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Assessing alfalfa stands for winter injury

Penn State Extension

Spring is starting rather quickly this year. While soil temperatures are still somewhat cool, they will be creeping up, encouraging plants to grow and soil microbes to start their processes.

One thing that is quickly taking shape are our forages. While we are not ready to graze or harvest yet, now is a great time to evaluate your stands, particularly alfalfa.

The winter survival of alfalfa is affected by many factors including stand age, plant variety, soil pH and fertility, winter and fall moisture status, snow cover and cutting management during the previous fall.

Signs of winter injury include slow green-up, uneven or asymmetrical growth of plants and root damage. Alfalfa buds can be especially affected by poor conditions, as are formed in the fall for spring growth. If buds are killed, the plant must form new ones in the spring, delaying growth and lowering first cutting yield.

Alfalfa field assessment can begin at the surface with a stand count. A healthy stand should have 55 stems/ft2. Early assessments made before stems are visible can be done with a plant count. A high yielding alfalfa stand seeded last year should have 20 plants/ft2. Counts as low as 12 will produce good yields but will likely result in shortened stand life. Stands seeded last spring or fall with less than 12 plants/ft2 should be disked and reseeded. A high yielding alfalfa stand over 1 year old should have at least 6 plants/ft2. If plant density is less than 6, oats (2 bu/a) or Italian ryegrass (10 lbs/a) can be overseeded to increase yield this year, but the stand should be terminated after cuttings are taken.

Root assessment is another way to determine stand condition. Alfalfa roots can be evaluated by digging a few plants 4 to 6 inches deep and assessing the taproot. If the root appears to be swelled and turgid it is alive and healthy. If the root is browned, dehydrated, and ropey it is dead or dying. If 50% or more of the root is blackened from root rot, the plant will most likely die during spring green-up or later in the year.

If winter injury was an issue for you this year, there are a few management tactics you can try for 2021.

First, allow plants to mature to mid- or full bloom before cutting. This will help plants restore needed carbohydrates for subsequent production. For severely injured stands, allow plants to go to nearly full bloom for the first cut and to early flower in subsequent cuttings. Stands with less injury could be harvested somewhat earlier depending on the extent of the injury.

Second, increase cutting height, which reduces damage to new, developing shoots.

Third, soil test and address any nutrient deficiencies. Finally, refrain from cutting injured stands after Sept. 1 to allow plants to build up food reserves prior to cold and potentially damaging winter months.

Zach Larson is the Penn State Extension agronomy educator for Blair County. He can be reached at 814-414-0582.

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