Forage Accumulation and Nutritive Value of Grazed Mulato II Brachiariagrass (Convert HD 364) in Response to Canopy Height and Nitrogen Rate.

Brachiaria spp. grasses are widely used as cultivated pastures by the Brazilian livestock industry. ‘Mulato II’ is a Brachiaria hybrid released in 2007 for both beef and dairy operations in Brazil and other tropical areas.  The objectives of this research were to describe and explain the effects of growth rate (applied as N fertilization rate) and canopy height (kept constant) on forage accumulation (FA) and nutritive of ‘Mulato II’ brachiarigrass under continuous stocking in southeastern Brazil, during two summer rainy seasons (2012/2013 and 2013/2014). A RCB design was used, with a 3 × 2 factorial arrangement, corresponding to three steady-state canopy heights (10, 25 and 40-cm) maintained by continuous stocking and two growth rates (“low” and “high”) imposed by N rates (50 and 250 kg N ha^-1 yr^-1), with three replications. Exclosure cages sampled every 21 days were used to estimate FA. Hand-plucked samples were taken every 21 to determine CP, IVDOM, and NDF concentrations. Data were analyzed using PROC MIXED of SAS, and means compared by Student test at 5%. The FA, CP and IVDOM were affected by growth rate (P<0.0001) and canopy height (P<0.0001). FA increased with canopy height (8600, 10100, 13400 kg DM ha^-1 yr^-1, for 10, 25 and 40 cm, respectively). Under low growth rate, ‘Mulato II’ was less productive (7900 vs. 13400 kg DM ha^-1). The IVOMD decreased with canopy height from 652 to 586 g kg^-1 probably due to differences in phyllochron and increased deposition of cell wall on taller canopies. The IVDOM was greater under high growth rate (652 vs. 581 g kg^-1), with similar trend for NDF (560 vs. 543 kg DM ha^-1), all likely associated with greater tissue renovation and frequency defoliation in order to maintain canopy height. CP decreased with canopy height from 150 to 121 g kg^-1 as result of a dilution effect of N in the total biomass. CP increased with N rate from (106 to 164 g kg^-1). The use of 40-cm stubble height results in greater FA, but the nutritive value is reduced. N is a tool to increase FA and the use of 250 kg ha^-1 yr^-1 results in greater nutritive value when the pasture is maintained under continuous stocking and variable stocking rate.