Modelling the effects of mulch application rates on cowpea growth, yield and soil water content using APSIM

Abstract

Early maturing crops that are drought-adapted together with mulch application have the potential to stabilize and increase dryland crop yields in semi-arid regions of the world. Agricultural systems models worldwide are increasingly being used to explore options and solutions for food security. Agricultural Production Systems Simulator (APSIM) is one such model that continues to be applied and adapted to meet this challenge. The APSIM model was used to assess the impact of different mulch application rates on cowpea grown under dry conditions in Vhembe district of South Africa.The objective of the study was to evaluate soil water content, crop water use efficiency (WUE), grain yield and the dry matter response of cowpea (Vigna unguiculata (L)) and validate the results using the model APSIM. Data taken from the study under optimum growth conditions was used to parametize the ASPSIM model. Soil water and fertility parameters measured were used for simulations while the same starting conditions were assumed for unmeasured parameters for all trials. Field experiments were conducted over two seasons (2016/17 and 2017/18). Treatments consisted of four levels of mulch application (0, 2, 4 and 8 t/ha) arranged in a randomized complete block design (RCBD) and replicated four times. The following parameters were measured: dry matter, grain yield, water use efficiency and soil water content. The data collected was subjected to analysis of variance using the randomised complete block design (RCBD) model and analyzed using the Statisticsl Analysis Systems (SAS) version: 9.4 at P ≤ 0.05 probability level. The perfomance of APSIM model was evaluated using root mean square error (RMSE), coefficient of efficiency (Ei), root mean square deviation (RMSD) and median unbiased absolute percentage error (MdUAPE). There were no significant differences (p>0.05) in dry matter accumulation at flowering stage and harvesting stage amongst seasons. The highest cowpea yields were achieved at 8 t/ha mulch rate in both the 2016/17 and 2017/18 cropping seasons (11144.0 kg/ha and 9705.6 kg/ha) respectively. Mulch application rates had significant (p<0.05) differences on grain yield in 2016/17 (2049 kg/ha <2232.5 kg/ha <3341 kg/ha <3631 kg/ha) and 2017/18 (1291 kg/ha <1774 kg/ha< 2965 kg/ha <3594 kg/ha). The season effect of grain yield was not significant. There were no significant differences in soil water content among treatments as well as between seasons. There were no significant differences among the treatments in terms of ET; however, there was a significant (p<0.05) difference between the seasons. In both seasons, the 4 t/ha mulch application rate recorded the highest ET (139.17 mm and 125.83 mm in 2016/17 and 2017/18 seasons, respectively). The effect of treatments in terms of water use efficiency was found to be significant (p<0.05), with the highest WUE found to be at the 4 t/ha and 8 t/ha mulch application rate, with 4 t/ha being the highest (24 kg/ha/mm and 24 kg/ha/mm, in 2016/17 and 2017/18 seasons, respectively). R2 for both seasons was 0.5 for yield and dry matter at harvest. RMSE for yield was found to be 0.3 in both the 2016/17 and 2017/18 cropping seasons. Ei was recorded at 0.9. RMSE for total dry matter at harvesting stage was calculated to be 0.3 and the 0.4, during the 2016/17 and 2017/18 seasons, respectively. At flowering RMSE was found to be 0.2 for both seasons and Ei to be 0.9.