The biology and management of waterhemp in Indiana

Waterhemp is a dioecious weed species indigenous to the Midwestern United states yet it has only recently become problematic in agronomic crop production in Indiana. Waterhemp is a small-seeded broadleaf which has increased in prevalence in conjunction with an increase in conservation tillage practices. Waterhemp germinates and emerges from the top 3 cm of soil and is known to exhibit extended periods of continual emergence, longer than most other summer annual weed species that are typically present in agronomic production settings. As a C4 species, waterhemp then grows rapidly and is capable of producing thousands of seeds, while effectively competing with corn and soybean crops. Corn and soybean yields can be reduced by 50–70% when competing with waterhemp for an entire growing season. There are also many herbicide-resistant biotypes of waterhemp, which create additional management challenges beyond the competitive nature of this weed. The objective of this research was to evaluate the emergence, growth and development, and the influence of tillage and herbicides on waterhemp biology and management in Indiana. The emergence characteristics of waterhemp were evaluated in a fallow field study where waterhemp emergence was monitored weekly throughout the growing season from three different tillage systems; no tillage, a single tillage event, and two tillage events 30 days apart. Waterhemp densities were low in 2014 and there were no differences in emergence from either tillage system. Higher waterhemp densities in 2015 produced more emergence from both the no-tillage and the two tillage treatments compared to the single tillage treatment. In both years, a flush of emergence was observed after the second tillage event in the two tillage treatment. Waterhemp emergence was first observed on April 24, 2014 and April 16, 2015 and 50% of the total emergence had occurred by May 22, 2014 and May 15, 2015. Waterhemp emergence can be decreased with a single tillage event in high-density waterhemp infestations. A second field study evaluated waterhemp emergence in a soybean environment. A factorial experiment evaluated the influence of no-tillage and conventional tillage combined with soil residual and foliar herbicides on waterhemp emergence. Soybean were planted on May 8, 2014 and May 14, 2015, and waterhemp emergence was then monitored biweekly throughout the growing season. Wherever soil residual herbicides were utilized, regardless on tillage, there was very little waterhemp emergence. In plots with no residual herbicides, emergence was 152% to 223% greater from the no-tillage treatment. There were no times throughout the season in which weekly emergence was higher in the conventionally tilled plots compared to the no-tillage plots. Waterhemp emerged for 10 and 12 weeks after planting in 2014 and 2015, respectively, with no difference in duration of emergence from either tillage treatment. Soil residual herbicides and conventional tillage were able to decrease waterhemp emergence in soybean. A third study investigated the growth and development of five waterhemp populations grown in a common garden. Populations from Indiana, Illinois, Missouri, Iowa and Nebraska were established in May, June, and July to simulate a discontinuous germination pattern and were measured weekly for plant height and flowering, and finally harvested for biomass and seed yield. Plant biomass accumulations from the May (1,120 g plant-1) and June (1,069 g plant-1) establishment dates was greater than the July (266 g plant-1) establishment date. There were no differences in the mean biomass accumulation among the five populations in either the May or June establishment but the July establishment ranged from 195 to 338 g plant-1 across the populations. Seed yields were higher in the May (926,629 seeds plant-1) and June (828,905 seeds plant-1) establishment dates than the July (276,258 seeds plant-1) establishment date. The Illinois population flowered the latest of all the populations but was also among the tallest in all three establishment dates. The July establishment flowered the quickest after establishment, accumulated the least biomass, and had the fewest, but the largest seeds. This experiment showed the effect of establishment timing on waterhemp growth and development and differences among populations when grown in a common environment