What to Look for in Properly Staging Pre-Harvest Glyphosate Application in Cereals
Staging a crop for preharvest glyphosate application for perennial weed control can be difficult when there is variability of crop staging within the targeted field. As well, kernels in the same spike will reach physiological maturity at different times, with the seeds in the middle of the head maturing first.
Basically, for wheat you want to apply the preharvest glyphosate when grain moisture of the wheat crop is less than 30%.
In terms of visual assessment, the wheat crop must be in the hard dough stage. This is when the kernel has become firm and hard and a thumbnail impression remains on the seed (see Figure 1). Remember, you can’t rely on the color of the field as an indicator. Walk the field and hand thresh heads to determine kernel staging.
Figure 1: Kernels at various times during grain filling: a) kernel at watery ripe, b) kernel at late milk, c) kernel at soft dough, d) kernel at hard dough showing loss of green color, and e) kernel ripe for harvest.
SOURCE: GROWTH AND DEVELOPMENT GUIDE FOR SPRING WHEAT. 1995. S.R. SIMMONS, E.A. OEKLE & P.M. ANDERSON. PHOTOGRAPHER: DAVE HANSEN.
Loss of green from the kernel
- The first indicator is the loss of green in the kernel, and the appearance of a dark layer of cells or pigment along the crease (Figure 1). Kernels in the same spike will reach physiological maturity at different times, with the middle of the head maturing first.
FIGURE 2: WHEAT KERNELS BEFORE (TOP) AND AT (BOTTOM) PHYSIOLOGICAL MATURITY. NOTE THE DARKENED PIGMENT STRAND.
Another visual indicator for wheat is a change in color of the peduncle, which is the part of the stem located just below the head. It will have turned very light green or yellow at physiological maturity (Figure3).
SOURCE: TOPICS ADDRESSING SMALL GRAIN CROP DRY-DOWN AND HARVEST. 2015. JOCHUM WIERSMA, SMALL GRAINS SPECIALIST; DOUG HOLEN, CROPS EXTENSION EDUCATOR AND PHYLLIS BONGARD, EDUCATIONAL DEVELOPMENT AND COMMUNICATIONS SPECIALIST
At the end of the hard dough stage, the kernel has reached its maximum dry weight and the wheat is therefore physiologically mature, i.e. no more weight is added to the grain. Therefore, final yield has been determined.
If application of a preharvest glyphosate occurs prior to the 30% moisture content, yield can be reduced, along with quality factors such as test weight. In addition, early application prior to the recommended timing may result in grain with glyphosate levels above maximum residue limits. This could have implications depending upon target market.
When it comes to timing an application on a variable field, this will be difficult but remember its likely better to apply on the later side than too early. Also remember that depending on weather conditions, glyphosate can take up to 2 weeks for optimal weed control. However, under hot, dry conditions harvest could commence is as little as 7 days after application. Therefore, keep harvest timing and weather forecasts in mind as well when planning your preharvest application.
Another thing to keep in mind is that not all glyphosate products are registered for preharvest application on all crop species – always refer to individual crop labels for a list of registered uses and crop species. Check with malt barley or milling oat buyers prior to application to confirm acceptance of glyphosate-treated grain.
Following label instructions and keeping in mind pre-harvest intervals are also key component in Cereals Canada’s Keep It Clean initiative. More information is available at http://www.cerealscanada.ca/keep-it-clean/
- Glyphosate can be applied when seed moisture is less than 30 per cent, in the greenest/least mature areas of the field. At this stage, the seed is firm and reasonable pressure from your thumbnail will leave a dent in the seed.
- Never harvest earlier than recommended; usually 7 days after application. Check the product label for the recommended PHI.
- Oats may not be accepted if treated pre-harvest – growers are advised to check with their grain buyer before applying glyphosate to oats.
- Malt barley will not be accepted if treated pre-harvest.
What happens if the cereal crop is terminated before physiological maturity?
Swathing or applying glyphosate before the crop reaches physiological maturity will result in:
Test weight losses.
Green kernels in the harvested grains.
Losses get progressively worse the earlier the crop is cut or treated. Research at North Dakota State University (NDSU) in spring wheat and durum showed that swathing the grain at 45 percent moisture caused a 1 to 2lb reduction in test weight and about a 10 percent reduction in grain yield.
Article revised by Jason Voogt, Owner/Lead Agronomist at Field 2 Field Agronomy Inc.
Original article written by Pam de Rocquigny, former Provincial Cereal Crops Specialist, Manitoba Agriculture (now MWBGA General Manager)