In 2018, Louisiana ranked fourth in production acreage of sweetpotato behind North Carolina, Mississippi, and California (LSU AgCenter, 2019). That year Louisiana harvested 7682 acres of sweetpotato with an estimated yield of 491 50-lb bushels per acre, resulting in $94.5 million in total production value (LSU AgCenter, 2019). Production and packing fresh market costs were about $4000/acre to $4600/acre, while production costs for the processing sector was about $2300/acre before storage (LSU AgCenter, 2019). Given this high level of production costs, there is little margin for error in terms of factors such as crop injury from off-target herbicide application or sprayer contamination events that can negatively impact yield.
Maximum sweetpotato yield production requires adventitious roots to effectively produce lateral roots that swell and produce mature storage roots (Villordon et al., 2014). Previous research has indicated that in pot studies at ≈5 to 15 d post-transplant, adventitious roots (representing 80% of the final yield) progressively grow and produce lateral roots depending on the internal auxin signaling (Villordon et al., 2014). Villordon et al. (2009) differentiated storage root development into a three-stage phenology scheme: SR1, SR2, and SR3. SR1 consists of the presence of at least one adventitious root greater than 0.5 cm in length, in at least 50% of transplanted slips. SR2 consists of the presence of anomalous cambium in at least one adventitious root on 50% of the plants. SR3 consists of at least one visible storage root, an adventitious root that is swollen 0.5 cm at its widest point, in at least 50% of the plants. Storage root formation begins between 13 and 20 d in the field. Lateral root development is fundamentally dependent on auxin signaling, and anything that interferes with this process interferes with storage root formation. This is the precise window for targeting negative impacts, such as herbicide injury, to determine maximum potential to reduce yield due to reduction in storage root number (A. Villordon, personal communication).
With increasing populations of weeds resistant to glyphosate herbicide, variety development has shifted focus to developing new product formulation technologies with older herbicides and using plant genetic modification to combat such populations. Two new technologies have been commercialized that allow application of 2,4-D choline (in combination with glyphosate) or diglycolamine (DGA)/N,N-Bis(3-aminopropyl) methylamine (BAPMA) salt of dicamba (3,6-dichloro-2-methoxybenzoic acid) (alone or in combination with glyphosate) over the top of crops that were previously intolerant to these two herbicides. Soybean (Glycine max), cotton (Gossypium hirsutum), and corn (Zea mays) varieties with these traits are readily available for purchase and use by producers. Dicamba and 2,4-D selectively control most dicotyledonous plants including morningglory (Ipomoea sp.) (Siebert et al., 2004), palmer amaranth (Amaranthus palmeri) (Norsworthy et al., 2008), and marestail (Conyza candensis) (Bruce and Kells, 1990), and therefore are more commonly used in monocotyledonous crops, such as pastures, turf, and in some instances corn and small grains. These new technologies use plant genetic resistance to these products so that applications may be made directly to the transformed crops.
Merchant et al. (2013) found that morningglories, when exposed to either 2,4-D at 1.2, 1.75, or 2.3 L·ha−1, or dicamba at 0.6, 1.2, and 2.3 L·ha−1, were completely controlled. Glyphosate applied at 1120 g·ha−1 controlled 2- to 5-cm entireleaf (Ipomoea hederacea) and pitted morningglory (Ipomoea lacunosa), while the same species at 8 to 10 cm were controlled 84% and 88%, respectively (Corbett et al., 2004). Because sweetpotato is also an Ipomoea species, off target movement of 2,4-D, dicamba, and glyphosate is a major cause for concern to producers. Research has shown sweetpotato to be highly sensitive to 2,4-D, with a very low concentration of only 100 ppm considered optimum to induce flowering (Mutasa et al., 2013). Previous research has shown that 1/4 of the recommended rate of 2,4-D applied at 27 d after transplant will result in complete kill of ‘Beauregard’ sweetpotato within 2 weeks (Clark and Braverman, 1998). At a similar rate, dicamba and triclopyr resulted in chlorosis and severe stunting of plants. Dicamba, 2,4-D, and triclopyr at 1/4 and 2,4-D at 1/10 of the recommended use rate evaluated resulted in almost nonexistent yield, while dicamba and 2,4-D applied at 1/100 of the use rate resulted in intermediate yield reduction. Clark and Braverman (1998) also demonstrated that stored roots from plants treated with dicamba at 1/10 of the use rate produced shoots with epinastic symptomology 8 months after application. In a separate study, Clark and Braverman in 1998 also reported that glyphosate applied at 1/2, 1/4, and 1/10 of the use rate 27 d after transplant reduced ‘Beauregard’ U.S. No. 1 and total marketable yield. When applied at 41 d after transplant, yield reduction was observed only with the 1/2 and 1/4x rates. Myers et al. in 2017 also indicated negative impacts with regards to injury and yield to sweetpotato exposed to simulated glyphosate drip rates encountered in wick weed control applications 4 to 8 weeks after planting.
No research has been conducted on the potential impacts on sweetpotato from 2,4-D (applied as Enlist Duo; Corteva Agriscience, Wilmington, DE) and dicamba (applied as Roundup Xtend; formerly Monsanto Company, St. Louis, MO) herbicide formulations that will be available for use in the Enlist or Xtend cropping systems. Off-target movement or sprayer contamination of 2,4-D, dicamba, or glyphosate is a major cause for concern. With this concern in mind, research was conducted in Louisiana to 1) determine differences in sweetpotato susceptibility to 2,4-D or dicamba applied at differing rates; 2) evaluate impact of reduced rates of these hormonal herbicides and glyphosate that may be encountered in off-target or sprayer contamination events; and 3) determine the impact of application timing on growth and yield of sweetpotato.
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