Forage Stands – Assessing Flood Injury
There is no precise way to predict the flood damage to perennial forage fields. Every field will react differently, depending on the degree, duration and timing of the flooding, type of species, stand age, soil type and fertility.
Forage species in Manitoba have different flooding tolerances ranging from a couple weeks to a couple of months. Timing has a lot to do with how injurious flooding will be. Flooding during early spring while soils are still frozen and the weather cool, will not evoke the same kind of damage as when forages are up and growing. Flooding that takes place in the winter and leaves the forages encased in ice can cause damage. Forages in this circumstance, even though dormant, can perish due to suffocation.
When the water has receded, it is time to start evaluating your forage stand, with the final decision made when fields are dry enough to work.
To start your assessment look for surviving forage plants. If the plant is dead the crown and/or roots may already be rotting and will pull or break apart easily. Dig up a number or plants over the area of concern. If the root and crown have discoloration or soft spots, it is likely dying or at the very least will not return to full vigor. Healthy roots are whitish colored and firm and the crown should have evidence of new tiny buds.
Alfalfa is our most common legume and has a very low tolerance to flooding. Any flooding or saturated soils that has lasted more than a couple weeks will kill alfalfa. Most commonly, we see this in fields with low spots or water runs which, in mixed seedings i.e. alfalfa and grass, fills in with grasses as the alfalfa dies out. Alfalfa is self-toxic so it will not come back from the seed bank. If the alfalfa stand was older than 2 years, then seed to annual and reseed back to alfalfa the following year.
Rhizomatous rooted grasses will have a better chance of filling in voids. Grasses are not self-toxic so reseeding in the same year is possible if weeds can be controlled and a good seedbed prepared. Spacing between the plants or large areas without any surviving alfalfa or grass is a sign that your field will need rejuvenating. Take care to check for noxious or invasive weeds that may have floated in with the floodwaters; treat weeds before reseeding.
Best seeding dates for grasses include spring to mid-June or early fall if moisture is sufficient. Alfalfa is best seeded in the spring to mid-June, as fall seeding is often unsuccessful.
Annual Forages
If you have lost your forage stand to flooding, annual forages can be used to fill the gap. Oats seeded at 1.5 to 3 bu/ac (55 to 115 kg/ha) can be cut at the late milk to early-dough stage (about 8 weeks after planting) for green feed or silage. For pasture, a mixture of 10 lbs./ac (11 kg/ha) of annual rye grass or 0.3 bu/ac (20 kg/ha) of fall rye seeded with 1.5 bu/ac (53 kg/ha) of oats will provide abundant high-quality pasture when fertilized and rotationally grazed. With reasonable growing conditions, either pasture mixture should be ready for grazing about 6 to 8 weeks after planting. Both mixtures will give excellent pasture late into the fall. The mixture containing the fall rye has an added advantage as it can provide an early pasture for the following spring if given some fall rest.