The High-value Crops Project at North Dakota State University in the US has been evaluating the potential injury to potato from glyphosate for almost 10 years. According to Prof Harlene Hatterman-Valenti – a High-Value Crops Specialist – research results have suggested that red-skinned cultivars were more sensitive to low rates of glyphosate compared to russet cultivars and that low glyphosate rates were more detrimental to the current year’s crop when herbicide uptake occurred around the tuber initiation stage. “In contrast, low glyphosate rates were more detrimental to next year’s crop (planting of daughter tubers as seed) when herbicide uptake occurred closer to harvest,” she says. “In 2015 and 2016, low glyphosate and/or dicamba rates were first evaluated on ‘Russet Burbank’, and in 2017 the same low rates were evaluated on ‘Atlantic’ as dicamba-tolerant soybeans were available to growers beginning in 2017, and effects to sub-lethal rates of each herbicide (dicamba and glyphosate) and their combination was unknown.”
She says that results from the 2017 spray applications indicated that visible injury was transient, decreasing to less than 15% by 20 days after treatment, with flower abortion as the most noticeable response to dicamba, while flower color bleaching (purple flower color changing to white) was the most noticeable response to glyphosate.
“Marketable and total yield differences were not significantly different at P = 0.05. However, the highest rate of dicamba, glyphosate, and dicamba+glyphosate did increase the yield of tubers < 4 oz., but other grade categories did not statistically differ.”
Results suggested that dicamba will cause more injury than glyphosate to the chipping cultivar, but that yield reduction will be minor and not differ from the untreated. The only problem with this conclusion is that two of four environments during 2015 and 2016 trials showed a similar response, prof Harlene Hatterman-Valenti notes. “It was concluded that stressful environmental conditions (higher maximum and minimum air temperatures) when the sub-lethal herbicide rates were applied caused less visible injury and less yield reduction. The total yields were 45% and 34% higher when plants under stressful environmental conditions received the highest sub-lethal dicamba and glyphosate rates, respectively.
What was more astonishing was that when the daughter tubers from these stressed plants that received the highest sub-lethal dicamba, glyphosate, and dicamba+glyphosate rates were planted the following year, emergence at five weeks after planting was reduced 53%, 71%, and 71%, respectively, compared to the untreated, while emergence at five weeks after planting was reduced 22%, 41%, and 28%, respectively, compared to the untreated for seed from the unstressed plants that received the highest sub-lethal dicamba, glyphosate, and dicamba+glyphosate rates. , Prof Harlene Hatterman-Valenti says.
The plant back results suggest that seed producers will need to watch for even subtle herbicide injury symptoms during tuber initiation because residues in the seed pieces may significantly affect emergence. Further research is needed to determine air temperatures that cause the observed plant stress responses and to determine if the environmental stress could result in herbicide residues in the tubers even when these herbicides drift before tuber initiation.
Prof Harlene Hatterman-Valenti can be reached at email@example.com for further information.