Availability of soybean with dicamba resistance will provide an alternative weed management option, but risk of dicamba injury to sensitive crops from off-target movement and spray tank contamination is of concern. Research conducted at multiple locations and years evaluated soybean injury and yield response to POST applications of the diglycolamine salt of dicamba. Dicamba was applied at the two to three trifoliate stage (V3/V4) at 4.4, 8.8, 17.5, 35, 70, 140, and 280 g ae ha−1(1/128 to 1/2 of the recommended use rate of 560 g ae ha−1). Soybean injury 7 d after application was 20% following dicamba at 4.4 g ha−1and increased to 89% at 280 g ha−1. At 14 d after application, injury for the same rates increased from 39 to 97%. In a separate study, dicamba was applied at first flower (R1) at 1.1, 2.2, 4.4, 8.8, 17.5, 35, and 70 g ha−1(1/512 to 1/8 of use rate). Soybean injury 7 d following dicamba application was 19% at 1.1 g ha−1and increased to 64% at 70 g ha−1. For the same rates of dicamba, injury from 7 to 14 d after application increased no more than 4 percentage points. For dicamba rates in common for the timing studies, soybean injury 14 d after treatment was greatest for application at V3/V4, but the negative effect on mature soybean height and yield was greatest for application at R1. For dicamba at 4.4 g ha−1(1/128th of use rate), soybean yield was reduced 4% when applied at V3/V4 and 10% when applied at R1. For 17.5 g ha−1dicamba (1/32 of use rate), yield was reduced 15% at V3/V4 and 36% at R1. Based on yield reductions for 4.4 and 17.5 g ha−1dicamba, soybean at flowering was around 2.5 times more sensitive compared with vegetative exposure.
Herbicides used as harvest aids are applied at crop maturity to desiccate weed and crop foliage. Weeds present in the harvested crop can increase moisture content and foreign material, reducing grade and market price. Weeds can also delay the harvest operation and reduce harvest efficiency. Glyphosate can be used to desiccate weeds in glyphosate-resistant crops without concern for crop injury. Carfentrazone and pyraflufen-ethyl used as harvest aids can be effective in desiccating broadleaf weeds in corn and soybean. Paraquat, although effective on grass and broadleaf weeds when applied late season, can cause significant crop injury if applied too early. With expanded production of early maturing soybean cultivars in the mid-South (Arkansas, Louisiana, Mississippi, Missouri bootheel, and west Tennessee), presence of green stems, green pods, or green leaf retention, or combinations of these at harvest has increased. Interest in harvest aids has shifted to use as a crop desiccant. Paraquat also is an effective soybean desiccant, but application timing differs for indeterminate and determinate cultivars. Paraquat applied after soybean seed reached physiological maturity reduced number of green stems, pods, and retained green leaves present, allowing harvest to proceed 1 to 2 wk earlier than nontreated soybean. Seed moisture, foreign material, and seed damage also were reduced when paraquat was applied.
Research was conducted over 2 yr to evaluate soybean response to harvest aid herbicide treatments paraquat at 0.28 kg ai ha−1, paraquat with carfentrazone at 0.014 kg ai ha−1, and sodium chlorate at 6.72 kg ai ha−1. Indeterminate and determinate soybean cultivars were treated when moisture of seed collected from the uppermost four nodes of plants averaged 60, 50, 40, 30, and 20% (± 2%). For each soybean cultivar, the harvest aid treatment by application timing interaction was not significant, and data for harvest aid treatments were averaged. Application of harvest aid at 60% average seed moisture reduced yield for the maturity group (MG) IV indeterminate cultivar 15.4% compared with the nontreated; 100-seed weight was reduced 12.4%. Yield and seed weight were not negatively affected when harvest aid was applied at 50% average seed moisture and soybean was harvested 14 and 15 d before the nontreated control. Although planting date in the 2 yr for the indeterminate cultivar differed by 26 d, number of days from planting to harvest aid application at 50% average seed moisture was 112 and 116 d. For MG V and MG VI determinate cultivars, application of harvest aid at 60% average seed moisture reduced yield compared with the nontreated control 22 and 18.1%, respectively, and at 50% average seed moisture 15.6 and 4%, respectively; seed weight reductions of 8.9 to 33.3% accompanied the yield reductions of the two cultivars. Reduction in soybean yield and seed weight was not observed when harvest aid was applied at 40% average seed moisture, and harvest for the 2 yr was 8 and 9 d earlier for the MG V cultivar and 10 and 14 d earlier for the MG VI cultivar.
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