Effect of cropping system and age of plant at harvest on tuber rot and performance of elite cassava varieties in derived savannah

Devastated tuber rot disease among farmers prompted the evaluation of the elite improved varieties in the intercrop and the practice of delaying harvesting when there is glut in the market necessitated this study. Trial was carried out at the Federal University of Agriculture, Abeokuta between 2011 and 2014 to evaluate yield performance of 21 elite cassava varieties planted as sole crop verse intercropped and harvested at different age. The 2 x 21 x 3 factorial experiment was laid out in randomized complete block design and replicated three times. The tuber yield obtained from sole plot in 2011/2012 cropping season was significantly higher than intercrop whereas those of 2012/2014 cropping season were similar. Land Equivalent Ratio was above one in both cropping seasons indicating that the performance of the improved varieties in intercrop was efficient. The pooled mean tuber yield showed that TMS 30572, 92/0326, 95/0211, 01/1371, 00/0338, 01/0046, 00/0098, 01/1097, 01/0085, 98/0581 and 98/510 were among the top eight varieties. Harvesting could be delayed up to 15 months after planting to reduce tuber rot.


INTRODUCTION
Cassava (Manihot spp.) belongs to the family of Euphorbiaceae.Cassava is one of the most important food crops in Africa, South America and Asia.It derives its importance from the fact that its starchy, thickened, tuberous roots are a valuable source of cheap calories, especially in the developing countries where calorie deficiency and malnutrition are widely spread.Its usage as a source of ethanol for fuel, energy in animal feed, and starch for industries is increasing.Cassava contributes the largest share of daily per capita food consumption (1.6 kg) in Nigeria (FAOSTAT, 2003) and ranked number one among the top 20 commodities produced in Nigeria (www.fao.org/faostat/en/#rankings/commodities_by_countries) for more than estimated 800 million people around the world (Akparobi et al., 1998;Lebot, 2009).
Nigeria, Thailand, Indonesia and Brazil were ranked as first, second, third and fourth respectively, among the top 20 countries producing cassava in the world.The current estimated cassava production in 2013 for Nigeria, Thailand, Indonesia and Brazil were 47.4, 30.2, 23.0 and 21.5million tonnes, respectively (www.fao.org/faostat/en/#rankings/countries_by_commodities).Total area harvested in 2009 was 3.13 million ha, with an average yield of 11.7 t ha -1 (FAO, 2010).It is produced predominantly (99 %) by small farmers with 1-5 ha of land intercropped with yams, maize, or legumes in the rainforest and savannah agro-ecologies of Southern, Central, and lately Northern Nigeria.The world production of cassava root was estimated to be 184 million tonnes in 2002.
IFSERAR, ( 2009) conducted a diagnostic survey in South West Nigeria and reported that the local varieties grown among the farmers were not only low yielders but their ability to tolerate, or resist new strains of diseases and pests occasioned, perhaps, by climate change.Mwangi et al. (2004) similarly reported that the root rots are an important constraint to cassava production in humid forest and forest transition of Central and West Africa and can impact negatively on food security to several millions people inhabiting the regions.Rotting is known to increase significantly if mature plants are left in the soil for extended period of time (Oyeka, 2004).Yield loss was estimated at 20 to 100 % in Democratic Republic of Congo (Mwangi et al., 2004).These challenges necessitated the evaluation of 21 promising varieties collected from International Institute of Tropical Agriculture (IITA) in maize/cassava intercrop.
IITA have released several high yielding varieties but there performances in the intercrop as well as their tolerant or resistant level to cassava root rot disease have not been documented.There is therefore the need to ascertain the performance of these elite cassava varieties under the predominant intercropping systems among the resource constraint farmers in the region.Besides, the highly perishable nature of cassava tubers has compelled the farmers to harvest only when there is availability of market or family need.This delay harvesting enables farmers to leave the mature plant in the soil as a form of storage.This storage period enable the farmers to keep the fresh tuber in good quality for an extended period.Growth and dry matter accumulated will continue since cassava is believed to mature 7-24 months.Most cassava varieties attain optimum weight at 18 months after planting when starch accumulation is highest (Ekanayake et al., 1997).Hammer et al. (1987), who evaluated sequential harvests to age 24 months, reported that root rot occurred in the second year.Sagrilo et al. (2006) quoted Sagrilo et al. (2002) that cassava harvested at 21 months could improve storage root yield compared to 12 months.Ebah-Djedji et al.(2012) who harvested cassava sequentially at 11, 13, 15 and 17 months after planting in Cote d' Ivoire recommended that tuberous root of improved cassava varieties should be harvested at 13 MAP to obtain optimum dry matter content.These inconsistencies in the appropriate time of harvesting to obtain optimum dry matter content and quality is further aggravated by the prevailing tuber rot.Consequently, there is the need to ascertain the appropriate time to harvest these elite cassava varieties.This will ensure maximum dry matter accumulation without losing much of the tuber to root rot, particularly when harvesting is delayed because of poor market arrangement.The objectives of this study therefore were to: evaluate the performance of the improved varieties, 2) investigate the effect of intercropping on the elite cassava varieties and 3) determine the effect of delay harvest on the cassava tuber rot.

MATERIALS AND METHODS
The study was carried out at the Institute of Food Security, Environmental Resource and Agricultural Research (IFSERAR) farm, Federal University of Agriculture, Abeokuta in 2011/2012 and repeated in 2012-2014 cropping seasons.The experiment was laid out in a Randomized Complete Block Design in split plot and replicated three times in the 2011/2012.Cropping systems and variety factors were assigned to the main plot and sub plot, respectively.However, in 2012-2014 cropping season, harvesting date (12, 15 and 18 months) was varied as the third factor i.e. sub subplot (split split-plot) to gain additional information.The intercrop proportion mixture and population adopted was additive series.Table 1 shows the twenty one elite cassava varieties collected from International Institute of Tropical Agriculture (IITA), Ibadan.Benlate treated stem cuttings of 25 cm were planted into plot size of 9 m x 7 m (1.2 ha experimental field) at spacing of 1 m x 1 m in July 2011 and harvested in July, 2012.The cuttings obtained from the harvest were replanted in July 2012 and harvested sequentially in July (12 months after planting MAP), October 2013 (15 MAP) and January 2014 (18 MAP).Three seeds per hole of treated maize 'SUWAN 1' variety was alternately planted inbetween cassava stands (in the intercrop plots only) to evaluate the performance of cassava under intercropping (i.e.additive series).Maize was harvested at green stage.Weeding was carried out at 3, 9 12 WAP.Other weedings were done once in a month.Fertilizer 400 kg/ha N: P: K: Mg (12:12:17:2) was applied in the 2011/2012 cropping season whereas 2012/2014 trial did not receive fertilizer because of circumstance beyond our control.

2.1.1
Plant height (cm): 5 randomly selected cassava plants within the plot were measured with aid of graduated meter rule from the ground level to the highest leaf.

Stem girth (mm):
Vernier caliper was used to determine the stem girth (at 10 cm above the ground) of 5 randomly selected cassava stems within the plot.
2.1.3Tuber girth (mm): Vernier caliper was used to determine the tuber girth of 5 randomly selected freshly harvested tubers from ten up rooted cassava stands samples

Number of tubers per plant:
Determined by average number of freshly harvested tuber from the ten samples uprooted.

Rot incidence (%):
This was done by dividing the rotted tubers by total tuber multiplied by 100.

2.1.6
Tuber yield (t/ha): The mass of uprooted tuber from the ten sampled cassava stand was converted to t/ha.(i.e.mass of sampled/sampled area*10000/1000}

Data analysis:
Data collected were subjected to analysis of variance using GenStat Edition 12. Significant means were separated by using DMRT at 5 % probability.3).TME 1, TMS 01/1097, TMS 30572 and TMS 00/0338 were constantly ranked amongst the top varieties with high number of tubers in 12 MAP of the two seasons.The variance in tuber number could be genetically inherent and was considered as vital yield attribute that contributes immensely to the increase in tuber yield.It has been documented that the increase in yield were attributed to increase in number of tuber/stand and single root mass (Kogram et al., 2002).

Influence of intercropping on the tuber girth of elites cassava varieties at 12 MAP
The cropping system and variety did not influence tuber girth in 2011/2012 (Table 2), however, in 2012/2014 the varieties varied significantly among each other in 2012/2014 at 12 MAP (Table 3).'TMS 98/0510' had the highest tuber girth while 'TMS 97/JW2' had the least.The stem girths of the varieties were influenced by cropping systems in both seasons of the trial at 12 MAP (Tables 2 and 4).However, the varieties TMS 00/0338 and TMS 98/0505 consistently recorded the highest and the lowest, respectively in 12 MAP of both cropping season.

Influence of intercropping on the root rot of elites cassava varieties at 12 MAP
The cropping systems did not influence tuber rot infection in both seasons but there were significant differences among the varieties in 12 MAP of 2011/2012 (Table 2), they were however similar in 2012/2014 (Table 4).The rot incidence observed in 2011/2012 was high and ranges between 9.8 and 22.5 % while that of 2012/2014 was low and range between 0.00 and 0.94 % at 12 MAP.The lost incurred during 2011/2012 cropping season is in consonant with the finding of Mwangi et al. (2004) who documented 20 to 100 % tuber lost.

Influence of intercropping on the LER and tuber fresh mass of elites cassava varieties at 12 MAP
The Land Equivalent Ratio (LER) was similar in both cropping season but above one suggesting that intercrop plots was more productive.The fresh tuber mass of the varieties varied in the two cropping seasons at 12 MAP (Tables 2 and 4).Sole cassava plots had significantly higher tuber mass than intercrop in 2011/2012 (  2 and 4) while the harvest made at 15 and 18 MAP had similar tuber yield.This is, perhaps, an indication that maturity had not been attained and thus dry matter accumulation were at variance at 12 MAP.Although, number of tuber, stem girth and tuber girth varies among the varieties at 15 and 18 MAP (Table 3) but all the varieties had similar tuber yield (Table 4).This is an indicative of the fact that all the varieties tested in this trial attained maturity period after 12 MAP.Based on the definition of maturity period of cassava by Benesi et al. (2008) is the point where maximum or near maximum yield is obtained.
The topmost 3 consistent varieties in the two cropping seasons were TMS 30572, TMS 92/0326 and TMS 98/0581.Although 'TMS 01/1371' and 'TMS 01/1386' were not listed among top yielder, but had beta carotene as an advantage and statistically comparative yield with the top varieties in the two cropping seasons.6).The wide gap in tuber yield between the two cropping seasons could be attributed to the fertilizer application.Although, farmers hardly use fertilizer for cassava production because of the notion that cassava can thrive on marginal soils that cannot sustain other crops.This trial connotes that the addition of fertilizer can substantially enhance tuber yield.Odedina et al. (2012) who worked on integrated nutrient management reported similar gap between control and other sources of nutrient.Ironically, appreciable quantity of root rot was observed in the first cropping season compared to the second, it was not quite clear if the addition of fertilizer was responsible for the tuber rot.Consequently, there is the need to validate whether or not fertilizer application to cassava influences root rot.

CONCLUSIONS
The study has shown that the cassava varieties were not affected by intercropping but Land Equivalent Ratio was above one in both cropping seasons indicating that the performance of the improved varieties in intercrop was efficient.Plant height and tuber girth were higher in 15 and 18 MAP than 12.

Table 1 :
Selected cassava varieties used for the experiment

Table 2 :
Influence of intercropping on the tuber rot, agronomic parameters and tuber yield performance of elites cassava varieties in 2011/2012 cropping season

Table 3 :
Influence of intercropping on agronomic performance of elite cassava varieties at different age of plant in 2012/2014 cropping season

Table 4 :
Influence of intercropping on root rot and tuber yield of elite cassava varieties at different age of plant in 2012/2014 cropping season

Influence of age at harvest and intercropping on the tuber yield of elites cassava varieties at 12, 15 and 18 MAP
Generally, the consistence in plant heights values at 12 MAP of varieties TMS 97/JW2, TME B 419 and TMS 01/1371 (which ranked among the top) and those of TMS 01/0131, TMS 01/0046 and TMS 98/0505 (at the bottom of the list) in both cropping seasons are indication of their stability despite differences in crop management.Besides, the ability of TMS 30572, TMS 92/0326 and TMS 98/0581 to constantly rank among the first six varieties at 12 MAP in the two seasons makes them candidates to be recommended to farmers (Table On the bases of their consistent performance at 12 MAP, in the two cropping seasons, TMS 30572, TMS 92/0326 and TMS 98/0581 are candidate varieties to be recommended to farmer with or without resource constraints.The pooled mean tuber yield showed that TMS 30572, TMS 92/0326, TMS 95/0211, TMS 01/1371, TMS 00/0338, TMS 01/0046, TMS 00/0098, TMS 01/1097, TMS 01/0085, TMS 98/0581 and TMS 98/510 are top eight varieties.The incidence of tuber rot was highest at 18 MAP hence; harvesting could be delayed up to 15 MAP to reduce