Weed competition is the major biological stress affecting cassava production in smallholder farms in West and Central Africa, where yields are low compared with those in Asia and Latin America. Options for improved weed management are crucial in increasing productivity. Selected pre- and post-emergence herbicides, integrated with appropriate tillage and plant spacing, were tested in 96 sites in four locations in Nigeria, 24 in 2016 and 72 in 2017. Trials were split plots with six pre-emergence herbicides and no post-emergence treatment as main plots. Subplot treatments were four post-emergence herbicides, weeding with a motorized rotary weeder, short- and long-handled hoes, and no post-emergence weed control, i.e., regardless of pre-emergence treatments. Indaziflam-based treatments, irrespective of post-emergence treatment, and flumioxazin + pyroxasulfone applied pre-emergence followed by one weeding with a long-handled hoe provided >80% control of major broadleaf and grass weeds. Compared with herbicide use, farmer control practices (53%) were not efficient in controlling weeds. The highest root yield was produced where (1) s-metolachlor was combined with atrazine, and one weeding with a long-handled hoe or clethodim with lactofen, and (2) indaziflam + isoxaflutole was combined with glyphosate. An increase in root yield from 3.41 to 14.2 t ha-¹ and from 3.0 to 11.99 t ha-¹ was obtained where herbicides were used compared with farmers’ practice and manual hoe weeding. Our results showed that integrating good agronomic practices with safe and effective use of appropriate herbicides can result in root yield >20 t ha^−1. i.e., twice the national average root yield of 8–12 t ha⁻¹, with >50% net profit. The use of appropriate herbicides can reduce the amount of manual labor required and improve livelihoods, specifically for women and children. Smallholder cassava farmers would require continuous training on the safe use and handling of herbicides to improve efficiency and prevent adverse effects on humans and the environment.

Weed competition severely constrains cassava root yield in sub-Saharan Africa; thus, good weed control measures, including the use of herbicides, are increasingly important. Herbicide trials were conducted at five locations across eastern, western, and north-central Nigeria over two cropping seasons (2014 and 2015). Nineteen premixed PRE herbicides applied at different rates were evaluated for efficacy on weeds and selectivity on cassava. Manual hoe-weeding at 4, 8, and 12 wk after planting (WAP) and two S-metolachlor + atrazine treatments commonly used by cassava growers were included for comparison. Six of the 19 PRE herbicide treatments (indaziflam + isoxaflutole, indaziflam + metribuzin, flumioxazin + pyroxasulfone, isoxaflutole, acetochlor + atrazine + terbuthylazine, and terbuthylazine + S-metolachlor) consistently provided 80% to 98% broadleaf and grass weed control up to 8 wk after treatment. Overall, PRE herbicide treatments and cassava yield were significantly positively correlated. Herbicide treatments terbuthylazine + S-metolachlor, flumioxazin + pyroxasulfone, diflufenican + flufenacet + flurtamone (respectively, 60 + 60 + 60, 120 + 120 + 120, 90 + 360 + 120, and 135 + 360 + 180 g ha⁻¹), acetochlor + atrazine + terbuthylazine (875 + 875 + 875 g ha⁻¹), S-metolachlor + atrazine (870 + 1,110 g ha⁻¹), oxyfluorfen (240 g ha⁻¹), indaziflam + isoxaflutole (75 + 225 g ha⁻¹), indaziflam + metribuzin (75 + 960 g ha⁻¹), and aclonifen + isoxaflutole (500 + 75 g ha⁻¹) contributed to yields exceeding twice the Nigerian national average of 8.76 tonnes ha⁻¹. These treatments had root yields of 1.4 to 2 times higher than plots that had been hoe-weeded three times. There were some adverse herbicide treatment effects such as delayed cassava sprouting and temporary leaf bleaching observed in indaziflam and diflufenican + flufenacet + flurtamone treatments, whereas sulfentrazone caused prolonged leaf crinkling. The PRE applications alone at rates safe for cassava did not provide adequate season-long weed control; supplemental POST weed control is needed about 10 WAP for satisfactory season-long control.

A number of improved varieties have been developed by the International Institute of Tropical Agriculture (IITA), combining diverse plant type with resistance or tolerance to several diseases, insect pests, and parasitic weeds and possessing other good agronomic traits. Trials were established with farmers to evaluate several new IITA-bred cowpea varieties on-farm in a process of participatory varietal selection. Central to this has been a ‘mother-daughter’ approach with researcher-managed ‘mother’ trials and farmer-managed ‘daughter’ trials in order to combine researchers' and farmers' criteria in evaluation of new varieties. In both set of trials, new varieties IT89KD-391, IT97K-499-35, and IT89KD-288 were favoured by farmers because of their high grain and fodder yields. Farmers have also shown interest in the continuous use of a local variety Kanannado Brown. It is suitable for relay intercropping; having a creeping habit, and the ability to smother weeds. The brown seeds fetch higher market prices. The implications are that improved cowpea varieties should be suitable for relay-intercropping and controlling weeds, be brown in colour, have large grain size, be pest resistant as well as give high yields of grain and fodder. At the same time, it is important that the farmer's criteria should be considered in breeding and varietal selection programmes.

Multilocational characterization of selected herbaceous and shrub legumes was carried out over a 3-yr period at lowland and midlatitude sites in the moist savanna agroecological zone in Nigeria where a south–north gradient exists for rainfall, length of growing season, and insolation. One study site each was located in coastal–derived savanna (DS), southern Guinea savanna (SGS), and northern Guinea savanna (NGS). A fourth site was located in the plateau of the NGS. Only one legume cover crop, lablab, produced adequate ground cover and good weed suppression in three locations irrespective of rainfall amount, duration, and distribution. Velvetbean was superior to other legume cover crops in the lowland savanna locations where rainfall exceeded 1,100 mm/yr but not in the plateau of the NGS where rainfall was less than 1,000 mm. Pigeonpea grew luxuriantly and produced canopy cover that effectively suppressed weeds in the DS where rainfall was high. Centurion grew well and suppressed weeds effectively only in the SGS site where rainfall of 1,120 mm was well distributed over a 5-mo period. Sunnhemp grew well and suppressed weeds only in the NGS study sites where rainfall duration was 4 mo and the length of the growing period was short. Weed density was negatively correlated (P < 0.05) with percentage ground cover of those legume cover crops that were effective in weed suppression in all locations. Redundancy analysis revealed significant associations between weed species and cover crops at each of the locations.Nomenclature: Centurion, Centrosema pascuorum Mart. Ex Benth. #3 CENPA; lablab, Lablab purpureus (L.) Sweet # LABPU; pigeonpea, Cajanus cajan (L.) Millsp. # CAJCA; sunnhemp, Crotalaria ochroleuca G. Don # CROOC; velvetbean, Mucuna pruriens (L.) DC. var. utilis (Wright) Burck # MUCPR.Additional index words: Legume ground cover, weed density, weed species composition.Abbreviation: WAP, weeks after planting; DS, coastal–derived savanna; SGS, southern Guinea savanna; NGS, northern Guinea savanna; RDA, redundancy analysis.

Field studies were conducted in northeast Nigeria to evaluate the response of early and late-maturing maize varieties infested with natural populations of Striga to different rates of nitrogen (N) fertilizer. There were significant reductions in the number of emerged Striga at 120 kg N ha⁻¹ for the early varieties and at 60 and 120 kg N ha⁻¹ for the late varieties. The number of emerged Striga only significantly differed among the late varieties. Averaged across varieties, grain yield of the early varieties was 144% higher at 60 kg N ha⁻¹ and 192% higher at 120 kg N ha⁻¹ than without added N. For the late varieties the increase in grain yield was 85% higher at 60 kg N ha⁻¹ and 144% higher at 120 kg N ha⁻¹ than without added N. Among the early varieties, TZE COMP4 C3 had significantly lower grain yield than the other varieties. Among the late varieties, grain yields of 8338-1-1 and TZB-SR were significantly lower than in the other varieties. Our results show that the application of 60–120 kg N/ha to Striga resistant or tolerant varieties may reduce damage and increase grain yield. Higher economic rates of return was obtained at N rates of 60–120 kg N/ha than the other rates.