Abstract:
Groundnut (Arachis hypogaea L.) is one of the most important cash oil crops for smallholder farmers in semi-arid parts of eastern Ethiopia. However, its productivity is low due to biotic and abiotic stresses as well as poor agronomic practices. Suboptimal plant population and soil fertility depletion are among the major constraints to sustainable groundnut production in the semi-arid ecology of Ethiopia. This problem is aggravated by lack of application of fertilizers at appropriate rate from required types and reliable sources to grow the crop productively. The general objective of the present study was, therefore, to investigate the influence of plant spacing and application of different types and rates of mineral NP fertilizers, vermicompost and Zn on growth, yield components and yields of groundnut varieties. Two field experiments were conducted during 2015 and 2016 cropping seasons in Babile district and one experiment was conducted during 2016 cropping seasons in Babile and Fedis districts, Eastern Ethiopia. The first experiment was undertaken to investigate the effects of plant spacing on the growth, yield attributes and yield of groundnut varieties. The experiment consisted of four groundnut varieties (BaHa-Jidu, BaHa-Gudo, Roba and Werer-962 and six plant spacing (30 cm x 15 cm, 40 cm x 10 cm, 50 cm x 10 cm, 50 cm x 15 cm, 60 cm x 10 and 60 cm x 15 cm), planted in Babile during the 2015 and 2016 main cropping seasons. In the second experiment, groundnut variety BaHa- Gudo was planted in Babile during 2015 and 2016 cropping seasons with the application of three rates of nitrogen (0, 23 and 46 kg ha-1), phosphorus (0, 46 and 92 kg P2O5 ha-1) and VC (0, 2.5, 5.0 t ha-1). For the third experiment, three groundnut varieties (BaHa-Jidu, BaHa-Gudo and Roba) found promising in the first season of the first experiment and three rates of fertilizers combination of nitrogen (kg N ha-1), phosphorus (kg P2O5 ha-1), VC (t ha-1) as (0: 0: 0, 46: 46: 0, 46: 46: 2.5) set based on the preliminary results from first season of the second experiment were used in combination with three rates of zinc (0, 5, 10 kg ha-1) to investigate the effect of combined application of mineral NP, vermicompost and Zn on growth, yield attributes, yield and oil content of groundnut. All experiments were laid out as a randomized complete block design in a factorial arrangement with three replications. The results of first experiment revealed that groundnut variety BaHa-Gudo planted at 60 cm x 10 cm had maximum hundred xx seed weight (79.8 g), pod yield (2.67 t ha-1) and seed yield (1.65 t ha-1) at par with the same variety planted at 40 cm x 10 cm and BaHa-Jidu planted at 50 cm x 15 cm spacing. Therefore, BaHa-Gudo grown at either 60 cm x 10 cm or 40 cm x 10 cm was found to be high yielder and BaHa-Jidu grown at 50 cm x 15 cm can also be taken as an alternative for yield among the varieties and plant spacing tested. In the second experiment, the result revealed that interaction of N, P and VC significantly (P < 0.001) affected number of branches per plant, plant height, nodulation, number of pods per plant, harvest index, pod and seed yield and seed oil content . The highest total (36.1) and matured (28.8) pods per plant, pod (51.9%) and seed (35.9%) harvest index, pod (3.05 t ha-1)and seed (2.11 t ha-1) yields, oil content (50.5%) were obtained at combined application of 46 kg N, 46 kg P2O5 and 2.5 t VC ha-1 which enhanced the pod yield, seed yield and oil content by about 100, 113, and 7%, respectively, over the control treatment in the test variety. The economic analysis also indicated that the highest net benefit of 50,018 Birr ha-1 with marginal rate of return of 671% was obtained from combined application of 46 kg N, 46 kg P2O5 with 2.5 t VC ha-1. It could, thus, be concluded that applying 46 kg N ha-1, 46 kg P2O5 ha-1, and 2.5 t VC ha-1 would enable farmers to increase yield and oil content and economic return from groundnut production. Similarly, results of the third experiment revealed that canopy spread, nodulation, pod per plant and hundred seed weight, dry biomass, pod and seed yields and seed oil content were significantly (P < 0.001 affected by the interaction of variety, fertilizers combination and Zn rate. The highest matured pod per plant (25.3), hundred seed weight (79.6 g), dry biomass (7.42 t ha-1), pod (3.19 t ha-1), seed (2.13 t ha-1) yields and seed oil content (51.3%), net benefit of 46521 Birr ha-1 with marginal rate of return of 242% were obtained from variety BaHa-Gudo that received combined application of 46 kg N ha-1, 46 kg P2O5 ha-1, and 2.5 t VC ha-1 with 5 kg Zn ha-1. Therefore, variety BaHa-Gudo could be preferably produced using combined application of 46 kg N ha-1, 46 kg P2O5 ha-1 and 2.5 t VC ha-1 with 5 kg Zn ha-1 for high groundnut productivity. In conclusion, cultivating variety BaHa- Gudo at 60 cm x 10 cm and/ or BaHa-Jidu at 50 cm x 15 cm plant spacing with combined application of 46 kg N: 46 kg P2O5: 2.5 t VC and 5 kg Zn ha-1 increased yields and seed content as well as economic returns compared to no fertilizer, or NP fertilizer alone or VC alone.