A sustainable strategy for alfalfa establishment: optimized reduced-rate tank mixtures for controlling Amaranthus retroflexus and Chenopodium album

Alfalfa (Medicago sativa L.), a globally significant perennial leguminous forage crop, is widely cultivated in Inner Mongolia. Problematic weeds present a significant challenge to alfalfa production during the seedling stage, especially redroot pigweed (Amaranthus retroflexus L.) and common lambsquarters (Chenopodium album L.). However, limited information is available on pre-emergence herbicides effective against both redroot pigweed and common lambsquarters in alfalfa fields during the establishment phase in Inner Mongolia. We examined the effects of 12 pre-emergence herbicide tank-mixture treatments on weed density and alfalfa emergence, fresh weight, and dry weight at 30 days after herbicide application. Acetochlor-imazethapyr (270 + 18.75 g a.i. ha⁻¹) resulted in higher alfalfa fresh and dry weights than the weedy treatment and reduced the density of common lambsquarters and redroot pigweed by more than 91%. Acetochlor + trifluralin (270 + 187.5 g a.i. ha⁻¹) treatment, corresponding to 75% of the recommended rate for each herbicide, provided effective weed control. It achieved over 93% weed density reduction against common lambsquarters, redroot pigweed, and grasses, while supporting alfalfa crop establishment. These findings highlight the potential of reduced-rate herbicide mixtures for sustainable weed management in alfalfa. In particular, the acetochlor + trifluralin combination (270 + 187.5 g a.i. ha⁻¹) offers an effective and lower-cost option for broad-spectrum weed control under local production conditions. Our results demonstrate that an optimized herbicide combination and application rate can effectively control redroot pigweed and common lambsquarters in alfalfa fields.