Marcelo Barcelo Gomes
Abstract:
The aim of this experiment was to evaluate combinations of two intermittent grazing intervals, corresponding to the time required for the canopy to reach heights of 25 and 35 cm and two nitrogen application rates (50 and 200 kg N/ha/year) on canopy structure, light interception and forage production in marandu grass pastures. The experiment was conducted at IZ - Institute of Zootechnics, located in Nova Odessa, SP. The treatments named by the nitrogen height/dose ratio 25/50, 25/200, 35/50 and 35/200 were allocated to experimental units consisting of six 0.5 ha pickets each according to a 2 x 2 factorial arrangement ( two heights and two doses of N) in a randomized complete block design with four replications, totaling 48 ha. The variables grazing cycle number, occupation days, rest days, light interception, foliage area, foliage angle, dry matter (MS) mass, leaf: stem ratio, spatial distribution of morphological components were evaluated. length of pasture vertical profile, total forage and morphological components density and forage accumulation rate. The data were analyzed in a subdivided plot in a mixed model, considering as random effect the block and the errors associated with each plot. The plot considered the effect of the season (summer I - period from December 2009 to January 2010 and summer II - period from February to March 2010) and the block, in the subplot the effect of height and interactions and in the sub-pairing. the effect of nitrogen dose and interactions. The means were estimated by the least squares method with the comparison of treatments by the Student “t” test using the SAS v.9.0 software at 5% significance. There was a higher number of grazing cycles (1.82) in the 25/200 treatment. There was an effect of height and dose of N on the occupation days (seven days for 25/200 treatment) and rest days (34 and 36 days for height 25 cm and dose 200 kg N/ha, respectively). There was effect of height and dose of N for light interception (96.03 and 98.10% for heights of 25 and 35 and 96.62 and 97.51% for doses of 50 and 200 kg N / ha, respectively). There was an effect of the interaction season x height x dose of N, and treatments 35/50 and 35/200 showed the highest values of foliage area index (IAFolhagem) in both summer I and summer II. The effect of season (41.77 and 44.91º for summer I and summer II, respectively) and height-dose N interaction (lower value - 41.78 °) were observed for foliage angle - for treatment 35/50). There was an effect of height on pre-grazing forage mass which presented higher values of total forage mass (12356 kg MS/ha), leaf forage mass (4668.76 kg MS/ha) and stem forage (3553,69 kg MS/ha) for the height of 35 cm. There was season effect (1.66 and 1.30 for summer I and summer II) and height (1.61 and 1.35 for 25 and 35 cm) on leaf: stem ratio. There was an effect of N dose on total dry matter after grazing, and the 200 kg N / ha dose presented higher value (8103.58 kg MS/ha). Post-grazing leaf forage mass was affected by height (1141, 87 and 669.14 kg DM / ha at 25 and 35 cm) and by season x dose N interaction showing difference between summer I and summer seasons. II for the 50 kg N/ha dose (709.71 and 1076.10 kg MS/ha for summer I and summer II). Post-grazing stem forage mass was affected by the season x height interaction with difference between heights of 25 and 35 cm in season I (1730.67 and 2372.04 kg MS/ha for 25 and 35 cm) and increase between summer I and summer II to 25 cm height (1730.67 to 2340.10 kg MS/ha). The forage mass of dead material after grazing was affected by height (4114.36 and 4936.97 kg MS/ha for 25 and 35 cm) and by the interaction season x dose with variation of 50 kg. of N/ha the summer I and summer II seasons (5015.49 and 3654.76 kg of MS/ha for summer I and summer II season) and between the doses of N during the summer II season (3654.76 and 4541.94 kg/ha for 50 and 200 kg/ha). The leaf: stem ratio in post grazing was affected by the season x dose x height interaction, presenting higher values for treatments 25/50 and 25/200. The canopy structure in the pre-grazing condition showed about 50% of its upper portion composed of leaf blades regardless of the time of year and treatment. The total volumetric density was affected by the season x dose N x height interaction, presenting higher values during the summer I season. The leaf volumetric density was affected by the season x dose x height interaction showing variation between the summer I and summer II seasons. There was no effect of season, N dose and height on the total forage accumulation rate. Grazing strategy with 25 cm height combined with 200 kg N/ha fertilization is recommended for the best control of pasture structure for animal production.
