Phosphorus management for grain cowpea (Vigna unguiculata (L.) Walp.) in summer rice fallows

Abstract

A study titled “Phosphorus management for grain cowpea (Vigna unguiculata (L.) Walp.) in summer rice fallows” was conducted at College of Agriculture, Vellayani, during 2019-2022 with the objectives of identifying grain cowpea varieties suited for summer rice fallows and to study the influence of beneficial phosphorus inoculants in enhancing phosphorus availability for the crop. The field experiment was carried out at the Integrated Farming System Research Station (IFSRS), Karamana from March to June, 2021. The design of experiment was Factorial Randomized Block Design (RBD) with three replications. The two factors studied were grain cowpea varieties (v) and phosphorus management (p). The grain cowpea varieties were v1- Kanakamony, v2- PGCP-6 and v3- DC-15. The five levels of P management were p1- Recommended dose of phosphorus (RDP) as per the KAU POP (30 kg P2O5 ha-1), p2- 75% RDP + AMF, p3- 75% RDP + PSB, p4- 75% RDP + AMF + PSB and p5- 50% RDP + AMF + PSB. All the treatments were applied with FYM @ 20 t ha-1, lime @ 250 kg ha-1, N @ 20 kg ha-1, K2O @ 10 kg ha-1 as per KAU POP. Rhizobium was applied @ 100 g per kg of seed as seed treatment. AMF at 5 g per pit and PSB mixed with FYM (20 g PSB per kg of FYM) @ 10 g per pit were applied to the treatment plots at sowing. Crop growth characters such as plant height and number of effective nodules were higher in PGCP-6 and was comparable with DC-15. PGCP-6 also recorded significantly the highest number of branches per plant, root mass and root volume per plant at 30 and 45 DAS. At 60 DAS, all the root parameters of PGCP-6 were comparable with DC-15. Higher number of leaves per plant was recorded in DC-15. Growth characters such as plant height, number of branches, leaves and effective nodules, root depth, mass and volume per plant were higher in p4 and were comparable with p5 and p2. AMF root colonization was higher in p4 and comparable to p5. The LAI of PGCP-6 was higher and comparable to DC-15 at 45 and 60 DAS. However, at 30 DAS, PGCP-6 recorded significantly the highest LAI (1.13). Among different levels of P management, higher LAI was observed in p4 at 45 and 60 DAS. 123 The variety PGCP-6 recorded significantly the highest number of pods per plant (16.29) and average pod weight (2.43 g). However, average pod length, number of seeds per pod and pod yield per plant were higher in DC-15 and were comparable with PGCP 6. Grain yield per plant, grain yield and harvest index were higher in PGCP-6 (13.65 g, 1111 kg ha-1 and 0.269 respectively) and was comparable to DC-15 (13.43 g, 1097 kg ha-1 and 0.264 respectively). The variety PGCP-6 exhibited an yield increase of 1.3 and 21.4 per cent over the varieties DC-15 and Kanakamony. The treatment p4 produced higher number of pods per plant (15.72) and it was on par with p5 (15.76) and p2 (14.32). The average pod weight was higher in treatment p4 (2.61 g) and was comparable with p2 (2.36 g). Other yield attributes including pod yield per plant, grain yield per plant and grain yield were found higher in treatment p4 (19.72 g, 13.71 g and 1174 kg ha-1 respectively) and was on par with p5 (19.49 g, 13.39 g and 1164 kg ha-1 respectively). Treatment p5 recorded higher harvest index (0.285) and was comparable to p4 (0.277). The treatment p4 ensued an yield increase of 8.8, 31.03 and 31.8 per cent over p2, p1 and p3 respectively. 1 Among different levels of P application, p1 recorded significantly higher soil available P (65.02 kg ha-1) at vegetative stage. However, at flowering stage higher soil available P was recorded in p4 (64.38 kg ha-1) and it was on par with p5 (62.20 kg ha-1) and p2 (62.59 kg ha-1). The highest uptake of N (99.17 kg ha-1) and P (13.69 kg ha-1) were noticed with PGCP-6 while the highest K uptake (61.46 kg ha-1) was recorded by DC-15. Among different P levels, the treatment p5 resulted in higher N (100.16 kg ha ) and K (63.67 kg ha-1) uptake and was comparable with p4 and p2. Higher and comparable P uptake was recorded with p4 (14.90 kg ha-1) and p5 (14.63 kg ha-1). Among interactions, the treatment combination v2p4 recorded higher number of branches per plant at 45 DAS (6.21) and was comparable with v2p5 (6.17). Higher grain yield per plant (14.50 g) and grain yield (1254 kg ha-1) were recorded in v2p4 and was comparable to v2p5, v3p4 and v3p5. The variety PGCP-6 recorded higher net returns (₹ 30402 ha-1) and B:C ratio (1.423). Among P management, p1 recorded higher net returns (₹ 31347 ha-1) and B:C 124 ratio (1.548). The highest net returns (₹ 40647 ha-1) and B:C ratio (1.72) were recorded by treatment combination v2p1. The study revealed that grain cowpea variety PGCP-6 performed better with respect to growth, yield and economics compared to varieties DC-15 and Kanakamony. Application of 75 per cent RDP + AMF + PSB recorded higher phosphorus uptake and availability and was comparable with the application of 50 per cent RDP + AMF + PSB. Better crop performance in terms of growth, nutrient uptake and yield and yield attributes along with a reduction in 50 per cent recommended dose of P has been achieved through the application of 50 per cent RDP + AMF @ 5 g per plant + PSB @ 20 g per kg of FYM @ 10 g per plant. Variety PGCP-6 managed with the application of 50 per cent RDP supplied with AMF @ 5 g per plant and PSB @ 20 g per kg of FYM @ 10 g per plant had the most favourable effects considering growth, nutrient uptake and yield. Considering the economics of cultivation, PGCP-6 cowpea managed with the RDP as per KAU POP was the best treatment compared to other treatments.