Many crop simulation models rely on the equation, AB = q (PAR), to estimate the potential accumulated biomass (AB) by the crop. Simply stated, AB is proportional to the accumulated photosynthetic active radiation absorbed by the crop (PAR). The constant, q, is often referred to as Radiation-Use Efficiency (RUE) and describes the efficiency of the plant canopy to convert PAR into plant biomass (g/mj). These models use Beer's Law to estimate PAR transmitted through crop canopies with the following equation: TPAR = exp(k(c)LAI), where TPAR is the percentage of photosynthetic active radiation transmitted through the canopy, LAI is the leaf area index, and k(c) is the canopy extinction coefficient. The objective of this study was to determine if RUE and K(c) differed among three commonly used U.S. cotton, Gossypium hirsutum L., varieties.

The study was conducted under irrigated field conditions in 1988 and 1989 at the Blackland Research Center, Temple, TX. The varieties studied, Acala SJ-2, Deltapine 50, and Tamcot CD3H, were grown under optimum management practices in rows spaced 70 cm apart and plant densities of 12 to 14 plants m^-2. Measurements of accumulated above-ground biomass, LAI, and intercepted PAR were made weekly throughout the season.

Data revealed the total accumulated above-ground biomass of the varieties did not differ between years, but Tamcot CD3H produced significantly less total biomass (900 g m^-2) on average than Acala SJ-2 (1400 g m^-2) and Deltapine 50 (1250 g m^-2) at harvest. The canopy extinction coefficient, k(c), was estimated to be -0.71 and did not differ among varieties or years. However, statistical differences were found in RUE among varieties. Averaged over years, RUE of Tamcot CD3H (1.31 g MJ^-1) was significantly lower than Acala Si-2 (1.61 g MJ^-1) but RUE of Deltapine 50 (1.46 g MJ^-2) did not differ significantly from Tamcot CD3H or Acala Si-2. Therefore, total above-ground biomass at harvest was correlated with RUE.

Varieties also differed in biomass partitioning between vegetative structures and bolls as indicated by the harvest index (e.g., g bolls / g total above-ground biomass). The harvest index of Deltapine 50 averaged over years (0.35) was significantly lower than Acala Si-2 (0.48) and Tamcot CD3H (0.54).

This study revealed that significant differences in RUE and biomass partitioning exist among cotton varieties commonly grown in the U.S.A. However, varietal differences were not found in the ability of the canopy to intercept photosynthetic active radiation. These results suggest that crop simulation models of cotton must account for both differences in RUE and biomass partitioning if they are to accurately simulate growth of different cotton varieties.