MAXIMIZING YIELD AND NET RETURNS TO NITROGEN, PHOSPHORUS AND POTASSIUM FERTILIZER APPLICATION IN RICE (Oryza sativa. L) PRODUCTION ON LOWLAND SOILS

ABSTRACT
 
Rice (Oryza sativa .L) is an important staple and source of income for farmers in Nigeria. Current fertilizer recommendations for rice do not commonly consider fertilizer cost relative to paddy price (CP). The rates needed for maximizing yield rather than profits have been rather estimated. As the cost of fertilizer increases relative to the price of paddy, the nutrient rate needed to maximize net returns, typically called the economically optimal rate (EOR), is expected to decrease. To this end, fertilizer trials were conducted at Wushishi in Niger state and Kadawa in Kano state in 2015 cropping season to: quantify the yield response of lowland rice to nitrogen, phosphorus and potassium applications; estimate the EOR at different CP; evaluate the components of Nitrogen Use Efficiency (NUE). The trials consisted of different rates of N (0, 40, 80, 120, 160 kg/ha), P (0, 7.5, 15, 22.5 kg/ha) and K (0, 10, 20, 30 kg/ha) arranged in an incomplete factorial combination and laid out in a randomized complete block design with three replicates. At plot level, paddy yield and components of yield were measured. Mean yield without nitrogen in Kadawa and Wushishi were 2.6 and 5.05 tons/ha respectively. There were significant increases in yield with N application in both locations. Paddy yield was increased by 3 tons/ha in Wushishi with the application of 80 kg/ha of N. Similarly, 40 kg/ha produced maximum paddy yield of 3.55 tons/ha in Kadawa. Estimated optimum nitrogen rates from asymptotic non-linear regression model were 114kg/ha and 106kg/ha for Wushishi and Kadawa respectively. As the CP increased, the range of profitability and EOR decreased. In Kadawa, EOR ranged from 61 to 32 kg ha^-1 of N at CP of 2 to 10. Values for EOR were 82 to 49 kg/ha for CP of 2 to 10 for Wushishi. Yield was not significantly increased with P and K applications in both locations. Agronomic Efficiency (AE) and Partial Factor Productivity decreased with increasing N rate in both locations. In Wushishi, AE and PFP at CP of 2 were 32.6 kg/kg and 95.6 kg/kg respectively. Similarly, AE and PFP at CP of 2 in Kadawa were 15.1 kg/kg and 59.6 kg/kg respectively. Nitrogen fertilization was observed to be more profitable in Wushishi due to greater crop response. Applying Nitrogen at economically optimal rate can help reduce expenditure on fertilizer while maximizing profits for rice growers. Environmental pollutions commonly associated with nitrogen fertilization can also be reduced.
 
CHAPTER ONE
 
1.0 INTRODUCTION
 
Rice (Oryza sativa L.) is an important staple for more than half of the world’s population. It accounts for 18.99% of calorie, 1.83% of fat and 12.73% of protein intake per day (FAOSTAT, 2011). The demand for rice may increase by 60% by 2025 (Fageria et al., 2003). The demand for rice is growing particularly in sub-Saharan Africa due to rapid population growth, consumer preference and rising income. Nigeria is the largest consumer of rice in sub-Saharan Africa with an estimated demand of 6.7 million metric tons of milled rice (USDA, 2017). Production is estimated at 3.8 million metric tons of milled rice and has failed to keep pace with the growing demand resulting to the importation of 2.6 million metric tons of milled rice to fill the gap. (USDA, 2017).
 
Low yield has been identified as major constraint to rice production in Nigeria where average yield is estimated at 1.88 tons per hectare (USDA, 2017). Soil nutrient depletion and low fertilizer use have been commonly cited as yield limiting factors in Nigeria (Ezui et al., 2010; Liverpool-Tasie et al., 2014). Farmers understand the need to apply nutrients to their land but little is used due to the economic and social factors surrounding access to fertilizer. High cost and delayed delivery of fertilizer are major limitations to its use in rice production. The expenses incurred in procuring fertilizer in Sub-Saharan Africa are two to six times the cost in Europe or the United States (Sanchez, 2002). Cost of importation, market inefficiencies and transportation cost are some of the factors responsible for high fertilizer prices. Excessively high fertilizer cost usually reduces the profitability of fertilizer use (Kaizzi et al., 2012a).
 
Several studies have reported significant increase in yield of rice with fertilizer application. Ishaya and Dauda (2010) reported optimum yield of 7 tons/ha with application of 130 kg/ha of
 
1
 
 
nitrogen in the sudan savanna. Kamara et al. (2011) observed yield increase of 3 tons/ha with the application of 100kg/ha of N to NERICA varieties. In a trial conducted by Jibrin et al. (2010) in the sudan savanna, 120kg/ha of N increased yield by 62.9%.
 
 
1.1 Statement of Problem
 
The Fertilizer recommendation for all irrigated lowland rice in Nigeria has been 100 kg/ha N, 60kg/ha P2O5 and 60kg/ha K2O (Chude et al., 2011) with the assumption that the need for applied nutrients is constant over varieties, seasons and diverse agro-ecological zones. Fertilizer recommendations are commonly developed without considering the cost of fertilizer relative to the price of paddy typically referred to as fertilizer cost to paddy price ratio (CP). Instead, the fertilizer rates needed to attain maximum yield have often been estimated. The CP is an indicator of how much of an output is required to purchase a kilogram of a nutrient (Liverpool-Tasie et al., 2014). As the fertilizer cost increases relative to price of paddy, the Economically Optimum Rate (EOR), described as the nutrient rate needed to maximize net return to fertilizer use, is expected to decrease (Wortmann et al., 2007). In Nigeria, Liverpool-Tasie et al. (2014) observed that the CP can be as low as 1.7 and high as 11.2 in some years and farming systems. Applying fertilizer at existing recommendations when the CP is high could lead to low net returns on fertilizer use. Kaizzi et al. (2012a) and Kaizzi et al. (2014) developed regression equations to relate CP and EOR for maize and rice respectively. This provided a means for estimating EOR at any CP value.
 
In addition, Nitrogen is the most limiting nutrient in rice production as it is prone to losses by volatilization, leaching and denitrification (Wortmann et al., 2010). The high cost of nitrogen in the developing world requires that higher yield are attained with minimum nitrogen fertilization
 
2
 
 
(Havlin et al., 2014). Adoption of improved agronomic practices to increase efficient use of fertilizer N is therefore critical to profitability and environmental sustainability. Agronomic efficiency and partial productivity are components of Nitrogen Use Efficiency that are closely related to profitability and are higher at EOR (Wortmann et al., 2007). In Uganda, application of N at EOR was observed to increase nitrogen use efficiency in upland rice compared to N rate that gave maximum yield (Kaizzi et al., 2014). Similarly, Kaizzi et al. (2012a) and also observed higher efficiency of N use by maize at EOR in Uganda.
 
 
1.2 Justification
 
Existing blanket fertilizer recommendations for lowland rice require periodic revision because the growth and need of crops for supplemental nutrients can vary greatly among fields, seasons and years as a result of differences in crop-growing conditions, crop and soil management, and climate. Furthermore, the variation across locations and seasons of CP (Liverpool-Tasie et al., 2014) requires that fertilizer recommendations be fine-tuned to ensure maximum profitability of fertilizer use. Additionally, the inability to accurately determine fertilizer rates leads to over fertilization in some years and locations and under fertilization in others with lower NUE. As a result, there is a clear need to improve fertilizer management strategies.
1.3 Aims and Objectives
The general objective of the study therefore was to provide recommendations for optimizing yield and profit from fertilizer use by rice farmers. The specific objectives were to;

1. Quantify the response of lowland rice to N, P and K
2. Determine the economically optimal nutrient rate for N, P, K at different fertilizer cost to paddy price ratios (CP).
3. Evaluate the Nitrogen Use Efficiency of lowland rice in the savanna.