Reduced atrazine doses combined with sorghum aqueous extracts inhibit emergence and growth of weeds

Combining low doses of herbicides with allelopathic plant extracts subject weeds to different mechanisms of action, which reduces herbicide resistance. The effects of reduced atrazine doses combined with sorghum aqueous extracts (sorgaab) from sorghum accessions IS9456, IS22320 and Mahube on emergence and growth of Bidens pilosa and Eleusine indica were evaluated in a greenhouse experiment at the University of Zimbabwe in 2017. Two experiments were set up as a 3 ˟ 5 factorial arrangement in a completely randomized design testing sorghum varieties as sources of sorgaab, and five atrazine-sorgaab mixtures (100% sorgaab, 10% of the label recommended dosage (LRD) of atrazine for maize + 90% sorgaab, 30% LRD atrazine + 70% sorgaab, 100% LRD atrazine, and untreated check). Percent emergence, height, and total chlorophyll content in leaves significantly decreased (P<0.001) as influenced by sorghum variety in the order IS22320>Mahube>IS9456, for B. pilosa and E. indica. Sorghum variety significantly (P<0.001) affected dry weight of B. pilosa similarly to other parameters but did not significantly (P>0.05) affect dry weight of E. indica. Percent emergence, height, total chlorophyll content and plant dry weight significantly (P<0.001) decreased in the order untreated control>100% sorgaab>10% LRD atrazine + 90% sorgaab>30% LRD atrazine + 70% sorgaab>100% LRD atrazine. There were significant (P<0.05) effects of sorghum variety as source of sorgaab ˟ atrazine-sorgaab mixture interactions on B. pilosa emergence and height and E. indica height. There is potential to exploit sorghum allelopathy using aqueous extracts alone, and in mixture with reduced doses of atrazine in controlling certain weeds. However, the allelopathic efficacy of sorgaab was dependent on sorghum variety. The sorghum variety IS9456 possibly produces high amounts of water soluble allelochemicals, making it a suitable candidate for use in integrated weed management.