Planting time and stand density effect on radiation interception and use efficiency of maize and sunn hemp intercropping in semi-arid South Africa

Solar radiation must be efficiently utilised to enhance dry matter accumulation and carbon sequestration capacity within dryland agroecosystems. The study evaluated the effects of sunn hemp (Crotalaria juncea L.) intercropping time and stand density on radiation interception and radiation use efficiency (RUE) in a maizebased system. Field experiments were conducted under the in-field rainwater harvesting (IRWH) technique for two consecutive growing seasons, 2019/20 and 2020/21. Radiation interception and productivity were measured and calculated for maize and sunn hemp, sole and intercrops. The intercrops were identified by the sunn hemp intercropping times [simultaneous with maize planting (P1), at V15 maize growth stage (P2), at R1 maize growth stage (P3)] and stand densities [16 plants m-2 (D1-low), 32 plants m-2 (D2-medium), 48 plants m-2 (D3-high)]. Total radiation interception was calculated using a row-crop radiation transmission (RCRT) model from strip width of component crops, extinction coefficient (k), plant height, and leaf area index (LAI) measurements taken throughout the growing season. The results showed that the LAI of sunn hemp was high when intercrop was planted simultaneous with maize planting and at 48 plants m-2. The overall k for the sole sunn hemp (SSH) cropping system was 0.37. The correlations (r2) between the modelled and measured fractions of photosynthetic active radiation (PAR) in simultaneous maize planting, V15 maize growth stage, and R1 maize growth stage intercropping treatments were 0.87, 0.97 and 0.93, respectively. Simultaneous planting of sunn hemp intercrop with maize captured the most growing season PAR of 798.70 MJ m-2 and had the highest RUE of 1.42 g MJ-1. These results suggest that intercropped sunn hemp planted simultaneously with maize would help to increase production through the more efficient use of solar radiation. Optimising planting time and density can help plan the planting time to increase radiation utilisation in maize/sunn hemp intercropping systems.

Automated summary

This study evaluated the effects of sunn hemp intercropping time and stand density on radiation interception and use efficiency in a maize-based system. The results showed that simultaneous planting of sunn hemp with maize captured the most growing season PAR and had the highest RUE, indicating that it can significantly enhance dry matter accumulation and carbon sequestration in dryland agroecosystems. Optimizing planting time and density can further increase radiation utilization in maize/sunn hemp intercropping systems.