Introduction

Kenya’s economy is mainly agriculture-based. Consequently low food crop production leads to food insecurity. Over eighty percent of Kenya’s landmass is hot and dry and therefore unsuitable for arable farming. Crop failures due to low and erratic rainfall are rampant in most parts of the country leading to perennial food shortage. Luckily, the country is endowed with fresh-water rivers which have traditionally been used mainly for production of paddy rice under irrigation. The Kenyan government having realized the huge potential the country has in improving food sufficiency through prudent use of irrigation water is putting a lot of emphasis into expanding the irrigation schemes in the country. In addition, there is need to increase productivity of these irrigation schemes through crop diversification. In most of these irrigation schemes, paddy rice is the main food crop; it is rotated with maize. The irrigation board has been seeking alternative crop with which to rotate with rice because maize takes a long time to mature; a short-duration crop such as potatoes could fill in the gap. Introduction of potatoes as one of the rotational crops to replace maize may play an important role in increasing productivity of the irrigation schemes since potato matures early and has many uses. Work to evaluate performance of potatoes at Perkerra irrigation scheme was initiated in October 2013-February 2014 season. Later, it was expanded to Bura irrigation scheme and Mwea Irrigation Agricultural Development (MIAD) Centre.

Experimental materials and procedure

At Mwea, the experiment was planted out on 1^st October 2014 while at Bura, it was planted on 27^th August 2014. At Perkerra, it was carried out for two seasons; first season was planted on 6^th November 2013. While the second season was planted on 21^st August 2014. At all the three sites, the crop was harvested sequentially as the varieties matured at different times. The experimental materials planted at the three sites are given below (Table 1).

Table 1 Potato genotypes used in the study

In each site, the experiment was laid out as a randomized complete block design replicated three times. Furrows, 75 cm apart were dug and potatoes were planted 30 cm apart on the shoulder of the furrows. Each plot consisted of 5 rows each row having 12 plants totalling to 60 plants per plot. During planting, Diammonium phosphate (18% N: 46% P₂O₅) was applied at the recommended rates of 500 kg ha^-1 in furrows before planting.Watering was done through furrow irrigation by gravity when need arose.  Weeding, earthing-up and spraying against pests and late blight were carried out as per recommendations for potato production in Kenya (KARI, 2008).

Data collection

During plant growth, data collected included number of emerged plants per plot (taken 45 days after planting) and number of plants that survived until harvest. This data was used to calculate % stand establishment and % plant survival, respectively. When the plants were mature (varieties matured at different times) all the plants that survived in a plot were harvested. Data collected on plot basis included number and weights of tubers of different sizes i.e. <30, 30-60 and 60< mm in diameter. This data was used to calculate total yields (ton/ha) and % ware potato yields.

Data analysis

Data was analysed using Genstat statistical package, 14^th edition (Payne et al., 2011) and means separated using Fisher’s Protected LSD Test at 5% (Steel and Torrie, 1980).

Results and Discussion

Soils in the three sites were slightly basic (Table 2). The pH is a little bit on the higher side because potatoes do best at pH 5.5 to 6.5 (KARI, 2008).

Table 2 Soil characteristics at the study sites

(a)    Mwea

Shangi reigned high in terms of emergence; survival rate was also high at Mwea (Table 3). Generally, survival rate of all potato varieties was more than 60 % while emergence ranged from 20 to 91% (Table 3). All the varieties took less than 100 days to mature.

Table 3 Potato field establishment, plant survival and days to maturity at Mwea

Based on the number of plants that survived, Pimpernel gave the highest yield of 21.14 ton/ha fffollowed by Shangi (Table 4). The six highest yielding varieties were Pimpernel (21.14), Shangi (19.71), Romano (19.6), Desiree (18.74), Kenya Mavuno (17.54) and Ngure (17.4 ton/ha). Most varieties gave a high percentage (over 70 %) ware sized tubers (Table 4). These are the table tubers used for processing and domestic consumption.

Table 4 Total yields (ton/ha) and the % ware sized tubers harvested at Mwea

For Pimpernel, Shangi and Ngure, over 60% of potential yields were realised (Table 5). There was a significant (P≤0.05) and positive correlation between potential yields and realised yield (r=0.64). This implies that most varieties that are potentially high yielding had most of it realised.

Table 5 Potential and realised yields at Mwea Note: *= Assuming 100% plant emergence and 100% survival

(b)    Bura

Percent plant emergence was generally high at Bura; Ngure had the highest emergence (93.89 %) followed by Kenya Karibu (Table 6). Shangi fared badly with 47% emergence and 47% plant survival. Although the average percent plant emergence was higher in Bura than in Mwea, the opposite was true for plant survival and total yields (Tables 3, 4 and 6). In Bura there were no ware-sized tubers. The lower average tuber yields realised at Bura (8.68 ton/ha) than at Mwea (13.44 ton/ha) could be due to high temperatures in Bura than at Mwea. The high temperatures could also be responsible for lack of ware-sized tubers at Bura. However, in both sites, potential yields realised were less than 50% (Tables 5 and 7).

Table 6 Potato field establishment, plant survival, days to maturity and yields at Bura

Table 7 Potential and realised yields at Bura Note: *= Assuming 100% plant emergence and survival

On average, only 45 % of potential yields was realised at Bura (Table 7). There was a significant (P≤0.05) and positive correlation between potential yields and realised yield (r=0.75). This implies that most varieties that have potentially high yields had most of it realised.

(c)   Perkerra

During the first season at Perkerra, mean % plant emergence was 59.4 with 8 varieties showing over 70 % mean plant emergence (Table 8). Yields were much higher (15. 92) than at Mwea and Bura. The six most productive varieties were Asante (28.64), Dutch Robyjn (27.46), Roslin Eburu (23.21), Kenya Mavuno (22.42), Pimpernel (22.92) and Roslin Bvumbwe (22.47 ton/ha).

Table 8 Field establishment, plant survival, days to maturity and yields of potato varieties during the first season at Perkerra

Thirteen varieties had over 50% of their potential yields realised (Table 9).

During the second season at Perkerra, mean % plant emergence was 52.07 with Sterling and Ngure leading the pack (Table 10). The plants took the longest time to mature (100.24) compared to Mwea and Bura (Table 3, 6, 8, 10). Total yields realised at Perkerra during the second season ranged from 14.796 ton/ha (Dutch Robyjn) to 2.106 ton/ha for Meru Mugaruro (Table 11). The mean yields were almost half of the yields realised during the first season (Table 8).

About 40% of potential yields were realised at Perkerra during the second season (Table 12).

There was a highly significant (P≤0.01) and positive correlation between total yields and plant survival at Mwea (Table 13).

There was a highly significant (P≤0.01) and positive correlation between total yields and plant emergence at Perkerra during the first season (Table 14). All the other correlations were positive but non-significant.

At all the sites, less than 50% of potential potato yields were realised. However, Mwea and Perkerra appear to be better sites for potato production than Bura in terms of realised and potential yields. Bura is hotter than the other two sites. The high temperatures at Bura could also be responsible for lack of ware-sized tubers. Temperature is the single most important uncontrollable factor affecting growth and yield of potato (Smith, 1968); above ground biomass and plant height is increased and tuber yield is reduced (Wolf et al., 1990). The shorter growth duration at Bura (65 days to maturity), which could have also be occasioned by high temperatures, could have further contributed to low yields.

In Mwea, the most productive varieties were Pimpernel (21.14), Shangi (19.71), Romano (19.6), Desiree (18.74), Kenya Mavuno (17.54) and Ngure (17.4 ton/ha). These varieties could be recommended for production in Mwea. During the first season at Perkerra, the six most productive varieties were Asante (28.64), Dutch Robyjn (27.46), Roslin Eburu (23.21), Kenya Mavuno (22.42), Pimpernel (22.92) and Roslin Bvumbwe (22.47 ton/ha). These varieties could be recommended for production at Perkerra. Adoption of a particular variety could be high if it meets local tastes and preferences as well as market demand in addition to high yield. The old varieties that were released in Kenya a long time ago such as Desiree, Romano, Pimpernel, Roslin Eburu and Roslin Bvumbwe performed well in Mwea and Perkerra. These varieties have been in cultivation for a long time and may have probably become more adapted to local agroclimatic conditions hence high yields.

The positive correlations between total yields and % plant emergence and between total yields and % plant survival means that the two traits are important determinants of yields. Lack of consistency in significance of above correlations could probably be due to differences in the study sites (environment), differences in genotypes used (Table 1) or the genotype x environment interactions.

Research shows that efficient irrigation management can increase marketable yield (Bradley and Jeffrey, 1997); proper irrigation management helps optimize yields, size distribution and quality of both seed- and consumption-grade tubers. Moisture stress (depending on the crop growth stage) can reduce tuber yields, produce misshapen tubers, and negatively affect processing quality. Water stress from tuber initiation until the end of tuber bulking stage is the most detrimental to potato yields (Steyn et al., 2007).

References

Jaetzold R., Schmidt H., Hornetz B., and Shisanya C., 2006, Farm Management Handbook of Kenya. Natural conditions and farm management information. Part B. Central Kenya. Subpart B2. South Rift. Vol. II. 2nd ed., Ministry of Agriculture, Nairobi,

KenyaUNESCO, 1977, FAO-UNESCO Soil Map of the World. Vol. VI. Africa. UNESCO, Paris, France

KARI, 2008, Production of food (ware) potatoes. KARI information brochure, Kenya Agricultural Research Institute, Nairobi, Kenya

Payne, R.W., Murray D.A., Harding S.A., Baird D.B., and Soutar D.M., 2011, GenStat for Windows 14th ed. Introduction, VSN International, Hemel Hempstead, UK

Steel R.G.D. and Torrie J.H., 1980, Principles and procedures of statistics: A biometrical approach, 2nd ed. McGraw-Hill, New York, USA

Smith O., 1968, Potatoes: Production, storing, processing, The Avi Publishing Company, Inc., Westport, Connecticut, USA

Wolf S., Marani A., and Rudich A., 1990, Effects of temperature and photoperiod on assimilate partitioning in potato plants, Annals of Botany, 66: 513-520

Bradley A.K., and Jeffrey C.S., 1997, Potato Irrigation Management, bulletin 789, University of Idaho Cooperative Extension System, Moscow, Idaho, USA

Steyn J.M., Kagabo D.M., and Annandale J.G., 2007, Potato growth and yield responses to irrigation regimes in contrasting seasons of a subtropical region, African Crop Science Conference Proceedings, 8: 1647-1651