Smooth pigweed is one of the most troublesome weeds in Argentina. The objective of this study was to evaluate the sensitivity of 50 smooth pigweed accessions to fomesafen, topramezone, glyphosate, 2,4-D, and dicamba. Accessions were collected from soybean fields in various cropping areas in Argentina. The herbicide treatments included 2,4-D (1,140 g ae ha−1), dicamba (560 g ae ha−1), fomesafen (250 g ai ha−1), topramezone (34 g ai ha−1), and glyphosate (1,080 g ae ha−1). Plant survival was evaluated 30 d after each treatment application. Of the smooth pigweed accessions tested, 84% and 76% were susceptible (0% survival) to 2,4-D and dicamba, respectively. More than 90% of the accessions showed high (>60%) survival to glyphosate. While none of the accessions showed total sensitivity (0% survival) to the other herbicides evaluated, 43% and 72% of the accessions showed greater than 60% survival to fomesafen and topramezone, respectively. The differences in survival among accessions confirm the existence of genetic variability in Argentinian smooth pigweed and suggest that weed management practices should be prioritized to preserve the efficacy of these commonly used herbicides.

Several populations of smooth pigweed with resistance to imidazolinone (IMI) herbicides have been identified in recent years. One IMI-resistant population (R2) was 10-fold more sensitive to cloransulam-methyl in the greenhouse when compared with a susceptible (S) population. Laboratory studies were conducted to determine if differences in the absorption, translocation, and metabolism of cloransulam-methyl existed between S and R2 populations and to determine if these differences could account for the whole-plant responses observed in the greenhouse. Enzyme assays were also conducted to determine if differences in acetolactate synthase (ALS) sensitivity to cloransulam-methyl existed between S and R2 populations. Absorption of foliar-applied cloransulam-methyl was rapid and similar in both populations of smooth pigweed. Translocation of the absorbed radioactivity out of the treated leaf was symplastic and generally similar in both populations. Translocated radioactivity was detected primarily in shoots above and below the treated leaf with little movement to the roots. 14C-cloransulam-methyl metabolism was also similar in S and R2 populations. Differential tolerance to cloransulam-methyl in S and R2 populations in the greenhouse cannot be explained by differences in the absorption, translocation, and metabolism of this herbicide. However, ALS from R2 was 25-fold more sensitive to inhibition by cloransulam-methyl than ALS from S.

The development of production systems that provide the benefits of reduced tillage and also reduce or eliminate herbicide inputs presents a challenge to practitioners of sustainable agriculture. This research was conducted to evaluate management approaches for minimum-tillage corn production in a hairy vetch cover crop, specifically, mechanical versus herbicidal methods for pre-plant cover-crop kill and for post-emergence control of emerged weeds. Pre-plant treatments included 2,4-D [(2,4-dichlorophenoxy)acetic acid] plus residual herbicides, 2,4-D alone, flail mower, corn stalk chopper, light disk and heavy disk. The pre-plant herbicide treatments were followed by a post-emergence treatment of dicamba, whereas the pre-plant mechanical treatments were followed by post-emergence cultivation, as needed, for weed control. The mechanical treatments that kept residue on the soil surface (mower, stalk chopper and light disk) killed hairy vetch when it was flowering, but not when vegetative. The herbicide treatment with pre-plant 2,4-D alone, followed by dicamba post-emergence, controlled annual broadleaf weeds (the dominant species in these experiments) similarly to treatment with 2,4-D plus residual herbicides, suggesting that residual herbicides may be eliminated in cover-crop-based no-tillage systems. The pre-plant mechanical treatments followed by cultivation did not control annual broadleaf weeds as well as herbicide treatments, but did maintain final populations below threshold levels in two of four experiments. The stalk chopper and light disk left high levels of vetch residue on the soil surface, and reduced initial broadleaf weed populations compared with the heavy disk that incorporated residue. However, broadleaf weed populations were reduced with less efficiency by cultivation of unfilled soil following the stalk chopper or light disk (38–69%) than by cultivation of tilled soil following the heavy disk (87–95%). Thus, although maintaining surface cover-crop residue without tillage initially reduced weed emergence, it also reduced the efficiency of cultivation, leading to similar final weed populations in all mechanical-based, pre-plant treatments.