Residual Effect of Imidazolinone Herbicides on Emergence and Early Development of Forage Species in Rice-Livestock Systems

The intensification of rice–livestock systems has encouraged the integration of forage species into rice cultivation. However, the use of imidazolinone-tolerant rice cultivars (imazapyr + imazapic, IMIs), essential for weedy rice control, may hinder the establishment of sensitive forages due to herbicide residues. This study evaluated the emergence and early growth of six forage species through bioassays in soils with different IMI use histories. Over three years (2021–2023), soils were collected from three rice-growing regions in Uruguay with contrasting soil properties, at depths of 0–15 and 16–30 cm, under three IMI conditions: Control (no prior use), IMI-1 (applied one season before sampling), and IMI-2 (applied in the season before sampling). Emergence, plant height, and shoot and root biomass were analyzed using linear mixed models and principal component analysis. Shoot biomass was reduced by up to 60% in IMI-2 soils. Poaceae species and T. pratense were the most affected, while T. repens and L. corniculatus exhibited higher tolerance. Multivariate analysis indicated stronger residual effects on sandy loam soils. These findings highlight that imidazolinone persistence varies with forage species and soil properties, underscoring the need for careful planning in rice–pasture rotations to prevent adverse effects on forage establishment and system sustainability.