MOI, PETER ELIA

Intercropping is an important sustainable cropping system in which two or more crops are grown in the same piece of land. Despite the development of high yield varieties, sorghum yields have remained low due to low soil fertility, inappropriate cropping practices and limited use of fertilizer nitrogen (N). The integration of cowpea into sorghum-based crop systems and N use are likely to increase yield. However, how sorghum-cowpea compatibility, N use and their interactions impact yield of the companion crops is only partially understood. Further, leaf senescence regulates grain yield and quality in sorghum. However, the effect of intercropping sorghum with cowpea on the patterns of leaf senescence of the former is not known. Therefore, an experiment was conducted in 2018/2019 short rain season at Katumani and Igoji KALRO research stations to: (i) determine the effect of intercropping and nitrogen rates on the growth and yield of selected varieties of sorghum and cowpea; (ii) investigate the effect of intercropping sorghum with cowpea on sorghum time-course of leaf senescence and its association with grain yield. Cropping systems (sole crops of two varieties each of sorghum and cowpea, and cereal-legume intercrop combinations of the two varieties of sorghum and cowpea), and three rates of N (0, 40, and 80 kg N ha-1) were laid out in a randomized complete block design with split plot arrangement, replicated three times. Cropping systems formed the main plots while N rate formed the sub-plots. Sorghum and cowpea growth data were collected every 10 days, which started at 4 weeks after planting throughout physiological maturity while grain yield and yield components data were collected at physiological maturity (harvest). Sorghum leaf senescence was assessed from flowering to maturity at both whole-plant level and flag-leaf level. At the whole-plant level, leaf senescence was scored visually by counting the number of leaves that presented more than 50% green leaf area while the greenness of the flag leaf was tracked using SPAD 502 chlorophyll meter. A logistic function in SigmaPlot was fitted to estimate four parameters of senescence in sorghum, including minimum and maximum SPAD units, time to loss of 50% maximum SPAD (EC50) and the rate of senescence (RS). Data were subjected to analysis of variance using Genstat and means were separated using
ii
the least significance difference test (p≤0.05). Intercropping significantly reduced leaf area index (LAI) of Gadam by 0.53 units but LAI of Serena was not affected by intercropping. Addition of 80 kg N ha-1 increased overall sorghum LAI by 0.08 units (28%) compared with control plots where no fertiliser was applied but no differences were detected between 40 and 80 kg N ha-1. Further, intercropping reduced the number of fertile tillers m-2 by 6 tillers but addition of N significantly increased the number of fertile tillers m-2 by 1 tiller. Similarly, intercropping significantly reduced CGR of sorghum by 54% for Serena but CGR of Gadam was not affected by intercropping however addition of 80 kg N ha-1 increased overall sorghum CGR by 30% but without difference between 40 and 80 kg N ha-1. Grain yield of Gadam exceeded Serena by 1.33 t ha-1 but irrespective of the cowpea variety, intercropping significantly reduced the grain yield of sorghum by 53% for Gadam and 42% for Serena in Igoji and by 54% for both varieties in Katumani. Addition of 40 kg N ha-1 significantly increased grain yield of sorghum by 0.53 t ha-1 (27%) compared with control plots were no fertiliser was applied but no difference was detected between addition of 40 and 80 kg N ha-1. The harvest index (HI) and N uptake of sole sorghum exceeded counterparts in an intercrop with cowpea by 30% and 0.01 kg m-2 respectively. Addition of N significantly increased N uptake by 0.006 kg m-2 but had no significant effect on HI. Sorghum grain yield was positively and significantly correlated with leaf area index, fertile tillers, panicle weight, harvest index and crop growth rate under sole cropping system however, sorghum grain yield was inconsistently correlated with these traits under intercrop system. Similarly, intercropping significantly reduced the CGR of cowpea by 50% for K80 and 25% for M66 and grain yield of K80 by 54% but grain yield of M66 was not affected by intercropping. On the other hand, addition of N had no significant effect on cowpea growth and yield. The total land equivalent ratio (LER) in both sites was greater unity: 1.4 in Igoji and 1.6 in Katumani. Intercropping reduced the peak leaf greenness (SPADmax) of the flag by 8 SPAD units but delayed leaf senescence at whole plant by 0.2 leaves plant-1 day-1 compared with sole crop system. On the other hand, fertilizer N delayed leaf senescence at both whole-plant and flag-leaf levels. While EC50 did not correlate with grain yield, sorghum yield was positively and significantly correlated with SPADmax, SPADmin and the rate of leaf senescence. The results therefore suggest that the peak leaf greenness of the flag leaf in the period bracketing flowering determined grain yield but the delay in leaf senescence at whole plant level might have been non-functional. Further, although intercropping reduced sorghum yield, present results show that there is potential to exploit cropping system x N
iii
interactions to increase yield, especially in wetter environments than in areas with low rainfall. Lack of significant differences in grain yield between the application of 40 and 80 kg N ha-1 suggests that sorghum yield could be maximized at lower N rates. However, further studies are needed to establish the economically optimal N rate in sorghum production. Gadam variety is recommended for commercial production under sole cropping system with addition of N at a rate of 40 kg N ha-1 as raw material for making malt and as food security crop in the study areas due to its high yielding traits, short maturity period compared with Serena however the growth and yield performance of Gadam across ecological zones deserve further investigation. Intercropping and N fertiliser application is only recommended for sorghum production to improve household food security since sorghum/cowpea intercropping was more productive than sole (LER˃1). Screening and breeding of more cowpea varieties compatible for sorghum intercropping is recommended. The effect of competition for resources in sorghum/legume intercropping system and source-sink relationship on sorghum leaf senescence and yield deserve further investigation.
Keywords: EC50, fertilizer nitrogen, intercrop system, leaf greenness, rate of senescence, SPAD, yield