The 2016 growing season will be remembered by many for the widespread detections of Palmer amaranth across Iowa. While native seed mixes contaminated with Palmer amaranth seed used in Conservation Reserve Program (CRP) fields were the largest source of introduction, it is important to recognize that the weed was also found in at least 7 counties in areas other than CRP fields. Palmer amaranth seed can be transported by machinery, in feed or bedding and by wildlife, thus all fields in Iowa are at risk of being invaded by Palmer amaranth.
Integrated Crop Management News
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Landscape diversification, including the use of cover crops, can provide habitat and forage for beneficial insects. This is especially true in the spring when there is a lack of food. Alternatively, cover crops can also support field crop pests, including moths, beetles, flies and slugs. The early spring vegetation, sometimes called a “green bridge,” provides resources until the row crops emerge.
Adult alfalfa weevils become active and start laying eggs as soon as temperatures exceed 48°F. Alfalfa weevil eggs develop based on temperature, or accumulating degree days, and hatching can start around 200-300 degree days. Start scouting alfalfa fields south of Interstate 80 at 200 degree days and fields north of Interstate 80 at 250 degree days. Based on accumulated temperatures since January, weevils could be active throughout southern and central Iowa (Figure. 1).
Bean leaf beetle adults (Photo 1) are susceptible to cold weather and most will die when the air temperature falls below 14°F (-10°C). However, they have adapted to winter by protecting themselves under plant debris and loose soil. Each spring, adult beetles emerge from overwintering habitat and migrate to available host plants, such as alfalfa, tick trefoil, and various clovers. As the season progresses, bean leaf beetles move to more preferred hosts, like soybean.
A warm weather pattern in late February caused soil temperatures across most of Iowa to rise above 40 degrees Fahrenheit (F). This warm period was likely long enough for alfalfa and some forage grasses (most ryegrass varieties and less winter-hardy orchardgrass and tall fescue varieties) to break dormancy. When low temperatures resume, alfalfa plants can reharden to a degree, but only to the extent that it still has stored carbohydrates available.
Soybean aphid remains the most important soybean insect pest in Iowa, and management over the last fifteen years has primarily relied on using foliar insecticides. The economic injury level was defined in 2007, and is approximately 675 aphids per plant or 5,560 cumulative aphid days. From that multi-state research, a conservative economic threshold was developed to protect yield: 250 aphids per plant with 80% of the plants infested through the seed set plant growth stage (R5.5).
More than 100 fields throughout Iowa were surveyed for the presence of the soybean cyst nematode (SCN) in 2016 in a project sponsored by the ISU Soybean Research Center and the Iowa Soybean Association (ISA). Soil samples were collected by ISU Extension and Outreach field agronomists and ISA staff and interns. Samples in the survey collected from Allamakee County were found to have low population densities of SCN. The presence of SCN in the samples was confirmed through greenhouse testing. This finding represents the first discovery of SCN in Allamakee County and confirms the presence of SCN in the last of Iowa’s 99 counties.
The anticipated increase in dicamba and 2,4-D use associated with XtendTM and EnlistTM crops will require enhanced stewardship to prevent problems with off-target movement. While many factors influence herbicide drift, high wind speeds pose the greatest threat of moving herbicides off the intended target.
Soybean varieties that are resistant to the soybean cyst nematode (SCN) are an essential tool for managing the pest. The SCN control provided by resistant soybean varieties can vary greatly, just as yield. Iowa State University evaluates the SCN control and yield of SCN-resistant soybean varieties in field experiments conducted throughout Iowa annually. Results of the 2016 experiments are now available online and will be distributed in print in January 2017.
Leaving crop residue on the soil surface improves nutrient cycling and, ultimately, soil quality that will increase and sustain soil productivity. Through conservation practices that include balanced residue management and soil fertility, environmental quality can be substantially enhanced. By retaining crop residue on the soil surface, soil organic carbon (SOC) and nutrient-holding capacity are increased while protecting the soil from wind and water erosion.