ABSTRACT- Field trials were conducted during the 2009-12 dry seasons at the Teaching and Research Farm of the Kebbi State University of Science and Technology located at Jega in the Sudan savannah zone of Nigeria. The aim was to evaluate the yield performance of three Irish potato (Solanum tuberosum L.) varieties and the economics of production under different irrigation intervals and NPK rates in the study area. Treatments consisted of three irrigation intervals (3, 6 and 9 days), four rates of NPK (20:10:10) fertilizer (0, 300, 600 and 900 kg NPK ha-1) and three varieties of potato (Bertita, Diamant and Nicola). Factorial combinations of irrigation intervals and fertilizer rates were allocated to the main-plots, while varieties were assigned to the sub-plots in a split plot design, replicated three times. The size of each sub-plot was 4.5 x 3m (13.5m). Result of the cost-benefit analysis revealed that the revenue per naira invested (RNI) was highest when 600 kg NPK ha^-1 was applied under 6 days irrigation interval (N 5.45), followed by 900 kg NPK ha^-1 under 6 days irrigation interval (N 5.24), while the least RNI (N 2.44) was by the untreated control under 3 days irrigation schedule. From the result of this study, the use of 600 kg NPK ha-1 under 6 days irrigation scheduling proved best for high tuber yield and economic returns.
Key words- Partial, Economic analysis, Irish potato, Production, Kebbi State

INTRODUCTION
Potato (Solanum tuberosum L.), popularly known as Irish potato originated in the high plains of the Andes Cordillera, Peru, where it is largely cultivated for food. The Spanish, who conquered Peru, discovered the crop and introduced it to Spain and the west of Europe in the mid 16^th century (Fergeria et al., 1991; Rolot, 2001). In Africa, it was not until the end of the 19^th century that potato was imported from Europe by the missionaries and the colonial administration (Rolot, 2001).
Potato was introduced to Nigeria in 1920 by Europeans involved in tin mining on the Jos Plateau (Rhodes et al., 2002). Production was limited to small garden plots until the Second World War, when the British colonial government encouraged potato cultivation to provide food for the servicemen in West Africa (Ifenkwe, 1989; Okwonko et al., 1995; Rhodes et al., 2002; Ugonna et al. 2013).
In tropical Africa, Malawi, Kenya, Ethiopia, Rwanda, Cameroon and Nigeria are the main potato producing countries and the crop is grown in high altitude which provides suitable temperature and photoperiod for growth (Harris, 1992; Okonkwo et al., 1995; Rolot, 2001). The high yield potential of potato, combined with reasonable yields even under stressful growing conditions, encourages its production, which is rapidly expanding in Africa (Rolot, 2001). Under normal growing conditions, potato yields are in the range of 40 – 60 t ha^-1 (Rolot, 2001). In Nigeria, potato is the most efficient tuber crop in terms of yield and days to maturity (Okonkwo et al., 1995). It matures in about 80 – 90 days as compared to 9 and 12 months for yam and cassava, respectively. Potato production in Nigeria in the year 2009, stood at 1.14 million tonnes cultivated on 145,680 hectares of land. Farmers yield was about 7.8 t ha^-1 (Ugonna et al. 2013; Anonymous, 2012).
The most important area of potato production in Nigeria is the Jos Plateau, which accounts for 85% of production in Nigeria. Biu and Mambila plateaus are two other areas where potato can be grown both in the dry and rainy seasons (Alhassan et al., 2004). In the low land areas of the northern states such as Kebbi, Kano, Kaduna, Borno, Sokoto and Adamawa, potato can be produced only during the harmattan period (November–February), when temperatures are sufficiently low (Okonkwo et al., 1995).
The most important factors that limit potato production in any region in Nigeria are insufficient water supply (rainfall or irrigation) and unfavorably high temperatures (Okonkwo et al., 2009). In Nigeria, while the high altitude regions of Jos, Mambila and Biu plateaus experience relatively low temperatures that are conducive for potato production in both rainy and dry seasons, the period of harmattan (dry and dusty wind blowing over West Africa between the end of November and middle of March) in the lowland northern states such as Kebbi provides low temperatures that support potato production, provided irrigation facilities are available (Okonkwo et al., 2009). It has been observed that almost all the potatoes consumed in the country are from the Jos plateau and Zaria areas. However, preliminary studies by the Kebbi State Agricultural Development Authority (KARDA) have revealed a huge potential for potato production in this axis of the Sudan Savannah. Incidentally, the cost of the commodity in the state is high, such that the crop is often considered as food for the rich, mainly because of the present restricted area of production. There is need to exploit other potential areas of production. The aim of this study was to evaluate the yield performance of some promising Irish potato varieties and the economics of production in the study area.

MATERIALS AND METHODS
The experiments were conducted during 2009/10, 2010/11 and 2011/12 dry seasons at the Teaching and Research Farm of the Kebbi State University of Science and Technology, Jega (lat. 120 11' N; long. 4⁰ 16' E) in the Sudan savanna ecological zone of Nigeria. The climate of the area is semi-arid with an average rainfall of about 550mm- 650mm per annum. The relative humidity ranges from 21- 47% and 51-79% during the dry and rainy seasons, respectively. The temperature ranges between 14-30^oC during the dry season and 27 – 41^oC during the rainy season (Anonymous, 2012).
The treatments consisted of three irrigation intervals (3, 6, and 9 days), four rates of NPK (20: 10: 10) fertilizer (0, 300, 600 and 900kg/ha) and three potato varieties (Nicola, Bertita, and Diamant). The treatments were laid out in a split-plot design with three replications. Irrigation intervals and fertilizer rates were combined and allocated to the main plots while variety was assigned to the subplots. The planting material (seed tubers) for the three varieties was sourced from the Potato Program Unit of the National Root Crop Research Institute (NRCRI) sub-station Vom, Jos, Plateau State. The seed tubers were pre-sprouted for six weeks before planting. The seed tubers were dressed with fungicide (Muncozeb powder) a day prior to planting. Planting was done manually with whole or cut tubers of approximately 30g weight per hill at intra-row spacing of 30cm and a depth of 8 – 10cm. Plots of 3.0 x 4.5m (13.5m2) were marked out, leaving a 1m space between main plots. Each subplot was made into six ridges, 75cm apart. Water channels were constructed for effective supply of water to each furrow during irrigation. The net plot area consisted of the two middle rows (3.0 x 1.5m) (4.5m²).
The source of water was a tube well. Water pump machine was used to draw water from the source to the field through the constructed water channels. Irrigation was scheduled according to the treatments, at 3, 6 and 9 days interval. The whole field, irrespective of the irrigation treatment, was watered 3 days before and after planting. The irrigation treatment was imposed after the crop has fully emerged [within 3 weeks after planting (WAP)]. Compound fertilizer (NPK 20: 10: 10) was used at the variable treatment rates of 0, 300, 600 and 900kg NPK/ha. These rates were applied according to the treatments in two split doses; the first and second doses were applied at planting and at 4WAP, respectively. The fertilizer was applied at about 10cm away from plant stand and 5cm deep and covered. Weeds were controlled manually using hand-hoe at 4 and 7 WAP. Karate (Lambda cyhalothrin) was sprayed at 4mlL-1 of water against insect pests. The crop was harvested on 16th February, 2010; 12th February, 2011; and 11th February, 2012; for the 2009/10, 2010/11 and 2011/12 trials, respectively. A light irrigation was given to all plots a day before harvesting irrespective of the irrigation treatment to facilitate easy lifting of tubers. Data generated were subjected to analysis of variance and means found to vary significantly were separated using Duncan’s Multiple Range Test (DMRT). The economics of potato production based on treatments imposed in the study area was estimated using Gross Margin (GM) analysis. The model is represented as:

Total Gross Margin (TGM) = Total Revenue (TR) - Total Variable Cost (TVC)

Total Variable Cost (TVC) per hectare was estimated at the rate of N1, 000.00 per man-day, N110.00 per kg of NPK (20:10:10) fertilizer, N150.00 per kg of seed potato, N100.00 per liter of petrol and N 550.00 per liter of engine oil.
The items of cost that made up the total cost of production include:

Seed tubers: Total of 1500kg of seed tuber was used per hectare, making N225, 000.00.

Fertilizer (NPK 20:10:10): Cost of fertilizer according to treatment were N0.00, N33,000.00, N66,000.00 and N99,000.00 for rates of 0, 300, 600 and 900kg NPK ha-1,      respectively.

Fuelling and servicing of water pump: Total of 30 litres of petrol was used per hectare at each irrigation day, making N3,000.00. 48 litres of engine oil was used for         servicing water pump throughout the season, which amounted to N8,800.00.

Chemicals: Fungicide (Muncozeb) and insecticide (lambdacyhalothrin) cost N3,000.00 and N4,200.00, respectively making N7,200.00. Labour:

• Land preparation including ploughing, harrowing, ridging and construction of water channels consumed 72 man-days, making N72, 000.00.
• Preparation of seed tubers for planting which in cluded cutting and dressing required 5 man-days, making N10,000.00.
• Irrigation used up to 4 man-days per irrigation, making N8,000.00.
• Fertilizer application (2 split application) used up to 4 man-days each, making 8 man-days, amounting to N8,000.00.
• Weeding (2 times) used up to 10 man days each, making 20 man-days, amounting to N20,000.00.
• Fungicide and insecticide spraying (2 times) used up to 2 man-days each, making N4,000.00.
• Harvesting used up to 20 man days, making N20,000.00.

The cost-benefit analysis was based on the interaction of irrigation and fertilizer of each trial and the combined data. Variety was not considered in the cost analysis because the cost of seed tubers of the three varieties was the same.

RESULTS AND DISCUSSION
The significant interaction between irrigation interval and fertilizer rates on fresh tuber yield for all seasons and the combined data is presented in Table 1. In 2009/10, the NPK untreated control gave similar fresh tuber yield with all the three irrigation intervals. With NPK rates of 300 kg ha^-1, 6 and 9 days intervals gave similar fresh tuber yield, with 9 days being lower than 3 days. With 600 and 900 kg NPK ha^-1, fresh tuber yield consistently decreased with widening irrigation interval from 3 - 9 days. On the other hand, at 3 days irrigation interval, fresh tuber yield was higher with 600 – 900 kg NPK ha^-1 than with 300 kg NPK ha^-1 and in turn lower with the untreated control. With 6 days interval, yield was higher with 600- 900 kg NPK ha^-1 than 300 kg NPK ha^-1 and the untreated control. With 9 days interval, NPK rates did not show significant effect on fresh tuber yield.
In 2010/11, the NPK untreated control gave higher yield with 3 and 6 days than with 9 days. With 300 kg NPK ha^-1, fresh tuber yield was similar with all the three irrigation intervals. With 600 kg ha^-1 rate, 3 and 6 days irrigation intervals gave higher fresh tuber yield than 9 days interval. With 900 kg NPK ha^-1, fresh tuber yield was higher by 3 and 6 days than by 9 days irrigation intervals. However, at irrigation interval of 3 days, NPK rate of 600 and 900 kg ha-1 gave higher fresh tuber yield than the untreated control. With 6 days interval, fresh tuber yield was higher with 600 – 900 kg NPK ha^-1 than 0 and 300 kg NPK ha^-1. With 9 days interval, NPK rates of 300 – 900 kg ha^-1 gave similar and higher fresh tuber yield than the untreated control.
In 2011/12, the NPK untreated control gave similar fresh tuber yield with all the three irrigation intervals. With NPK rates of 300 kg ha-1, 3 days intervals gave higher yield than 9 days. With 600 and 900 kg NPK ha^-1, irrigation at 6 days interval gave higher fresh tuber yield than 9 days. On the other hand, with 3 days interval fresh tuber yield was not affected by irrigation interval. With 6 days interval, tuber yield was higher with 600 -900 kg NPK ha^-1 than 0 and 300 kg NPK ha^-1. With 9 days interval, tuber yield was not affected by NPK rate.
In the combined data, the NPK untreated control and 300 kg NPK ha-1 each gave similar fresh tuber yield with all the three irrigation intervals. With 600 - 900 kg NPK ha^-1, 3 and 6 days irrigation interval gave higher fresh tuber yield than 9 days. On the other hand, with 3 and 6 days irrigation intervals, 600 – 900 kg NPK ha^-1 gave higher yield than the untreated control. With 9 days interval, NPK rate did not affect fresh tuber yield.
Table 2 and 3 present the costs of producing potato and revenue per naira invested for each treatment during the three trials and the combined data. Apart from the costs of fertilizer, irrigation and engine fueling, all other costs incurred in the course of production were the same for each treatment combination. Therefore the costs of fertilizer were N0/ha, N 33,000, N 66,000 and N 99,000 for 0, 300, 600 and 900kgNPK/ha, respectively; those for irrigation were N 240,000, N 156,000 and N 128,000 for 3, 6 and 9 days intervals, respectively and those for engine fuelling were N 90,000 N 58,500 and N 48,000 for 3, 6 and 9 days intervals, respectively. Another item of cost was the labour for fertilizer application which was also N0/ha for all treatments, with no fertilizer.
The total cost of production (TCP), the total revenue (TR), the total gross margin (TGM) and revenue per naira invested (RNI) are presented in Table 68. In 2009/10, the highest RNI was (N5.00) obtained when 900kgNPK/ha with 3 days irrigation schedule was employed, followed by 600kgNPK/ha with 3 days intervals (N 4.93); and the smallest was in the NPK untreated control with 3 days intervals (N 1.85) . In 2010/11, treatment with 600kgNPK/ha under 6 days irrigation intervals gave the highest RNI (N 5.26), followed by 900kgNPK/ha with 6 days intervals (N 5.02); ); and the smallest was in the NPK untreated control with 9 days intervals (N 2.19). In 2011/12, treatment with 900kgNPK/ha under 6 days irrigation intervals gave the highest RNI (N 6.29), followed by 600kgNPK/ha with 6 days intervals (N 6.28); and the smallest was in the NPK untreated control with 3 days intervals (N 3.28). For the mean of the three trials, RNI was highest when 600kgNPK/ha was applied under 6 days irrigation interval (N 5.45), followed by 900kg NPK/ha under 6 days irrigation interval (N 5.24), while the least RNI (N 2.44) was by the untreated control under 3 days irrigation scheduling.

Table 1: Irrigation x NPK interaction on fresh tuber yield (t ha^-1) in 2009-12 dry seasons and the combined data

NPK rates (kg ha-1)	Irrigation interval (days)
	3	6	9
2009/10
0	8.88d	8.84d	10.53d
300	20.51bc	14.51cd	13.48d
600	26.11a	20.88b	14.27d
900	27.57a	20.94b	14.00d
SE±	1.82
2010/11
0	10.53fg	11.44efg	9.12h
	300	17.77bcd	14.72def
600	23.09ab	23.80a	16.26cde
900	20.33ab	23.79a	18.43cd
SE±	1.63
2011/12
0	15.74c	15.34c	16.40c
300	21.89abc	21.22cd	16.40c
600	22.63abc	28.40ab	18.94bc
900	22.93abc	29.82a	17.17c
SE±	2.96
Combined years
0	11.72c	11.88c	12.02c
300	20.06ab	17.34bc	14.87bc
600	23.94a	24.65a	16.49bc
900	23.61a	24.85a	16.54bc
SE±	1.77

Within a year, means followed by the same letter(s) are not significantly different using DMRT at 5%

Table 2: Costs of potato production under different fertilizer and irrigation levels for the average of the three trials at Jega, Sudan savanna, Nigeria

Treatments	Seed potato (N)	Fertilizer (N)	Irrigation (N)	Eng. Fuel (N)	Eng. Mant. (N)	Seed prep. (N)	Land prep. (N)	Spray (N)	Planting (N)	Weeding (N)	Fert appl (N)	Harvesting (N)	Total cost (N)
No fert+3days	225,000	0	240,000	90,000	26,400	5000	72,000	11,200	10,000	20000	0	20,000	719600
No fert+6days	225,000	0	156,000	58,500	26,400	5000	72,000	11,200	10,000	20000	0	20,000	604100
No fert+9days	225,000	0	128,000	48,000	26,400	5000	72,000	11,200	10,000	20000	0	20,000	565600
300kg+3days	225,000	33000	240,000	90,000	26,400	5000	72,000	11,200	10,000	20000	8000	20,000	760600
300kg+6days	225,000	33000	156,000	58,500	26,400	5000	72,000	11,200	10,000	20000	8000	20,000	645100
300kg+9days	225,000	33000	128,000	48,000	26,400	5000	72,000	11,200	10,000	20000	8000	20,000	606600
600kg+3days	225,000	66000	240,000	90,000	26,400	5000	72,000	11,200	10,000	20000	8000	20,000	793600
600kg+6days	225,000	66000	156,000	58,500	26,400	5000	72,000	11,200	10,000	20000	8000	20,000	678100
600kg+9days	225,000	66000	128,000	48,000	26,400	5000	72,000	11,200	10,000	20000	8000	20,000	639600
900kg+3days	225,000	99000	240,000	90,000	26,400	5000	72,000	11,200	10,000	20000	8000	20,000	826600
900kg+6days	225,000	99000	156,000	58,000	26,400	5000	72,000	11,200	10,000	20000	8000	20,000	710600
900kg+9days	225,000	99000	128,000	48,000	26,400	5000	72,000	11,200	10,000	20000	8000	20,000	672600

Table 3: Revenue per naira invested on potato production under different fertilizer and irrigation levels for the 2009/10, 2010/11, 2011/12 and three-year mean data at Jega, Sudan savanna, Nigeria

Treatments	Total cost (N)	Total revenue (N)				Total gross margin (N)				Revenue per Naira Invested
		2009/10	2010/11	2011/12	Mean	2009/10	2010/11	2011/12	Mean	2009/10	2010/11	2011/12	Mean
No fert+3days	719,600	1,332,000	1,579,500	2,361,000	1,758,000	612,400	859,900	1,641,400	1,038,400	1.85	2.19	3.28	2.44
No fert+6days	604,100	1,326,000	1,716,000	2,301,000	1,782,000	721,900	1,111,900	1,696,900	1,177,900	2.19	2.84	3.8	2.94
No fert+9days	565,600	1,579,500	1,368,000	2,460,000	1,803,000	1,013,900	802,400	1,894,400	1,237,400	2.79	2.41	4.34	3.18
300kg+3days	760,600	3,076,500	2,665,500	3,283,500	3,009,000	2,315,900	1,904,900	2,522,900	2,248,400	4.04	3.5	4.31	3.95
300kg+6days	645,100	2,176,500	2,440,500	3,183,000	2,601,000	1,531,400	1,795,400	2,537,900	1,955,900	3.37	3.78	4.93	4.03
300kg+9days	606,600	2,022,000	2,208,000	2,460,000	2,230,500	1,415,400	1,601,400	1,853,400	1,623,900	3.33	3.63	4.05	3.67
600kg+3days	793,600	3,916,500	3,463,500	3,394,500	3,591,000	3,122,900	2,669,900	2,600,900	2,797,400	4.93	4.36	4.27	4.52
600kg+6days	678,100	3,132,000	3,570,000	4,260,000	3,697,500	2,453,900	2,891,900	3,581,900	3,019,400	4.62	5.26	6.28	5.45
600kg+9days	639,600	2,140,500	2,439,000	2,841,000	2,473,500	1,500,900	1,799,400	2,201,400	1,833,900	3.34	3.81	4.44	3.86
900kg+3days	826,600	4,135,500	3,049,500	3,439,500	3,541,500	3,308,900	2,222,900	2,612,900	2,714,900	5	3.68	4.16	4.28
900kg+6days	710,600	3,141,000	3,568,500	4,473,000	3,727,500	2,430,400	2,857,900	3,762,400	3,016,900	4.42	5.02	6.29	5.24
900kg+9days	672,600	2,100,000	2,764,500	2,575,500	2,481,000	1,427,400	2,091,900	1,902,900	.1,808,400	3.12	4.11	3.82	3.68

Based on the findings of this research, it could be concluded that production of Irish potato in the study area is profitable and the best results are obtained under irrigation interval of 6 days with 600kg NPK ha^-1.

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