Managing Forage Stands under Water

Prepared by Glenn Friesen, MAFRI Forage Specialist (BDS), Manitoba Agriculture, Food and Rural Initiatives

Flooding a forage stand limits the amount of oxygen in the soil profile, and since plant roots require oxygen to remain healthy, plant productivity and survival is reduced when soil moisture levels are too high. There is no precise way to predict the flood damage to perennial forage fields. It is known that alfalfa is not very resistant to flooding, but reed canary grass can withstand prolonged flooding. Commonly used forage species are between these two extremes. Every field will react differently depending on the degree and duration of flooding, the species present, the age of stand, the health and vigor, fertility level, stage of development of the plant at the time of flooding and the temperature. Most alfalfa hay stands and pastures in the eastern and central parts of Manitoba will be delayed in harvesting due to either slowed spring growth or saturated soils. Below are some tips for accessing the condition of your pasture or hay land.

Currently, above average spring precipitation has left some alfalfa stands suffering this spring. Alfalfa can generally withstand 1 to 2 weeks of fully saturated soils, whereas alsike clover is a little more tolerant at 2 to 3 weeks, and red clover likely the most tolerant at around 3 to 4 weeks. The extent of flood damage to fields can best be determined when the fields become dry enough to walk on. If stands are beginning to yellow from water logged soils, be prepared to assess the stand once growth continues. Assess your stand when alfalfa is about 6 inches in height using stems per square foot as your density measure. A stem density of 55 per square foot has good yield potential. There may be some yield loss when stem counts are between 40 and 50, and consider replacing the stand if there are less than 35-40 stems per square foot and the crown and root health is poor.

If the stand has been severely damaged, the only practical solution is to cultivate the field and re-seed. Seeding in the same field is possible if it is less than 2 years old; however it is recommended to use another field if the stand is older. Where the stand has been only partially damaged and is judged worth saving, weed control will become a problem because of the thinning-out of the stand. More herbicide options exist for grassy weed control in alfalfa than broadleaf weeds. Many fields have seen dandelion populations progress over the last few growing seasons, and unfortunately, there is no good option available for controlling them in alfalfa. Consider planting a new hay stand rather than spending good money and attempting to control them.

Re-seeding alfalfa–if necessary–should be done by mid-June. To increase the chances of establishing a stand at this later-than-normal seeding date, these fields should be seeded with a companion crop to protect the seedlings from hot weather. To minimize the competition effect, a half rate of oats or barley should be used. The cover crop should be harvested as silage to minimize seedling damage beneath the swath. Late summer seeding (August) of alfalfa may also be an option for soils with sufficient moisture; however does come with risk. In this case, it is recommended to seed without a cover crop, and seed early enough to ensure the plant receives at least 6 weeks of good growth before a killing frost.

Grasses as a group tolerate fully saturated soils more than alfalfa. Bromegrass will generally withstand over 3 weeks of saturation, while meadow fescue, meadow foxtail and timothy can withstand 6 weeks or more. However, if the stand has been severely damaged and cultivation is deemed the only solution, re-seeding of the grasses should be delayed until August. This will allow for good seedbed preparation.

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