Growth responses, competitiveness and control of Digitaria nuda (Schumach.) in maize (Zea mays)

Abstract

Digitaria species infestation levels have recently increased in South Africa due to the prevailing misconception amongst producers and herbicide agents that acetanilide herbicides will control all Digitaria spp. equally effective, irrespective of species differences. Since 2008 a relatively unknown Digitaria spp was noticed in maize fields and was positively identified as D. nuda (naked crabgrass). Research on naked crabgrass world-wide is limited; it has been reported to be of importance in sugarcane in Brazil and is considered as a serious grass weed in West Africa. Growth chamber trials were conducted to elucidate the germination characteristics of naked crabgrass. Germination of fresh naked crabgrass seed was less than 20%. Pre-treatment of fresh seed, by means of soaking seed for 24 h in distilled water, increased germination to 99%. Stored seed (1-yr old) germinated best (100%) in a 0.2 M KNO3 solution. Naked crabgrass germinated best at constant temperature greater than 25 °C and at fluctuating regimens of 30/15 °C, with alternating light/dark conditions. Naked crabgrass emerged faster and total seedling emergence was 20% higher on clay soil. Above-ground biomass was 5.0 g per plant on clay soil compared to 2.3 g on sandy soil. Seedling emergence was reduced by 27% after burial at a depth of 1 cm and only 5% seeds emerged at a depth of 6 cm. In a replacement series glasshouse trial naked crabgrass was more aggressive with regard to root mass (AI=0.3) and large crabgrass (D. sanguinalis) with regard to shoot mass (AI=0.04). Naked crabgrass was more competitive in a wet soil profile (CR=1.88) and large crabgrass in a dry soil profile (CR=2.02). Both grass weeds are making similar demands on the available resources (RYT=1). The competition effect of both naked and large crabgrass was determined in glasshouse trials in two soil types at different watering regimens. A negative linear relationship was recorded between grass density and maize plant height, shoot- and kernel weight. Estimated yield loss of maize varied between 29 and 76% and was described by the hyperbolic equation of Cousens (1985). Large crabgrass had a higher damage coefficient (4.1 on sandy soil) compared to naked crabgrass (3.1 on clay soil). A critical period of weed control was established in field trials for naked crabgrass. The beginning of the critical period of weed control when a 10% yield loss is estimated varied between the two and six leaf stage of maize, ending at the twelve leaf stage or two weeks after tasseling, indicating a need for season-long control of naked crabgrass. Yield loss of maize in the field trials ranged between 28 and 82% in the seasonlong weedy control treatments. In field and glasshouse trials the efficacy of naked crabgrass control was evaluated with different herbicides registered on maize. Naked crabgrass is more tolerant to acetochlor and s-metolachlor and started to emerge two weeks after applications. Large crabgrass is still effectively controlled by these herbicides. More than 85% of naked crabgrass was controlled when PRE applications of acetanilide herbicides were followed by triketone herbicides. Indiscriminate identification of all crabgrass species as “large crabgrass” can cause a shift from large crabgrass to naked crabgrass since the latter is more difficult to control. Extrapolation of characteristics and factors affecting germination and growth between similar species is perilous and should be verified. Results of this study proved that both grass weeds are severe competitors of maize, causing significant yield losses, but species specific characteristics could be distinguished and will improve decision making processes significantly to control naked crabgrass effectively in maize.