Herbicides affect forage crops in rotation with rice in Uruguay

Imidazolinone herbicide residues negatively impact the emergence and early development of forage crops in rice-growing areas in Uruguay. This was the main conclusion of a study conducted between 2021 and 2023 by researchers from the University of the Republic (Udelar) and the National Institute of Agricultural Research (INIA).

The study evaluated soil samples from three rice-growing regions with varying textures and classified the environments into three groups: control (no previous use), IMI-1 (application 17 months prior), and IMI-2 (application 5 months prior). Indicators such as emergence, plant height, and dry biomass of shoots and roots were analyzed.

Affected forages

The species studied included annual ryegrass (Annual ryegrass), perennial ryegrass (L. perenne), fescue (Tall fescue), white clover (trifolium repens), red clover (T. pratense) and birdsfoot trefoil (lotus corniculatus).

Fescue was the most sensitive grass, with an average 30% reduction in emergence in soils with a history of herbicide application. Ryegrass was less sensitive, with losses close to 20%.

Among legumes, red clover suffered the greatest losses. Emergence fell by up to 35% in IMI-2 soils. White clover and birdsfoot trefoil showed greater tolerance. These results demonstrate that sensitivity to residual effects varies not only among functional groups but also among specific species.

Height and biomass

Plant height showed a more pronounced reduction in IMI-2 soils, indicating a stronger residual effect when the interval between application and sowing is shorter.

At 70 days after sowing, partial height recovery occurred in samples from the deepest layer (16–30 cm), suggesting lower herbicide concentrations at this depth.

Dry biomass confirmed the observed trends in emergence and height. In IMI-2, shoot mass was up to 60% lower. Annual ryegrass and fescue showed the greatest losses. Bird's-eye parsley and white clover maintained biomass similar to that of the control, reinforcing their greater tolerance.

Variation according to soil type

Multivariate analysis indicated that sandier soils with acidic pH—such as those found in Rio Branco and Tacuarembó—favored residue availability, increasing the risk of phytotoxicity. Clayey soils with higher organic matter content—such as in Treinta y Tres—promoted greater herbicide adsorption, prolonging their persistence but reducing their immediate availability to plants.

These interactions reveal that soil type can enhance or attenuate the effects of residues, and should be considered when choosing forage species.

Rotation planning

The research highlighted that species sensitivity to residues varies more between species than between functional groups (grasses versus legumes). Therefore, forage selection should consider individual tolerance.

the species lotus corniculatus e trifolium repens showed greater tolerance in bioassays. Even in soils recently applied to herbicides, they maintained good emergence, height, and biomass indices. These legumes also have the potential to accelerate residue degradation in the soil due to their nitrogen contribution and increased lignification.

Further information at doi.org/10.3390/ijpb16030110