Imidazolinone-resistant Maize Research Articles

Effective Striga control and yield intensification on maize farms in western Kenya with N fertilizer and herbicide-resistant variety.

Maize production in western Kenya is limited by the spread of parasitic weed Striga hermonthica and depletion of soil nutrient stocks. Nitrogen (N) fertilizer and imidazolinone resistant (IR) maize are key elements in the agronomic toolbox to control infestations and enhance yields. The circumstances under which their use, individually or combined, is most effective on farmer fields have not been well documented. Inappropriate management decisions and low returns on investments arise from this knowledge gap, causing hunger and poverty in smallholder communities to persist. Experiments were carried out on 60 fields in three different agroecosystems of western Kenya using full-factorial treatments with non-herbicide treated maize (DH) and herbicide treated maize (IR), and N fertilizer omission and application. Trials were stratified on a field with low and high soil fertility within individual farms and repeated over two seasons. Cultivating IR maize instead of DH maize decreased the emergence of Striga with 13 shoots m-2 on average while applying N fertilizer on DH maize led to a reduction of 5 shoots m-2 on average. Decreases of Striga by use of IR maize and N fertilizer were between 6 and 23 shoots m-2 larger at the site with high levels of infestation than at the sites with medium or low emergence. Input of N fertilizer increased grain harvests by 0.59 ton ha-1 on average while use of IR maize enhanced the productivity with 0.33 ton ha-1 on average. Use of N fertilizer had similar yield effects in all three sites, whereas use of IR maize at the site with high Striga emergence increased maize production by 0.26-0.39ton ha-1 more than at the sites with medium or low emergence. The greater Striga responses to IR maize and the greater yield responses to N fertilizer demonstrate their use could be optimized according to field conditions and management goals. Combining IR maize and N fertilizer has larger added yield benefits where their individual effects on grain productivity are smaller. Findings from this study indicate that farmers in western Kenya require guidance on how to align the use of herbicide resistant maize and inorganic N inputs with the level of Striga infestation and maize yield on their fields for effectively controlling the pernicious weed and enhancing food production.

Agronomic Performance and Genotype × Environment Interaction of Herbicide‐Resistant Maize Varieties in Eastern Africa

ABSTRACTStriga hermonthica (Del.) Benth. and Striga asiatica (L.) Kuntze severely affect maize (Zea mays L.) production in sub‐Saharan Africa. A single Striga plant produces a large number of seeds that form a bank of viable but dormant seed in the soil until they get a chemical signal from suitable maize host roots. Imidazolinone‐resistant (IR) open‐pollinated maize varieties (OPVs) developed for Striga control were tested in diverse environments in four countries of eastern Africa in 2004. The objective of the study was to assess the agronomic performance of IR maize and genotype × environment interactions (GE) for grain yield (GY) and the number of emerged Striga plants across 17 environments under Striga‐infested and Striga‐free conditions. In the combined analysis of variance across Striga‐infested and Striga‐free locations, mean squares for genotypes and GE were significant for most measured traits. The best IR maize variety (STR‐VE‐216) outyielded the Striga‐tolerant and commercial genotypes by 113 and 89%, respectively, under Striga‐infested conditions. IR OPVs supported significantly fewer emerged Striga plants relative to the check varieties. Under Striga‐free conditions, IR OPVs showed GY advantage over commercial varieties. Under Striga‐infested conditions genotypic variance () was larger than genotype × location variance () for GY and number of emerged Striga plants at 12 wk after planting. The genetic correlations among locations under Striga‐infested conditions were high (0.990), suggesting little GE between most environments used. Cluster analysis of genotypes under Striga‐infested locations revealed two major groups that separated the IR OPVs from the check varieties. The outstanding performance of selected IR OPVs indicates that their use for Striga control would reduce the Striga seed bank while benefiting farmers with high GY.

The U.S. Witchweed Eradication Effort Turns 50: A Retrospective and Look-Ahead on Parasitic Weed Management

The U.S. Witchweed Eradication Effort Turns 50: A Retrospective and Look-Ahead on Parasitic Weed Management

Participatory evaluation of integrated pest and soil fertility management options using ordered categorical data analysis

Maize is becoming the major food crop around Lake Victoria. Major constraints to its production are Striga, stem borer, and declining soil fertility. Innovative integrated technologies have been developed: the ‘push–pull’ system (intercropping with Desmodium and surrounded by Napier grass), soybean and Crotalaria rotations, and imidazolinone-resistant (IR) maize seed. In 12 demonstration trials in four villages in Siaya and Vihiga districts (Kenya) and two villages in Busia (Uganda) in 2003 and 2004, 504 farmers evaluated all cropping systems and a mono-cropped continuous maize, each cropped with IR or local maize, and supplemented or not with fertilizer, totaling 16 treatments. Farmers evaluated all treatments for yield, resistance to Striga and stem borer, improvement of soil fertility, and provided an overall evaluation score, using an ordered scale of 1 (very poor) to 5 (very good). Data were analyzed using ordinal regression, estimating log odds ratios. The results show significant preferences for all treatments over the control. Push–pull with IR and fertilizer had the highest log odds ratio (2.93), so the odds of farmers preferring this treatment are 18.7 times the odds that farmers prefer the control. The odds ratios for the other push–pull combinations were generally highest (9–15), followed by the rotation systems with Crotalaria (3.5–7.0), and soybeans, especially with IR maize and fertilizer (odds ratio of 5.7). In mono-cropping systems, IR maize was only appreciated in combination with fertilizer, and then only in 2004. Push–pull and Crotalaria were more appreciated in 2004 than in 2003. Farmers in Vihiga had a stronger preference for push–pull, and those in Busia for soybean rotations. Significant differences among farmers were observed, but the effects were small. Women appreciated push–pull more than men, while other technologies were gender-neutral. Older farmers were more likely to prefer push–pull and Crotalaria with fertilizer. Livestock ownership was not found to have an effect on technology preferences. Measured yield, stem borer and Striga infestation all had significant but small effects, although their inclusion did not eliminate the treatment effects, indicating that other factors are still important. OLS of the scores for different criteria on the overall score shows yield to be the most important criterion (coefficient of 0.40), followed by soil fertility enhancement (0.25) and Striga resistance (0.13). Labor saving (0.09) and stem borer resistance (0.03) are relatively minor criteria. This research shows that scoring and analysis with ordinal regression is a convenient way to solicit and analyze farmers’ preferences for new technologies, with wide applicability in farming systems and participatory research. Its application here shows that farmers like the new technologies, especially push–pull, but that there are substantial differences between years, sites and farmers. The use of this method can be very helpful to define and focus further research and formulate specific and targeted recommendations for agricultural extension.

Crops with target‐site herbicide resistance for Orobanche and Striga control

It is necessary to control root parasitic weeds before or as they attach to the crop. This can only be easily achieved chemically with herbicides that are systemic, or with herbicides that are active in soil. Long-term control can only be attained if the crops do not metabolise the herbicide, i.e. have target-site resistance. Such target-site resistances have allowed foliar applications of herbicides inhibiting enol-pyruvylshikimate phosphate synthase (EPSPS) (glyphosate), acetolactate synthase (ALS) (e.g. chlorsulfuron, imazapyr) and dihydropteroate synthase (asulam) for Orobanche control in experimental conditions with various crops. Large-scale use of imazapyr as a seed dressing of imidazolinone-resistant maize has been commercialised for Striga control. Crops with two target-site resistances will be more resilient to the evolution of resistance in the parasite, if well managed.

Herbicide Seed Treatments for Control of Purple Witchweed (Striga hermonthica) in Sorghum and Millet

A promising approach for the control of parasitic weeds is herbicide seed priming, which consists of soaking crop seeds in a herbicide solution such that the herbicide is later present in the crop seedling to inhibit growth of attaching parasites. This technique is effective where selectivity exists between crop and parasite; for example, varieties of imidazolinone-resistant maize. However, seed priming has not been reported for sorghum or pearl millet, two crops that are greatly affected by purple witchweed. Research was initiated to evaluate herbicides for potential use as seed priming agents in these crops. Auxin-mimic and acetolactate synthase-inhibitor class herbicides were evaluated; specifically, clopyralid, 2,4-DB, dicamba, picloram, and prosulfuron. For sorghum, immersion of seed in 0.5% (w/v) ae 2,4-DB for 5 min 1 d before planting reduced purple witchweed densities to levels 20 to 50% of nontreated controls. However, this concentration was at the threshold of crop toxicity and reduced sorghum yields in some experiments. None of the herbicides tested consistently reduced purple witchweed on pearl millet. This research illustrates both the potential and limitations of adapting seed priming technology for sorghum and pearl millet.

Efficacy of imidazolinone herbicides applied to imidazolinone-resistant maize and their carryover effect on rotational crops

A field study was performed in imidazolinone-resistant maize in order to evaluate the effectiveness and carryover effect on rotational crops of two formulated mixtures of imidazolinone herbicides: imazapyr plus imazapic and imazapyr plus imazethapyr. During the 2003 season the mixtures were applied broadcast to five leaves maize plants at doses of 114 and 228 g ha −1, both tanks mixed with a 31% petroleum hydrocarbon adjuvant at 125 and 250 mL ha −1, respectively. Overall weed control varied from 85%, up to 95%. Weed species controlled were Setaria sp., Chenopodium album, Solanum sp., Amaranthus retroflexus and Digitaria sanguinalis, and Echinochloa cruss-galli whilst Eragrostis sp. was not controlled. Soil activity of the herbicides (carryover) was detected over 10 months after application, imazapyr/imazapic mixture being the most persistent. The sensitivity of rotational crops, from high to low, was the following: Beta vulgaris> Capsicum annum> Lycopersicum esculentum> Cucumis melo> Hordeum vulgare> Medicago sativa> Lolium multiflorum> Avena sativa> Pisum sativum> Allium cepa> Zea mays.

Imazapyr and pyrithiobac movement in soil and from maize seed coats to control Striga in legume intercropping

Imazapyr and pyrithiobac dressed to seeds of imidazolinone-resistant maize effectively control Striga hermonthica (witchweed). The effects of the movement of these herbicides in the soil and in maize plants was measured on Striga germination and on legumes intercropped with maize. Striga seeds were killed when high rates of either herbicide percolated through simulated soil columns; almost no viable Striga seeds remained in the upper 10 cm, and >80% were killed at 30 cm. The herbicides applied to maize leaf whorls moved systemically out of roots, killing attached and germinating Striga. Sensitive crops (beans, cowpea, and yellow gram) were unaffected when planted at >15 cm from maize coated with 0.4 mg a.i. pyrithiobac or 0.84 mg a.i. imazapyr seed −1, but were severely inhibited when planted within 12 cm. Simple herbicide seed coatings are thus compatible with commonly used African intercropping systems, while facilitating maize growth and depleting the Striga seed bank.

Time-scale for transgenic product development

Recent rapid progress on the production of transgenic plants has led to an increasing number of field trials throughout the world by both commercial and academic groups. A brief, selective, review of progress achieved by conventional plant breeding and the expected life time of cultivars developed through this process is followed by details of recent experience gained from mutagenesis programmes, in particular the generation and marketing of imidazolinone-resistant maize in the USA. The role of RFLP-aided backcrossing to speed the development of such products is emphasised. Three specific types of transgenic product development are examined in some detail. The first two, namely herbicide resistance and tomatoes with improved processing characteristics are already close to market. The third example, that provided by woody species (either fruit or forest trees) is inevitably longer term because of the prolonged duration of the breeding cycle. Nevertheless, it may in the future provide some of the most valuable benefits such as improvements in the efficiency of paper production.