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When Does Waterhemp Emerge and are there Tools to Forecast? (05/13/21)

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Controlling waterhemp in sugarbeet has been my most important weed control challenge since arriving at NDSU in 2014. Early on, we decided we needed to better understand waterhemp biology including the calendar date when waterhemp emerged in sugarbeet. Dr. Bob Hartzler, Iowa State University, suggested we track growing degree days (GDD) using a technique successfully implemented in Iowa, to simply scout by grouping weeds (Iowa State University IPM-64). Hartzler postulated it might be possible to estimate waterhemp emergence date in order to time application of lay-by residual herbicides in sugarbeet by tracking GDDs.

We have had mixed results tracking GDDs to forecast waterhemp emergence. For example, cumulative GDD ranged from 42 to 189 from January 1 to May 10 between the years 2016 to 2021 at the NDAWN station near Sabin, MN. Two data clusters were observed across years, the first averaging 150 GDDs and representing years 2016 to 2018 (Table 1). The second cluster averaged 54 GDDs and represented years 2019 to 2021. The earliest waterhemp observed was near Sabin on May 11, 2020, a low GDD cluster season.

Table 1. Cumulative growing degree days, base 48F (CGGD), January 1 to May 10, Campbell and Sabin, MN, 2016 to 2021
Location 2016 2017 2018 2019 2020 2021
Campbell, MN 206 CGDD 132 CGDD 123 CGDD 57 CGDD 66 CGDD 50 CGDD
Sabin, MN 189 CGDD 133 CGDD 129CGDD 42 CGDD 64 CGDD 57 CGDD

We also attempted to correlate waterhemp emergence with soil temperature. Soil temperature instrumentation measures temperature 4-inch (NDAWN) or 6-inch (Minnesota) or much deeper than weed seed emergence. We observed a stratification of soil temperature, the warmest temperature near the soil surface and the coldest 4 or 6-inch deep where bare soil temperature measurement is collected.

Soil temperature at Sabin was tracked in 2020 and 2021 (Figure 1). The only meaningful conclusion I can make, with all other factors being the same, waterhemp emergence likely would occur in 2021 on a similar date compared to 2020. However, there were differences between seasons. Fall 2019 was extremely wet with late season waterhemp seed production. These small plants made seed which was deposited on the soil surface. Moreover, fall tillage was not completed due to late season rainfall and wet fields. Finally, there was significantly more snowfall covering fields in winter 2019-2020 which protected soils from deep frost, enabling soils to warm sooner and potentially allowing waterhemp to emerge sooner in 2020 (Figure 1). Thus, one would predict waterhemp emergence in 2021 will be later than 2020.

Where do we stand and what are our best recommendations to forecast waterhemp emergence? We continue to use GDD and soil temperature but only as aids. In general, waterhemp emergence in sugarbeet seems to be geographic and season specific. For example, we anticipate and plan for waterhemp emergence on approximately May 10 in the Southern Minnesota Beet Sugar Coop region near Renville, MN. We anticipate waterhemp emergence on approximately May 15 along the boundary between Minn-Dak Farmers Coop and American Crystal Sugar Coop near Sabin, MN and Kindred, ND. Finally, we anticipate waterhemp emergence on approximately May 20 east and west along US 2 near Grand Forks. Emergence probably is even later along North Dakota 5 and Minnesota 175.

Line chart showing the bare soil temperature at 4-inches in Sabin, MN from April 1 to May 8 in 2020 and 2021

Tom Peters
Extension Sugarbeet Agronomist
NDSU & U of MN