ABSTRACT
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 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 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
TABLE OF CONTENTS
DECLARATION ii
DEDICATION iii
ACKNOWLEDGEMENTS iv
TABLE OF CONTENTS vi
LIST OF TABLES x
LIST OF FIGURES xii
LIST OF ABBREVIATIONS AND ACRONYMS xiv
ABSTRACT xv
CHAPTER ONE: INTRODUCTION
1.1. Background 1
1.2. Problem statement and justification 5
1.3. Objectives 7
1.4. Hypotheses 7
CHAPTER TWO: LITERATURE REVIEW
2.1. Importance and ecology of sorghum 8
2.2. Sorghum varieties in kenya 10
2.3. Current status of sorghum production and constraints to sorghum production 11
2.4. Consumption of sorghum in kenya and the region 13
2.5. Importance and ecology of cowpea 14
2.6. Nitrogen nutrition of sorghum 17
2.7. Nitrogen nutrition of cowpea 17
2.8. Nitrogen use efficiency in intercrop systems 18
2.9. Water use efficiency in intercrop systems 18
2.10. Radiation use efficiency in intercrop systems 19
2.11. Compatibility of intercrop systems 20
2.12. Intercropping effects on growth and yield of sorghum and cowpea 20
2.13. Patterns of leaf senescence in sorghum 22
2.14. Assessment of the productivity of intercrop systems 23
CHAPTER 3: EFFECT OF SORGHUM-COWPEA INTERCROPPING AND FERTILIZER NITROGEN ON GROWTH AND YIELD OF THE ASSOCIATED CROPS
3.1. Abstract 25
3.2. Introduction 26
3.3. Materials and methods 28
3.3.1. Sites 28
3.3.2. Treatments and experiment design 29
3.3.3. Experiment management 29
3.3.4. Data collection 30
3.3.5. Data analysis 36
3.4. Results 37
3.4.1. Effect of cropping system and nitrogen rates on sorghum phenology 37
3.4.2. Effect of cropping system and nitrogen rate on sorghum growth parameters 37
3.4.3. Effect of cropping system and nitrogen rate on sorghum yield and yield components 45
3.4.4. Effect of cropping system on cowpea phenology 54
3.4.5. Effect of cropping system and nitrogen rate on cowpea growth parameters 54
3.4.6. Effect of cropping system and nitrogen rate on cowpea yield and yield parameters 62
3.4.7. Land equivalent ratio 64
3.4.8. Relationships between sorghum grain yield and yield components 66
3.5. Discussion 67
3.5.1. Effect of intercropping and nitrogen rate on sorghum phenology and growth 67
3.5.2. Effect of intercropping and nitrogen rate on sorghum yield and yield components72
3.5.3. Effect of intercropping and nitrogen rate on cowpea phenology and growth 79
3.5.4. Effect of intercropping and nitrogen rate on cowpea yield and yield components 82
3.5.5. Effects of cropping system × N rate interaction on growth and yield of the sorghum and cowpea 85
3.5.6. Land equivalent ratio 85
3.6. Conclusion 87
CHAPTER 4: INTERCROPPING AND NITROGEN FERTILIZATION ALTERED THE PATTERNS OF LEAF SENESCENCE IN THE CANOPY OF SORGHUM
4.1. Abstract 88
4.2. Introduction 89
4.3. Materials and methods 93
4.3.1. Site 93
4.3.2. Treatments and experiment design 93
4.3.3. Experiment management 93
4.3.4. Data collection 94
4.3.5. Data analysis 97
4.4. Results 97
4.4.1. Senescence at the whole plant level 97
4.4.2. Senescence of the flag leaf 101
4.4.3. Relationships between traits of leaf senescence and grain yield 104
4.5. Discussion 106
4.5.1. Leaf senescence at the whole-plant level 106
4.5.2. Senescence of the flag leaf 107
4.5.3. Association between traits of leaf senescence and grain yield 110
4.5.4. Senescence and the modulation of grain yield 110
4.6. Conclusions 112
CHAPTER 5: GENERAL DISCUSSION, CONCLUSION AND RECOMMENDATION
5.1. General discussion 114
5.2. Conclusion 117
5.3. Recommendations 118
REFERENCES 120
LIST OF TABLES
Table 2.1. Area under sorghum cultivation, altitude, temperatures and annual rainfall in selected areas in Kenya where sorghum is mainly produced 10
Table 2.2. Plant height, time to flowering, time to maturity, prospective yield, resistance to biotic and abiotic stress and suitable ecological conditions for Serena, Seredo, Gadam, mtama1 sorghum varieties in Kenya 11
Table 2. 3. Sorghum trade and production in Kenya, Uganda, Tanzania and Rwanda 2016-2019 (tonnes). 14
Table 3.1. Soil chemical properties at depth of 0 – 30 cm before planting at Igoji and Katumani KALRO research stations during 2018/2019 short rain season 31
Table 3.2. Leaf area index, dry biomass (DM) at flowering and harvest and crop growth rate (CGR) of two sorghum varieties (Gadam and Serena) grown in sole and intercrop system with two varieties of cowpea (K80 and M66) and at three N rates (0, 40 and 80 kg N ha-1) at Igoji and Katumani KALRO research stations during 2018/2019 short rain season 43
Table 3.3. Sorghum grain yield, harvest index (HI), panicle width and panicle length, and N uptake of two sorghum varieties (Gadam and Serena) grown under sole and intercrop system with two varieties of cowpea (K80 and M66) and at three N rates (0, 40 and 80 kg N ha-1) at Igoji and Katumani KALRO research stations during 2018/2019 short rain season 49
Table 3.4. Number of fertile tillers m-2, panicle weight, Number of spikelets panicle-1 and weight of 1000 seeds of two sorghum varieties (Gadam and Serena) grown in sole and intercrop system with two varieties of cowpea (K80 and M66) and at three N rates (0, 40 and 80 kg N ha-1) at Igoji and Katumani KALRO research stations during 2018/2019 short rain season 53
Table 3.5. Cowpea plant height at 4, 6 and 8 weeks after planting (WAP) of two cowpea varieties (K80 and M66) grown in sole and intercrop system with two varieties of Sorghum (Gadam and Serena) and at three rates of N (0, 40, 80 kg N ha-1) at Igoji and Katumani KALRO research stations during 2018/2019 short rain season 55
Table 3.6. Number of effective root nodules at 4, 6, 8 WAP of two cowpea varieties (K80 and M66) grown in sole and in an intercrop system with two varieties of sorghum (Gadam and Serena) and at three rates of N (0, 40, 80 kg N ha-1) at Igoji and Katumani KALRO research stations during 2018/2019 short rain season 56
Table 3.7. Leaf area index, dry mass (DM) at flowering and harvest, crop growth rate (CGR) of two cowpea varieties (K80 and M66) grown in sole and intercrop system with two varieties of sorghum (Gadam and Serena) and at three N rates (0, 40, 80 kg N ha-1) at Igoji and Katumani KALRO research stations during 2018/2019 short rain season 60
Table 3.8. Cowpea grain yield, number of pods plant-1, number of seeds pod-1 and 100 seed weight of two cowpea varieties (K80 and M66) grown in sole and intercrop system with two varieties of sorghum (Gadam and Serena) and at three N rates (0, 40, 80 kg N ha-1) at Igoji and Katumani KALRO research stations during 2018/2019 short rain season 64
Table 3.9. Partial and total LER of two varieties of sorghum (Gadam and Serena) grown in an intercrop system with two cowpea varieties (K80 and M66) at Igoji and Katumani KALRO research stations during 2018/2019 short rain season 65
Table 3.10. Correlation coefficients between sorghum yield parameters and grain yield grown under sole and intercrop system at Igoji and Katumani KALRO research station during the 2018/2019 short rain season 66
Table 4.1. Number of green leaves at 10, 20, 30, 40 and 50 days after flowering (DAF) of two sorghum varieties (Gadam and Serena) grown under sole and intercrop system with two varieties of cowpea (K80 and M66) and three N rates at Igoji and Katumani KALRO research stations during 2018/2019 short rain season 99
Table 4.2. Maximum leaf greenness (SPADmax), minimum leaf greenness (SPADmin), time to loss of 50% SPADmax (EC50) and the rate of leaf senescence (RS) means of two sorghum varieties (Gadam and Serena) grown under sole and intercrop system with two varieties of cowpea (K80 and M66) and N rate at Igoji and Katumani KALRO research stations during 2018/2019 short rain season 103
Table 4. 3. Correlation coefficients between senescence traits and sorghum grain yield and yield parameters of two sorghum varieties (Gadam and Serena) grown in an intercrop system with two varieties of cowpea (K80 and M66) at 0, 40 and 80 kg N ha-1 at Igoji and Katumani KALRO research stations during the 2018/2019 short rain season 105
LIST OF FIGURES
Figure 2.1. Sorghum production, area harvested and grain yield in Kenya, 1990-2017. Source: MoA, 2017 12
Figure 3.1. Weather conditions from sowing to physiological maturity of sorghum at Igoji (a) and Katumani (b) Kenya Agricultural and Livestock Research Organization research stations during 2018/2019 short rain season 32
Figure 3.2. Phenology of two sorghum varieties (Gadam and Serena) grown in an intercrop system with two varieties of cowpea (K80 and M66) at Igoji and Katumani KALRO research stations during 2018/2019 short rain season 37
Figure 3.3. Plant height at 4, 6, 8 and 10 WAP of two sorghum varieties (Gadam and Serena) at three N rates (0,40,80 kg N ha-1) at Igoji (a, c) and Katumani (b, d) KALRO research stations during 2018/2019 short rain season 38
Figure 3. 4. Leaf area index, dry biomass at flowering and maturity and crop growth rate (CGR) of two sorghum varieties (Gadam and Serena) grown under sole and intercrop system with two varieties of cowpea (K80 and M66) and in interactions with 0, 40 and 80 kg N ha-1 at Igoji (a, c, e, g) and Katumani (b, d, f, h) KALRO research stations during 2018/2019 short rain season 44
Figure 3.5. Sorghum grain yield, panicle width and length, and harvest index of two sorghum varieties (Gadam and Serena) grown under sole and intercrop system with two varieties of cowpea (K80 and M66) and in interaction with three N rates (0, 40 and 80 kg N ha-1) at Igoji (a, c, e, g) and Katumani (b, d, f, h) KALRO research stations during 2018/2019 short rain season 50
Figure 3.6. N uptake of two sorghum varieties (Gadam and Serena) grown under sole and intercrop system with two varieties of cowpea (K80 and M66) and in interaction with three N rates (0, 40 and 80 kg N ha-1) at Igoji (a) and Katumani (b) KALRO research stations during 2018/2019 short rain season 51
Figure 3.7. Phenology of two cowpea varieties (M66 and K80) grown under sole and an intercrop system with two varieties of sorghum (Gadam and Serena) at Igoji and Katumani KALRO research stations during 2018/2019 short rain season 54
Figure 3.8. Dry biomass (DM) at branching and physiological maturity of two cowpea varieties (K80 and M66) grown under sole and intercrop system with two varieties of Sorghum (Gadam and Serena) and in interaction with three N rates (0, 40, 80 kg N ha-1) at the Igoji (a, c, e) and Katumani (b, d, f) KALRO research stations during 2018/2019 short rain season 61
Figure 4.1. Monthly rainfall and temperature from flowering to physiological maturity of sorghum at Igoji (a) and Katumani (b) KALRO research stations during 2018/2019 short rain season 95
Figure 4.2. An illustration of the fitted logistic curve in SigmaPlot and the estimated traits of leaf senescence of two sorghum varieties (Gadam and Serena) grown in sole and an intercrop system with two varieties of cowpea (K80 and M66) at Igoji and Katumani KALRO research stations during 2018/2019 short rain season 96
Figure 4.3. Number leaves that presented more than 50% green leaf area at 10, 20, 40 and 50 days after flowering (DAF) of two sorghum varieties (Gadam and Serena) grown in sole and intercrop system with two varieties of cowpea (K80 and M66) and interactions with 0, 40 and 80 kg N ha-1 at the KALRO research stations in Igoji (a, c, e, g) and Katumani (b, d, f, h) during 2018/2019 short rains season 100
Figure 4.4. Peak leaf greenness (SPADmax) of two sorghum varieties (Gadam and Serena) grown under sole and intercrop system with two varieties of cowpea (K80 and M66) and interactions with 0, 40 and 80 kg N ha-1 at Igoji (a) and Katumani (b) KALRO research stations during 2018/2019 short rain season 104
LIST OF ABBREVIATIONS AND ACRONYMS
ASALs - Arid and semi-arid lands
CGR - Crop growth rate
CS - Cropping system
DAF - Days after planting
EC50 - Time to loss of 50% maximum SPAD (peak leaf greenness)
FAO - Food and Agriculture Organization of the United Nations
GOK - Government of Kenya
HI - Harvest index
ICRISAT - International Crops Research Institute for the Semi-Arid Tropics
KALRO - Kenyan Agricultural and Livestock Research Organization
LAI - Leaf area index
LER - Land equivalent ratio
NUE - Nitrogen use efficiency
RUE - Radiation use efficiency
RS - Rate of senescence
SPADmax - Maximum SPAD (peak leaf greenness)
SPADmin - Minimum SPAD
TSP - Triple supper phosphate
UN - United Nations
WAP - Weeks after planting
WUE - Water use efficiency
CHAPTER ONE: INTRODUCTION
1.1. Background
The current population of Africa is 1.2 billion people however, by 2050 Africa’s population is estimated to rise to 2.4 billion (UN, 2017). Hence, food production should be improved further without adversely affecting the fertility of the soil and the environment (Layek et al., 2018). However, poor management practices among small holder farmers can not realize a good balance between nutrient supply and plant demand which often causes environmental pollution and low crop yields (Dobermann, 2007). Additionally, yield losses of cereals like sorghum (Sorghum bicolor (L.) Moench) are high in dry environments resulting from moisture stress especially at grain filling stage (Kassahun et al., 2010). Land fragmentation practices due to increased population growth have also limited the available land for crop production amidst increasing food demands (Karanja et al., 2014). Therefore, cereal-legume intercropping is considered an appropriate and sustainable practices to increase crop productivity per unit area with reduced external inorganic fertilizer N supply due to the legume ability in the intercrop system to replenish soil nutrients by fixing N in the soil (Ladha and Chakraborty, 2016).
Intercropping system involves growing multiple crops simultaneously in the same piece of land (Iqbal et al., 2019). This practice has been in use for a long time and has contributed to achieve sustainability of the agriculture systems (Layek et al., 2018). Integration of legume such as cowpea into cereal-based cropping systems provides sustainable enrichment of soil physio- chemical properties due to its nitrogen fixing capacity in the soil and helps to stabilize yields by increasing the productivity of land hence protect farmers from the risk of crop failure (Ndiso et al., 2016). The legume improves the nitrogen economy of the cereal by either contributing nitrogen to the soil or removing less amount of soil nitrogen (Layek et al., 2018). Additionally, intercropping contributes to subsequent prevention and reduction of soil erosion and land deterioration through the effective ground cover (Nawal, 1997). Further, intercropping helps to achieve crop diversity in an agricultural system (Baulcombe et al., 2009). The productivity and profitability of intercrop systems can be assessed using various indices including aggressivity ratio, competition ratio, monetary advantage index and using the land equivalent ratio (LER) where yield attained in an intercrop system is expressed relative to yield in a sole crop system (Sibhatu and Belete, 2015).
Sorghum is an essential crop grown globally for food and feed (Deb et al., 2004). The crop is majorly consumed as a grain, but can also be processed into porridges, breads and largely used as raw material for making alcohol (Mundia et al., 2019). Sorghum annual production in Africa is estimated at 20 million metric tonnes representing about 61% of the global total land cultivated and 41%of total global sorghum production (ICRISAT, 2013; Mundia et al., 2019). However, Kenya is among the least sorghum producing countries in Africa where its overall annual production is only 0.6% of Africa’s total annual production (Mitaru et al., 2012). Of the total sorghum annually produced in Kenya, 53% is utilised as food (either as grain or flour), 24% is processed to make malt, 11% is lost as waste, 10% for animal feed and 2% used as seed (Kilambya and Witwer, 2013).
Further, Kenya’s sorghum productivity remains at 0.8 t ha-1 despite development of high yield varieties with expected potential yield of between 2 and 5 t ha-1 making Kenya a net importer of sorghum in the region (Ochieng, 2011; Kilambya and Witwer, 2013). This is because of low soil fertility (N deficiency), poor management practices, continuous nutrient mining without replenishment, unpredictably low rainfalls, pests and diseases, birds infestation and weeds such as striga with capacity to cause 40% to 100% crop loss in the Sub-Saharan region (FAO and ICRISAT, 1996; Mitaru et al., 2012). Further, sorghum production is characterized by low use of inputs due to high and unaffordable costs by smallholder farmers (Muui et al., 2013).
Additionally, sorghum inability to meet its nitrogen requirement through own fixation has contributed to major yield constraints (Franzmann, 1993; Dorcas et al., 2019). However, the demand for sorghum has increased in the recent past due to its use as raw material for beer production; however, the current production cannot meet this demand (Kilambya and Witwer, 2013). Therefore, intercropping sorghum with legume like cowpea with effective biological nitrogen fixation (BNF), would increase nitrogen (N) availability through ‘N’ fixation to be utilized by sorghum hence increasing overall crop productivity (Egesa et al., 2016). Further, integration of fertilizer N at reduced rate to supplement N fixed by the legume symbiotically would help fully meet the N requirements of sorghum, reduce cost of fertilizers N and increase grain yield (Shamme and Raghavaiah, 2016).
Primarily, cereal-legume intercropping aims at increasing productivity of crops per unit land area by ensuring growth resources are efficiently utilized (Layek et al., 2018). The legumes in an intercrop improve soil fertility through BNF and decrease the competition for nitrogen in soil (Egesa et al., 2016). Additionally, soil conservation can be achieved through intercropping due to increased ground cover, thus, will reduce soil erosion and excessive rate of evaporation (Layek et al., 2018).
Despite the wide practice of intercropping, crop yields have remained low. Further, the success of an intercrop system largely depends on the compatibility of the companion crops, cropping density and intensity of competition for growth resources (Vasilakoglou et al., 2008). For instance, sorghum grain yield was significantly reduced in an intercrop system attributed to inter- species competition for growth resources and space (Karanja et al., 2014). Additionally, Sibhatu and Belete (2015) reported that sole sorghum exceeded 31% of the intercropped sorghum yield. Other limitations are attributed to nutrient-depleting nature of cereals like sorghum hence the N symbiotically fixed by the legume in an intercrop system alone may not fully meet its N requirement without external fertilizer N supply (Layek et al., 2018).
Nitrogen (N) is among the most deficient nutrients in many agricultural soils for cereal production on a global basis but is essential in crop growth (Yagoub and Abdelsalam, 2010). Higher crop yields have been attained by increasing N addition and improving fertilizer N efficacy (Dobermann, 2007). Further, increased growth and yield of sorghum with addition of N in the form of urea has been reported (Ahmed and Tanki, 1997). However, N losses remain a challenge in agricultural systems where, 30-50% of the applied nitrogen fertiliser continue being lost through leaching, denitrification and runoff (Shamme and Raghavaiah, 2016). Therefore, agricultural best management practices are required to reduce nutrient losses and prevent negative impact on the environment (Roberts, 2007).
Further, the use of N-fertilizer is expected to rise to match the increasing food demand of a rapidly growing world-wide population hence optimization strategies such as precision application of N-fertilizer practices are required (Sawargaonkar et al., 2013). Additionally, the costs of inorganic fertilizers are continuously rising and unaffordable to most small holder farmers hence integrating legumes like cowpea with effective biological nitrogen fixation (BNF) in sorghum cropping systems could reduce on the amounts of fertilizer N to be externally supplied and will cushion farmers from the high costs (Sibhatu and Belete, 2015). However, information on the appropriate N rates for sorghum production in an intercrop system to improve nitrogen use efficiency (NUE) remains limited (Kanampiu et al., 1997). Further, while previous studies have reported that prolonged leaf greenness has been correlated with higher grain yield in monocarpic crops like wheat (Kitonyo et al., 2017) and maize (Kitonyo et al., 2018), the current knowledge on the effect of sorghum-cowpea intercropping and varying levels of nitrogen application on leaf senescence in sorghum and its association with sorghum grain yield remains limited.
1.2. Problem statement and justification
Sorghum (Sorghum bicolor (L.) Moench) is an essential cereal as a food security crop and a raw material for making malt thus, increasing its productivity could end severe food insecurity and increase incomes of smallholder farmers in the dryland environments due to its unique traits of tolerating moisture stress and high yielding ability in a wide range of soils (Mwadalu and Mwangi, 2013). However, despite the development of improved varieties, the yield of sorghum has remained significantly low in the dryland environments (0.8 t ha-1) in comparison to expected grain yield of between 2 and 5 t ha-1 due to soil infertility and inappropriate cropping practices (Kilambya and Witwer, 2013). The former has been attributed to nitrogen deficiency resulting from constant loss of soil nutrients (N) without replenishment and high cost of inputs affecting farmers’ ability to apply sufficient N fertilisers to improve soil fertility while the latter results from limited information on appropriate cropping systems for sorghum production and limited skills among smallholder sorghum farmers (Kilambya and Witwer, 2013; Mwadalu and Mwangi, 2013). In Kenya, nitrogen deficiency and late water deficit account for yield losses of 37,000 and 11,000 tonnes per year (T yr-1) respectively (Wortmann et al., 2009; Kassahun et al., 2010). As a result of the low sorghum yields ha-1, most farmers engage in subsistence sorghum production making Kenya a net importer of sorghum to meet increased market demand (Ochieng, 2011). Therefore, in order to increase sorghum productivity to offset the current sorghum deficit, enhance food and nutrition security due to its nutritional importance as well as increase income of sorghum farmers through sale of sorghum as raw material for making malt, its critical to address the challenge of soil infertility in the dryland environments due to N deficiency and inappropriate cropping practices which are the main l root causes of low sorghum grain yield especially in the ASALs.
Intercropping sorghum with cowpea and nitrogen application may have the ability to enhance soil fertility. Additionally, cereal-legume intercropping could be a remedy to address moisture stress in the ASALs through improved land cover by the legumes which leads to retention of moisture and increased crop productivity per unit area of land available (Sibhatu and Belete, 2015). Also, a combination of intercropping and application of varying nitrogen rates would provide information on the appropriate nitrogen rates in an intercropping system that would optimize sorghum yields and ensure improved nitrogen use efficiency and profitability. Further, legume intercropping and fertilizer N application could prolong sorghum leaf senescence which has been reported to profoundly impact grain yield and quality by regulating source-sink relationships for nutrient demand (Feller et al., 2008; Gong et al., 2019). Further, prolonged leaf greenness has been correlated with higher grain yield in sorghum (Kassahun et al., 2010; Christopher et al., 2014), wheat (Triticum aestivum L.) (Kitonyo et al., 2017) and maize (Zea mays L.) (Kitonyo et al., 2018).
1.3. Objectives
The main objective was to improve the productivity of sorghum through intercropping and nitrogen fertilizer application. The study specific objectives were:
i. To determine the effect of intercropping and nitrogen on crop growth and yield of selected varieties of sorghum and cowpea
ii. To investigate the effect of intercropping sorghum with cowpea and fertilizer nitrogen on the time-course of sorghum leaf senescence
1.4. Hypotheses
i. Intercropping sorghum with cowpea and fertilizer N increases the yield of sorghum.
ii. Intercropping sorghum with cowpea and fertilizer N delays senescence of sorghum plants.
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