Manejo de Amaranthus spp. en praderas de Festuca arundinacea y Trifolium pratense

The agricultural-livestock production systems of Uruguay are characterized by rotating winter and summer agricultural cycles, with cultivated grasslands. Red clover (TR) (Trifolium pratense) and fescue (Festuca arundinacea) are two of the most used forages. The alternation between crops and pastures generates benefits in the use of resources and improves the sustainability of the system. In recent years it has become considerably more difficult to control the Amaranthus hybridus, A. palmeri and A. tuberculatus (yuyos colorados, YC) that coexist in the farms. The objective of the work was to evaluate the control of YC with pre- and post-emergent herbicides and the selectivity of these on already established TR and fescue. A trial for each forage species was carried out in the field between December 2019 and March 2020 in the town of Palmitas, Soriano, Uruguay. The treatments tested in RT were: diflufenican, s-metolachlor, diflufenican+s-metolachlor, flumetsulam, 2,4-db and MCPA. And in fescue they were: diclosulam, diflufenican, s-metolachlor, metribuzin, fluroxypyr+halauxifen, 2,4-d, picloram, fluroxypyr and MCPA+clopyralid. The same treatments were tested individually for each YC species (A. hybridus; A. palmeri and A. tuberculatus) under greenhouse conditions at INIA La Estanzuela, Colonia, Uruguay; in pre-emergence (pots planted with 30 YC seeds) or post-emergence (plants with 4 true leaves). The doses of the herbicides and their use as pre or post-emergents followed the label recommendation, and in all the trials 4 repetitions per treatment and a control without application were used. The field trials did not present the expected YC infestation, and only the phytotoxicity of herbicides on forage plants could be evaluated. TR presented approximately 25% phytotoxicity 30 days after the application of 2.4 db and MCPA, while the other herbicides did not exceed 15% phytotoxicity. For fescue only 2,4-D, picloram and fluroxypyr showed significant phytotoxicity, with values not higher than 20%. In greenhouse trials with pre-emergent herbicides, only diclosulam and flumetsulam will allow YC to germinate. The most effective post-emergents were the hormonal ones with controls greater than 85%, with the exception of MCPA, which did not exceed 62%. The post-emergent herbicide diclosulam was not effective in controlling YC. There are still control options for these YC populations in TR and fescue, but management must be extreme in order not to aggravate existing resistance problems.