Smallholder farmers in southern African countries rely primarily on cultural control and hoe weeding to combat weeds, but often times, they are unable to keep up with the weeding requirements of the crop because of its laboriousness, causing them to incur major yield losses. Optimisation of crop planting pattern could help to increase yield and suppress weeds and to reduce the critical period of weed control and the weeding requirements to attain maximum yield. Experiments were carried out in Zimbabwe during two growing seasons to assess the effect of maize density and spatial arrangement on crop yield, growth and seed production of weeds and to determine the critical period for weeding. Planting maize at 60 cm row distance achieved higher yields and better weed suppression than planting at 75 or 90 cm row distance. Increasing crop densities beyond the customary three to four plants m−2 gave modest reductions in weed biomass but also diminished crop yields, probably because of increased competition for water and nutrient resources. Maize planted in narrow rows (60 cm) intercepted more radiation and suffered less yield reduction from delaying hoe weeding than those planted in wider rows (75 or 90 cm), and the duration of the weed‐free period required to attain maximum grain yield was 3 weeks shorter in the narrow spacing than that in the 75‐ and 90‐cm row spacings. Weeding was more effective in curtailing weed seed production in the narrow row spatial arrangements than in the wide row planting. The results of these studies show that narrow row spacings may reduce weeding requirements and increase yields.

ABSTRACT

Spinacia oleracea L. (baby spinach) is a relatively new leaf vegetable crop in Zimbabwe, so the agronomic performance is unknown. A 3-year field experiment was done at the Seke Teachers College research farm, Zimbabwe. The research evaluated the response of baby spinach to different types of organic manure sources, days after fertilizer application (DAS), and growing season. A 3 × 2 factorial in a completely randomized block design (CRBD) with three replicates was used. Baby spinach cultivar, Dash, was grown on three organic manures (goat applied at 14.894 t ha−1, cattle at 17.789 t ha−1, and poultry at 13.807 t ha−1) in winter 2018, 2019, and 2020. Compound D (7% N, 14% P, and 7% K) at 300 kg ha−1 was included as a control. Crop growth rate, leaf area index, leaf area ratio, net assimilation rate, total dry matter production, and harvest index were measured on 7-day intervals from 14 to 35 days after transplanting. Analysis of variance (ANOVA) to compare the effects of the treatments on the baby spinach growth parameters was done. Significant p < 0. 05 interactions on the type of organic manure × DAS × growing season were observed on all the measured response variables. There were varied effects of organic manures on growth performance at 14 to 28 DAS where poultry manure had higher growth performance compared to cattle and goat manure. Generally, there was a significant p < 0. 05 gradual increase in growth from 14 to 35 DAS on the organic manures and a decline from day 28 to 35 DAS under compound D. Low quality (large C/N) cattle and goat manure had lower effects than high quality (small C/N) poultry manure at early growth stages of the baby spinach. Unlike compound D, organic manure continuously supplied adequate nutrients throughout the life cycle (35 DAS) of the baby spinach.

Farmland productivity is low in the semi-arid regions (NR IV and V) of Zimbabwe due to desertification and land degradation. Nevertheless, demand for food is increasing geometrically hence the need to increase output per unit area. Agroforestry (AF) which is an ecologically based and dynamic system that integrates multi-purpose trees on farms can increase productivity and offer resilience to climate change vagaries. However, the role of AF in Zimbabwean smallholder farming systems is still not well investigated. Therefore, this review explores the role of agroforestry on agricultural productivity in the semi-arid regions of Zimbabwe. The aim was to enhance sustainable food security among the rural poor through sustainable agriculture. Incorporating multi-purpose trees on agricultural lands can significantly restore soil productivity and offer soil resilience to erosion by water and wind. If well implemented, the AF can be a viable option in mitigating the impacts of drought on agriculture in these drier and marginalized areas.

Poor soil fertility and weed infestation are among major constraints to maize production in southern Africa. Nutrient and weed management strategies that are products of empirical research, are needed to improve efficiencies on farms. A field experiment was carried out in Eastern Zimbabwe on three smallholder farms positioned on upper, middle and lower catena. The farms differed in soil organic carbon (SOC) content, 3.9, 6.4 and 8.9 g kg À1 (hereafter referred to as low, medium and high), respectively, and are located within one km distance. The objective of the study was to investigate short-term (6 years) repeated application of soil nutrient amendments on maize productivity and weed dynamics across a soil fertility gradient. Treatments included strategic combinations of NPK fertiliser, cattle manure, and lime. On each farm, a randomised complete block design with three replicates was used. Multivariate, Principal Component Analysis, was used to establish the relationship between season, SOC content, nutrient management, and weed density. Maize yield was strongly linked to SOC content, with six-year mean maize grain yields of 1.31, 2.47, and 2.75 Mg ha À1 for low, medium, and high SOC content, respectively. Maize grain yields with cattle manure (CM) or NPK application were only 0.25 and 0.60 Mg ha À1 , respectively for the poorest SOC content field. However, when manure was combined with NPK fertiliser, yields at the site substantially increased to 1.5 Mg ha À1 while in medium and high SOC recorded 2.47 and 2.75 Mg ha À1 respectively. Weed density, and biomass were larger in the medium, and high SOC content. Richardia scabra, Melinis repens, and Cyperus sp. were associated with low SOC. Luecus martinicensis, Bidens pilosa, and Galinsoga parviflora were linked to medium and high SOC content. Results from this study suggest site-specific weed management approach. On soils critically poor in SOC content, maize yield gains are only achieved when organic and mineral fertilisers are combined. Our results also suggest that farmers must increase vigilance and intensity of weed management in soils with medium and high SOC content, particularly after application of CM.

Sorghum (Sorghum bicolor L. Moench) production in sub-Saharan Africa is seriously constrained by both biotic and abiotic stresses. Among the biotic stresses is witchweed (Striga spp.), a noxious parasitic weed causing major damage in cereal crops, such as sorghum. However, resistance through reduced germination stimulant production or altered germination stimulant composition provides a sustainable and most effective way for managing the parasitic weeds. Laboratory and glasshouse experiments were conducted using seven (7) sorghum genotypes to evaluate their resistance or tolerance the witch weed (Striga asiatica L. Kuntze). The first experiment was a laboratory agar gel assay arranged in a completely randomized design with six (6) replications to evaluate the effects of the seven (7) sorghum genotypes on the production of strigolactones by determining the percentage germination and the furthest germination distance of the Striga seeds. The second experiment was a seven (7) (sorghum genotypes)∗two (2) (Striga treatments) factorial glasshouse experiment conducted to evaluate the effects of Striga on sorghum growth, physiological and yield components of sorghum, Striga syndrome rating, and number of Striga per plant. The genotypes showed a significant (p<0.05) difference in germination percentage and furthest germination of Striga seeds in the agar gel assay. Genotypes SV4, Mahube, and ICSV 111 IN showed the least germination percentage and lowest germination distance, implying that these varieties either produced low strigolactones or altered their composition. In contrast, Kuyuma, Wahi, SV2, and Macia caused high Striga seed germinations and high furthest germination distances, suggesting that these sorghum genotypes were susceptible to Striga infection. The sorghum × Striga × time interactions were significant (p<0.05) on sorghum height. It was found that the heights of sorghum genotypes ICSV 111 IN and Mahube were not altered by Striga infection, but the heights of Kuyuma, Macia, SV2, SV4, and Wahi were reduced by Striga infection. The interaction of sorghum∗Striga effects was significant (p<0.05) on chlorophyll fluorescence. Striga infection did not alter the chlorophyll content of ICSV 111 IN and SV4. The sorghum∗Striga interaction effects were significant (p<0.05) on head index, leaf biomass, leaf index, root biomass, root index, plant biomass, and root : shoot ratio. Assessing Striga tolerance based on sorghum heights, chlorophyll content, and root : shoot ratio parameters, it could be concluded that the sorghum genotypes Mahube, ICSV 111 IN, and SV4 tolerated Striga infection, whereas Kuyuma and SV2 could be susceptible.

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