North Dakota State University previously has been relying on north-central U.S. university data for estimating soybean yield loss due to delays with initial weed control.  A NDSU study was conducted during 2011 through 2014 at Carrington, Doyon, Langdon and Minot to document soybean yield response based on timing of initial application of herbicides. The targeted timing of herbicide treatments in Roundup Ready soybean included: 1) at soybean planting, 2) early POST (post-emergence) - average weed height at 2- to 4-inches, 3) mid POST - average weed height at 6- to 8-inches, 4) late POST - average weed height 10 inches or greater, and 5) untreated check. Soil-applied and/or burndown herbicides were applied preplant or pre-emergence. Glyphosate alone or with a herbicide tank mixture was POST applied during the study’s targeted herbicide application periods, with a second POST application used as required to maintain weed control with the various treatments.

Table 1 shows soybean yield averaged across the 8 site-years of the study. Weed control at planting provided a 5% (2.2 bu/A) yield advantage compared to the early POST initial application of herbicide. The study results confirm the importance of weed control starting at soybean planting. Also, use of soil-applied herbicides at planting will provide the opportunity to use herbicides with multiple sites-of-action to assist with managing herbicide-resistant weeds. Yield with the early POST application improved yield 4% (1.3 bu/A) and 14% (5.7 bu/A), respectively, compared to the mid- and late-POST initial application of herbicides. Substantial weed competition was evident in the study as indicated by the yield of 22.7 bu/A with the untreated check.