Phosphorus Uptake and Balance in a Soyabean-Maize Rotation in the Moist Savanna of West Africa

This study investigated the effect of maturity class on phosphorus (P) balance in a soyabean-maize rotation in the West African moist savanna.F our soyabean varieties of different maturity classes were grown with or without P fertilizer followed by a maize crop. Soyabean stover P content averaged 1.77 kg ha−1 compared with 5.13 kg ha−1 in the grain. The late soyabean variety TGx1670-1F accumulated a significantly higher P in the grain (6.56 kg ha−1), and stover (2.57 kg ha−1) than the others. While P harvest index averaged 79%, P application increased grain P by 63-81% and stover P by 100% or more. When either soyabean grain or grain+stover was exported, P balance was negative and was not statistically different for varieties when no P was applied. At 30 or 60 kg P ha−1, P balance was negative but significantly lower in TGx1670-1F compared with other varieties. In creasing P rate applied to soyabean significantly (p 0.01) increased maize grain P by 35-66% in the second year. When P was exported only in soyabean grain, cumulative P balances after maize grain harvest (with no P or 30 kg ha−1 applied previous year) were not significantly different for previous soyabean crops. At 60 kg ha−1, however, P balance in previous TGx1670-1F plot was significantly lower than for other varieties. A further export of soyabean stover reduced P balance. Significant residual P effect was observed emphasizing the need to focus P fertilizer application in the cropping system rather than on the single crop. Also with more P in soyabean grain, a reduction in the extent of P depletion will be achieved by returning soyabean stover to the field after threshing

Cowpea yield gain from resistance to Striga gesnerioides parasitism in southern Benin

Striga gesnerioides is a serious parasite of cowpea (Vigna unguiculata) on the Abomey plateau of southern Bénin. S. gesnerioides in southern Bénin has been characterized as a race that is different from those found in the dry savanna of West Africa. IITA breeding line IT93KZ-4-5-6-1-5 was used in a trial to study fertilizer response of cowpea on two farmers' fields in 1998. No emerged S. gesnerioides plants were observed on this breeding line while heavy parasite loads were observed on local cultivars growing in adjacent fields. In 1999, IT93KZ-4-5-6-1-5 was grown again on seven fields and no emerged S. gesnerioides plants were observed. In 2000 the same line was grown on 18 farmers' fields with local cowpea cultivars in adjacent control plots. The mean number of S. gesnerioides plants on the local cultivars was 0.92 per cowpea plant (s.d. 0.72) while the improved line carried none. The mean yield gain from the improved cowpea line was 156% and varied from 20 to 680 kg pods ha−1. Thus it appears that progress has been made in developing cowpea lines that are resistant to S. gesnerioides and the potential impact of adoption of resistant cowpea is high

ESTABLISHMENT OF OPTIMUM PLANT DENSITIES FOR DRY SEASON SORGHUM GROWN ON VERTISOLS IN THE SEMI-ARID ZONE OF CAMEROON

Dry season transplanted sorghum is grown on Vertisols in the Lake Chad Basin at approximately 10,000 plants ha-1. Increasing plant density was hypothesised to be one way of increasing yields in this cropping system. To test this hypothesis, a trial was conducted in four environments near Maroua in northern Cameroon (one year at Yoldeo and three years at Salak) examining densities ranging from 10,000 to 50,000 plants ha-1. Grain yields were not significantly increased by increasing planting density in any of the environments because of reduced panicle size. For example, as planting density was doubled from 10,000 to 20,000 ha-1, the mean density of panicles harvested was increased by 85% but mean grain weight per panicle was decreased by 45%. Thus, in all environments, mean grain yields increased by 100 kg ha-1 (9%) at the transplant density of 20,000 ha-1 and 150 kg ha-1 at 26,667 plants ha-1. A comparison of results from three years at Salak suggests that the fraction of plants bearing panicles is influenced by the annual rainfall and, especially, the amount of rain during August and September. However, even after a season of adequate rainfall, panicle grain weight decreased with increasing panicle density, suggesting that there is little scope for increasing dry season sorghum transplant density without supplemental irrigation. Considering increases in labour input for nursery establishment, transplanting and harvest, the increased revenue from increasing planting density does not compensate for increased costs. The economic optimum is around 10,000 ha-1, which is similar to the current farmers' practice

Effect of soybean on subsequent maize grain yield in the Guinea savanna zone of West Africa

(African Crop Science Journal, 1997 5(1): 31-38