliation increases SR without reducing DMI.