Wheat growers in western Nebraska face several serious challenges: Wheat stem sawfly. Wheat streak mosaic. Weeds, including kochia. Making best use of scarce moisture.Research is providing answers to meet some of the challenges, and still searching for answers to others, according to speakers at the Wheat Production Workshop this month at Scottsbluff.
For their part, producers should respond to these challenges with a range of tools and stay tuned for new information, the researchers said.
Responses include careful selection of wheat variety; using a broad approach to weed control including several types of herbicide, crop rotation and perhaps tillage; killing volunteer wheat before planting season; and reconsidering some cultural practices that might be contributing to pests such as wheat stem sawfly.
Experts from the University of Nebraska-Lincoln, Kansas State University and the U.S. Department of Agriculture spoke to producers, crop consultants and others interested in wheat production. Funding for the workshop was provided by USDA Agriculture and Food Research Initiative (USDA-AFRI); UNL’s Institute of Agriculture and Natural Resources; and the Nebraska Wheat Board.
Wheat stem sawfly has become one of the most serious wheat pests in Nebraska in recent years. Sawfly larvae overwinter in the stubble of the previous year’s crop and emerge in May and June to attack the developing crop during stem elongation.
The larvae feed and tunnel through the developing wheat to girdle and weaken the stem for its exit from the remaining stub the following spring. After harvest and through the winter, the larvae live in a pupal chamber inside the stub at the very base of the stem. The girdling causes many of the wheat plants to lodge, or lay over on their sides.
UNL Extension Entomologist Jeff Bradshaw of the Panhandle Research and Extension Center said heavy infestations and extensive lodging were observed in 2013 throughout the Panhandle.
Sweep net samples collected in 2014 point to another heavy infestation. The 2014 samples are still being analyzed, but lodging doesn’t appear to be as widespread this year as in 2013 because many fields have thick stands of wheat due to favorable growing conditions.
Bradshaw said UNL personnel continue to study the extent of sawfly infestation, while also initiating studies to learn more about movement and seasonal dynamics of the pest. To date, it appears that adult wheat stem sawflies tend to aggregate along field borders, he said.
As scientists continue to learn more, it appears the problem needs to be addressed with integrated pest management that employs multiple tactics, according to Bradshaw. These include crop rotation, resistant varieties with solid stem characteristics, tillage, field width, and trap crops.
Bradshaw said certain varieties of wheat seem to experience less lodging then others when infested in the limited observations made so far. Goodstreak and Freeman are two hollow-stem varieties that have stood out as possibly having resistance to sawfly; however, these data are still very preliminary.
Another challenge to producers is wheat streak mosaic virus, spread by wheat curl mites.
Several speakers provided updates on different efforts to respond. UNL graduate student Justin McMechan and visiting scientist Everlyne Wosula discussed research into learning more about the wheat-mite-virus complex. In addition to WSMV, the other viruses are wheat mosaic and triticum mosaic virus.
McMechan is studying factors related to widespread outbreaks of the virus complex. They are often linked to pre-harvest volunteer wheat that results from hail storms that occur prior to wheat harvest.
Producers can reduce the risk of viruses by planting at the recommended planting date or later; using a resistant variety; and controlling pre-harvest volunteer wheat for at least 10 days prior to fall planting of winter wheat.
Developing more virus-resistant lines of wheat is what Robert Graybosch, research geneticist with the U.S. Department of Agriculture’s Agricultural Research Service is trying to accomplish. Graybosch said the task has been difficult. One focus is on resistance to the mites that carry the viruses. He said mites have mutated to adapt to mite resistant genes.
Researchers have identified several genes that provide resistance to the viruses, but breeding acceptable wheat lines with those genes has been a challenge because the genes limit yields, or resistance breaks down at higher temperatures.
Several cultivars have been developed and made commercially available, but Graybosch said USDA-ARS continues to coordinate breeding efforts to identify lines have improved agronomic performance.
Wheat producers also are challenged by weed resistance to herbicides.
Robert Wilson, weed specialist at the Panhandle Research and Extension Center, spoke about kochia, a major problem for wheat and a number of other crops in western Nebraska. Kochia, which affects both dryland and irrigated crops, is just one of more than 400 examples of weeds that have developed resistance to herbicides since the dawn of herbicide use in the 1950s.
The weeds have become resistant to more than 150 different herbicides that fall into in 21 of the 25 different known modes of herbicide action, according to Wilson. Some weeds have developed resistance to as many as nine different herbicide modes of action.
Much of the focus in recent years has been on weed resistance to glyphosate herbicides. Use of glyphosates has increased dramatically in the last two decades with the introduction of Roundup Ready crops.
In a survey funded by Western Sugar Cooperative, Wilson is conducting greenhouse tests to determine how kochia responds to six different rates of glyphosate. In many cases, a lethal dose of herbicide is much higher than the recommended application rate, he said.
Wilson urges producers to consider their entire farm when thinking about herbicide resistance.
Weed-control practices that will be effective and prevent or slow down the process of herbicide resistance include using herbicides with multiple modes of action; considering tillage; and studying crop rotations.