In practice, many producers seek to optimize the feed utilization in the year of termination. However, termination strategies that consider plant physiology are often the most successful, as they will target either the point in the cycle where the plant is at its weakest energy status (tillage operation) or the point in the cycle where energy and nutrients are actively translocated to the root and storage organs (herbicide application).
The best time to initiate forage termination is the year prior to seeding an annual crop. Herbicides should be applied when target plants are actively growing. When using glyphosate, it should be applied at least five days prior to cultivation. If the forage is to be harvested, glyphosate should be applied as a pre-harvest treatment three to seven days prior to the final harvest. This allows adequate time for uptake and translocation of the herbicide within the plant. The challenge with terminating forage stands is that environmental conditions vary from year to year, which can cause challenges not only with forage termination, but also affect the success of the annual crop the next year. If the forage plants are not actively growing at the time of spraying, the chemical uptake will be reduced and herbicide applications can be ineffective. Fall termination has limitations in the Brown and Dark Brown Soil Zones or when conditions are dry. During periods of drought there may be a minimal amount of actively growing forage material present, resulting in poor uptake of chemicals. In these areas it may be more effective to apply glyphosate products or other herbicides prior to the first harvest (i.e. summer termination). In wet years, tillage alone will not likely result in effective control regardless of stand composition, and may result in plant regrowth before and after the next crop is established.
Forage plants should be at or near the normal stage of cutting when herbicides are applied. Depending on the timing of the first forage harvest, this may be a pre-harvest application (late-bud to early bloom) or a post-harvest application on the regrowth (early bud). Glyphosate can be applied to forage stands three to seven days prior to a final hay cut or grazing event without adverse effects on forage quality or animal health. This will give the glyphosate time to translocate to the roots. Waiting more than seven to 10 days after spraying forage with glyphosate will result in reduced forage quality and quantity.
Termination during the growing season (July to September) allows for the decomposition of existing roots and sod, making seeding easier the following year. The disadvantage with summer termination is the potential for incomplete kill and forage and weed regrowth. Summer termination generally requires a follow-up herbicide treatment later in the growing season or the following spring prior to seeding.
Alternatively, producers may apply glyphosate to forage stands three to seven days prior to the last cut (August to September) before rotation to annual crops. Forage stands at the time of spraying should be at or near the normal stage of cutting. Perennial legumes, such as alfalfa, should be at the early bud to early bloom stage (20 to 25 cm high). Perennial grasses such as orchard grass, meadow brome, meadow foxtail, meadow fescue, smooth brome, and quackgrass should have at least three to four leaves. Late summer (August) to early fall (September) herbicide applications to actively growing plants are recommended for best results, as herbicides are readily translocated to the roots, crowns, and buds at this time. Another advantage of spraying in the fall is the potential for a second (or third) hay harvest in the final year of production, which may offset some of the costs of stand termination.
Glyphosate works best when temperatures are near 20 C and weeds and forage are actively growing. Research at Agriculture and Agri-Food Canada’s Brandon Research Station indicates that glyphosate effectiveness may be reduced when air temperatures are below 15 C, particularly if applications rates are less than 0.2 litres per acre. As application rates increase, the effect of temperature decreases. For herbicide applications in late September, there is an increased potential for frost to impact plant growth and herbicide uptake and translocation. A light frost of 2 to 3 C just before or after a glyphosate application will not reduce control, provided daytime temperatures return to the mid-teens. If a heavy frost (-4 C or lower) occurs, wait three days and then only apply glyphosate if the plants have recovered and are actively growing. Glyphosate control will be reduced when applied after a frost that has killed more than 40 per cent of the above-ground plant tissue.
Timing of termination using herbicides also has an impact on re-cropping options. If you are planning to seed a broadleaf crop following the removal of the alfalfa, then fall treatment with glyphosate is required to get adequate control and to minimize herbicide residue concerns. If you are seeding a cereal, use a glyphosate + 2,4-D/ dicamba mix in the spring after the alfalfa has broken dormancy and has begun to produce new leaf tissues. As long as there is enough late summer moisture to keep the forage from going dormant, control levels achieved using fall applications on actively growing plants are generally more consistent than spring applications.
Spring termination of forage stands either with or without herbicides do not generally work as well as termination in the year prior to seeding. In order for spring herbicide applications to be effective, sufficient plant material must be present. Grasses should be at the three to four leaf stage to allow for good herbicide coverage. For effective control of legumes, consider mixing products such as 2,4-D or dicamba with glyphosate. Follow label recommendations and ensure products are registered for tank mixing.
Spring termination of forages will delay the date of seeding of the next crop. Three to five days are needed after herbicide application to allow thorough translocation into the plant. Allowing for sufficient forage growth plus time for the herbicide to translocate in the plant results in seeding delays of two to three weeks as compared to fall termination. This is significant, as early seeding is known to increase yield potential of grain crops in Western Canada.
Field experiments in central Alberta have shown that spring termination of alfalfa/grass forages using a mixture of glyphosate and 2,4-D (0.67 litres and 0.3 litres per acre, respectively) or glyphosate alone resulted in lower yields of the subsequent annual crop (barley) compared to fall-applied herbicides or conventional tilled plots. The lower yield in spring herbicide-terminated forage plots may be due to poor control of forage plants and perennial and annual weeds. Like many weeds, both alfalfa and forage grasses are strong competitors and can reduce crop yield. Lower crop yield following spring terminated forage stands may also be a result of delayed decomposition of glyphosate-treated forage residues, reducing the availability of nitrogen, and a higher incidence of scald and net-blotch. It should also be noted that available soil moisture may be reduced, as the forage species needs to grow to an appropriate stage for spraying. Since soils will be drier with spring termination, crop yield is very dependent on timely and adequate rainfall following seeding. Furthermore, herbicide options may be more limited because of soil residual concerns.