Research Highlights

Computing Soybean Yield Loss From Palmer Amaranth and Waterhemp Interference

By Carol Brown

Crops battle turf wars with weeds each growing season. But when weed pressure begins to take over the turf and declare victory, how are crop yields impacted? North Dakota State University assistant professor Quincy Law is conducting a research project to measure soybean yield loss from waterhemp and Palmer amaranth interference and the economic threshold associated with their control.

The invasive and noxious weed specialist is working with colleague Joe Ikley on a two-year project, supported by the North Dakota Soybean Council and the North Dakota State Board of Agriculture Research and Education. The project goals are to utilize a statistical model that computes yield loss from weed interference with actual weed density scenarios from research plots. 

“We designed this experiment by following what previous scientific papers determined to be the best model to estimate yield loss from weed presence,” says Law. “We’re looking for the maximum yield loss that Palmer amaranth and waterhemp can cause, and the economic cost of these weeds.”

Waterhemp has been an issue in North Dakota crop fields for some time, but Palmer amaranth is a fairly new. It was first identified in the state in 2018, Law says, and has been confirmed in nearly 20 counties. These weed specialists are helping farmers with identification, and Ikley is leading an awareness campaign to bring attention about how Palmer amaranth can spread.

“Part of this project is to provide state officials with more data about Palmer amaranth because it’s considered noxious, which means by law it must be controlled,” Law says. “If we can also provide the potential yield loss this weed can cause, it can help justify its noxious status. Unlike weeds we have dealt with for some time, such as waterhemp, we’re not sure about the herbicide resistance of these new Palmer amaranth infestations.”

To compute the yield loss model, Law needed actual numbers to enter into the equation, so the team established their own weed density scenarios within the soybean research plots. They conducted this at two sites: one for Palmer amaranth and one for waterhemp.

“We attempted to reach our intended waterhemp densities via natural emergence from the soil seedbank in year one. The first flush didn’t reach our goal so we seeded waterhemp into the plots in mid-June and allowed existing waterhemp plants to grow,” he explains. “Conversely, we germinated Palmer amaranth in the greenhouse and transplanted them into the soybean plots at the cotyledon to true-leaf stage.”

The Palmer amaranth was planted in the inter-rows with 0, then 0.5, 1, 2, 4, and 8 plants per meter to achieve the different plant densities. Law says the weeds started to get dense at 4 and 8 plants per meter.

The team harvested the Palmer amaranth and waterhemp at each site, which entailed cutting the plants by hand and measuring their biomass. Once the rows were clean of weeds, they harvested the soybeans to measure yield and estimate loss as it related to weed density. 

The team recently finished harvesting for year two and are working on the data analysis. After the first year of the experiment in the Palmer amaranth plots, they saw a yield loss range from 27% per plant as weed density neared 0 to an estimated 57% maximum yield loss due to the weed’s presence in the worst-case scenario (Figure 1). Waterhemp density wasn’t uniform in the treatments, but data showed an inverse relationship between the weed biomass and soybean yield (Figure 2).

“Timing is an important factor in this experiment as well as in real-life. We transplanted the weeds into the plots 24 days after planting the soybeans, so they had nearly four weeks to germinate and grow before the Palmer amaranth was there,” he says. “In North Dakota, farmers are ideally planting in early May. Palmer amaranth and waterhemp usually emerge around the end of May. We modeled up to a 57% yield loss in this scenario. What happens with weed pressure if farmers must plant late and they don’t have several weeks to allow for soybean emergence and growth?”

In one set of plots in the second year of the experiment, they transplanted the weeds less than one week after planting soybeans. Although the data analysis hasn’t been completed, Law observed the earlier-planted Palmer amaranth grew larger with the earlier emergence opportunity. 

“We also saw that the early-transplanted weeds, at 4 plants per meter, created more biomass and caused a greater yield loss than the 8 plants per meter that were planted later,” Law adds. “Based on this preliminary observation, weed emergence timing can be more important than weed density.”

Law and Ikley have been sharing this experiment and first-year outcomes with soybean farmers and with non-soybean growers. Other crops may not have the herbicide options that soybeans do, so Palmer amaranth and waterhemp presence in other crop fields may be even more of an issue.

“I consider this yield loss research a ‘cautionary tale’ for other crop growers,” Law comments. “I want to show farmers that might be dealing with small or early weed infestations, that it is important to control and to not let it go unmitigated.”

Published: Feb 4, 2024