Weeds in corn and soybean fields are practically a certainty. No matter how much time, how many resources farmers used on weed control in 2015, it's more than likely weed issues will happen again next year. Here are some considerations from experts at University of Illinois for weed management in 2016.
Challenges caused by weather
Weather patterns during portions of the 2015 growing season once again demonstrated the inherent perils of weed management programs that rely exclusively on one tool or tactic. These perils were often highlighted in soybean fields not treated with soil-residual herbicides. Applications of postemergence herbicides were often delayed by frequent precipitation and persistently wet field conditions until well beyond the point when weed interference began to reduce soybean yield.
This provides a reminder of a very important central tenet of weed management: resources expended to keep weeds under control do not increase crop yield. Increases in crop yields are accomplished though plant breeding; weed management, on the other hand, preserves the genetic yield potential achieved through breeding.
Put another way, weeds and crop plants require the same resources for growth. Any resource consumed by competing weeds becomes a resource unavailable for the crop to use to express its genetic yield potential. Once weed interference has persisted long enough to adversely impact crop yields, nothing can restore the lost yield.
Lower commodity prices have many contemplating ways to reduce input costs in 2016. There are several viable options to reduce herbicide costs, but remember that hybrids and varieties, even those with the highest yield potential, will not realize their yield potential if weeds are not adequately and timely controlled.
For example, assume a soybean farmer did not realize (or refuses to accept the fact) that glyphosate-resistant waterhemp infests a particular 60-acre field. The decision is made not to invest in an effective soil-residual herbicide; the farmer believes glyphosate alone will control the waterhemp. Following the in-crop application of glyphosate, two outcomes of this decision become rather obvious: poor control of waterhemp, and weed interference that continues to reduce soybean yield for much of the growing season. With a soybean yield potential of 65 bushels per acre, a modest yield loss of 20% due to weed interference, and a soybean market price of $9 per bushel, the decision to save a few dollars in weed control costs at the beginning of the season actually resulted in a revenue loss of over $100 per acre through reduced soybean yield.
Keep in mind, especially while planning 2016 weed management programs, that wise investments to manage weeds before interference reduces crop yields will realize a return through more bushels harvested at the end of the growing season. An investment in high-yielding hybrids and varieties should be coupled with an investment in weed management that adequately protects yield potential.
Herbicide-resistant weed populations continue to be a common occurrence across most areas of Illinois. Waterhemp and horseweed (marestail) are the two most common herbicide-resistant weed species in Illinois, and observations during 2015 suggest these species are likely to remain prevalent in 2016. Approximately 1,700 waterhemp samples (representing 338 fields) were submitted to the University of Illinois Plant Clinic in 2015 for herbicide resistance screening. Although summary data for these samples are not yet available, the sheer number of samples submitted suggests herbicide-resistant waterhemp continues to be a significant management challenge for farmers. Waterhemp plants and/or populations resistant to herbicides from more than one site-of-action group are not uncommon, and we anticipate this phenomenon will continue. Data from 2014 indicated resistance occurred in close to 90% of the fields sampled, and multiple resistance to glyphosate and PPO inhibitors was confirmed in 52% of the fields sampled.
It’s not just the POST herbicides
Soil-applied PPO-inhibiting herbicides (sulfentrazone, flumioxazin, saflufenacil, etc.) can be effective against several small-seeded broadleaf weed species. These active ingredients are often selected to provide residual control of pigweed species in soybean, and frequently are applied with other active ingredients to broaden the weed-control spectrum. The foliar-applied PPO-inhibiting herbicides (lactofen, fomesafen, acifluorfen) constitute one site-of-action group for postemergence control of waterhemp and Palmer amaranth in soybean. As previously mentioned, resistance to PPO-inhibiting herbicides in waterhemp is becoming more common across Illinois. Some mistakenly believe this type of resistance is only to the foliar-applied PPO inhibitors, but resistance to PPO inhibitors occurs regardless of whether the herbicide is applied to the soil or to plant foliage. Read more about this resistance issue.