Leaf and Canopy Carbon Assimilation Rates of 'Mulato Ii' Brachiariagrass (Convert HD 364) in Response to Canopy Height and Growth Rate.

Carbon assimilation is the basis of grass growth and can be affected by several factors, including canopy management. The objective of this experiment was to quantify leaf photosynthesis and estimate canopy photosynthesis of Mulato II brachiariagrass pastures managed at different canopy heights and growth rates under continuous stocking. Treatments were the factorial combinations of three steady-state canopy heights (10, 25, and 40 cm) and two growth rates (“low” and “high”), the latter imposed by applying two N rates (50 and 250 kg N ha^-1 yr^-1). The treatments were replicated three times in a randomized complete block design. The experimental period was from 10 Dec. 2013 to 16 Apr. 2014.  Carbon assimilation in leaves and canopy, leaf area index (LAI), light interception (LI), and k were evaluated. Canopy photosynthesis was estimated using the sunlit and shaded leaves"model that uses LAI, LI, and k. Leaf and canopy assimilation and LAI were affected by canopy height and N rate. The LAI increased linearly from 2.5 to 5.8 with increasing canopy height and was greater when more N was applied (4.8 vs. 3.7). Leaf carbon assimilation rate was greater for pastures grazed to 10 cm (1.04 mg m^-2 s^-1 CO₂) and was similar for 25- and 40-cm swards (0.95 mg m^-2 s^-1 CO₂). The canopy carbon assimilation rate, on the other hand, was greater for 25- and 40-cm canopies vs. 10 cm canopies (1.95 vs.1.46 mg m^-2 s^-1 CO₂, respectively). The greater N rate resulted in greater leaf (1.12 vs. 0.85 mg m^-2 s^-1 CO₂) and canopy assimilation rates (2.03 vs. 1.54 mg m^-2 s^-1 CO₂). The greater leaf carbon assimilation on pastures managed at 10 cm under continuous stocking did not result in greater canopy assimilation rate, primarily as a result of reduced LAI compared with taller canopies.