By Dusty Sonnenberg, CCA, Field Leader, a project of the Ohio Soybean Council and Soybean check-off.
Over the past few years waterhemp has spread across the state of Ohio reducing yields in soybeans. In order to best manage waterhemp populations, it is helpful to understand the plant’s life cycle and characteristics.
Waterhemp is a summer annual weed, which means that it will typically emerge in the spring or early summer, grow during the summer, produce seed in mid to late summer, and be killed by frost in the fall. Waterhemp however can emerge throughout the growing season and even when soybeans are at the V5 growth stage, the later emerging plants can reduce yields by as much as 10%. Waterhemp grows quickly, increasing in size by 1 to 1.25 inches per day, and outcompetes soybeans and even other weeds for sunlight. It is a prolific seed producer adding over 250,000 seeds to the seedbank per plant. In areas where the weed does not have competition, it has been found to produce up to 1 million seeds per plant. The seeds are very small and remain viable in the soil for many years, however the number of viable seeds decreases over time with research showing that less than 12% of the seeds are viable after 4 years.
Waterhemp has developed herbicide resistance to as many as six modes of action in parts of the country. Many plants have developed multiple resistances. Waterhemp has been found to be resistant to Group 5 (e.g., triazines like atrazine and simazine), Group 2 (e.g., ALS-inhibiting herbicides like Pursuit and Classic), Group 14 (e.g., PPO-inhibiting herbicides like Ultra Blazer, Cobra and Flexstar), Group 9 (e.g., glyphosate), Group 27 (e.g., HPPD-inhibiting herbicides like Callisto, Laudis and Impact) and Group 4 (e.g., 2,4-D). It has been noted that Group 14 PPO-inhibitor herbicides with residual activity are likely to have utility in controlling PPO-resistant waterhemp when applied pre-emergence.
A characteristic that has allowed waterhemp to rapidly develop resistance to multiple herbicides is that it is dioecious. This means that some waterhemp plants produce male flowers and others produce female flowers, so there is more genetic diversity from the outcrossing of the different plants. This crossing increases the likelihood of evolving and spreading the herbicide resistance genes, and other traits.
To manage waterhemp, a multifaceted approach is recommended. This will include cultural practices, chemical application and mechanical controls. Keeping waterhemp out of fields is the first objective. Thoroughly cleaning equipment that has worked in fields with waterhemp present is important. Attention should be given to removing any soil that could contain the weed seed from planting or tillage equipment, or cleaning harvest equipment that ran in fields with waterhemp previously.
When planting soybeans, using narrow row spacing and optimum planting populations will increase the soybean’s ability to outcompete waterhemp for nutrients and sunlight. Deep tillage will also reduce the amount of waterhemp seed that germinates. Deep tillage buries the seed at a depth that makes it difficult to emerge. Using deep tillage combined with a residual herbicide program further enhances control. Fall seeded cover crops like rye can also reduce early-season watherhemp emergence during the following spring.
Chemical control of waterhemp includes using a full rate of an effective pre-emerge, soil residual herbicide very close to the time of planting. The over reliance of post-emerge herbicides for waterhemp control has actually contributed to the development of herbicide resistance. A full rate is important because waterhemp emergence extends late into the growing season. The later that waterhemp emergence is delayed, the better the potential is to maximize soybean yield. Using reduced rates are likely to reduce the percentage of the waterhemp population that will be controlled by the post-emergent products. Full rates are also more likely to delay the onset of herbicide resistance when compared to reduced rates.
For more information visit Take Action Pesticide-Resistance Management, sponsored by the United Soybean Board.