The effects of rhizobial inoculation, phosphorus application and cowpea-cowpea sequential cropping system on some varieties of cowpea on farmers’ fields in Minna, southern Guinea savanna of Nigeria

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The trials conducted in 2016 were repeated in 2017 to determine the effects of phosphorus fertilizer application and rhizobial inoculation on photosynthetic efficiency, nodulation, growth and productivity of three cowpea varieties and secondly to evaluate the performance of some  varieties of cowpea in cowpea-cowpea sequential cropping system on three farmers’ fields in Minna, southern Guinea savanna of Nigeria.  

The treatments of the first trial included three phosphorus rates (0, 20 and 40 kg P ha-1), five nitrogen sources (uninoculated, application of 90 kg N ha-1, inoculation with BR 3262, BR 3267 and USDA 3451 rhizobial strain) and three cowpea varieties (IT99K-573-1-1, IT93K-452-1 and TVX 3236). These were factorially combined and laid out in a randomized complete block design. The treatments of the second trial included six varieties of cowpea viz: IT93K-452-1, IT90K-76, Oloyin, IT99K-573-1-1, TVX-3236 and Kanannado planted in two sequence.

The results revealed that:

  • Phosphorus fertilizer application significantly increased the quantum yield of photosystem II (photosynthetic efficiency), growth, nodulation and yield of the three cowpea varieties used. Plants without P-fertilizer consistently gave the lowest values in all the parameters measured and the values obtained in plants that received 20 and 40 kg P ha-1 were at par except for grain yield (Figure 1)
Figure 1: Effect of phosphorus application on the performance of cowpea
  • Variety IT99K-573-1-1 and IT93K-452-1 plants had similar performance in biomass yield and nodule weight which was significantly higher than the values recorded in TVX 3236. However, IT99K-573-1-1 plants produced significantly higher grain yield than the remaining two varieties.
  • There was no significant difference among the inoculated, 90 kg N ha-1 fertilized and uninoculated plants with respect to photosynthetic efficiency and grain yield. However, plants fertilized with 90 kg N ha-1 had significantly higher shoot biomass yield than the inoculated plants (Figure 2).
Figure 2: Effect of rhizobial inoculation on the performance of cowpea
  • Interaction effect of rhizobial inoculation and phosphorus application on nodule number revealed that plants inoculated with BR 3262 required more phosphorus than others for maximum nodulation (Figure 3).
Figure 3: Interaction effect of rhizobial inoculation and phosphorus application on nodule number of cowpea

 The result of the second trial revealed that:

  • All the varieties were successfully planted twice in a season except Kanannado in which only one sequence was achieved (Figure 4)
Figure 4: Grain yield of some cowpea varieties planted in sequence
  • Of the five varieties that were planted twice IT90K-76 had the highest cumulative yield, closely followed by TVX 3236. The least was recorded in oloyin variety.


  1. P application is important for optimum performance of cowpea in the study area.
  2. The symbiotic effectiveness of the rhizobial inoculants used was not better than the indigenous strain present in the study area.
  3. IT99K-573-1-1 performed relatively better in grain yield than the other two varieties used.
  4. TVX 3236 and IT90K-76 are the best cowpea varieties among those tested for early planting in the study area.
  5. IT90K-76, TVX 3236, IT99K-573-1-1 and IT93K-452-1 can be recommended for cultivation in cowpea-cowpea sequential cropping system in the study area.

Adediran Olaotan Abimbola, Federal University of Technology, Minna, Nigeria (Click here for her 2017 update)