Alfalfa news: Summer 2024

Craig Sheaffer, Extension forage specialist, and Bruce Potter, Extension IPM specialist, and and Deborah Samac, Forage pathologist, USDA-ARS.

This year’s above normal rainfall in June and July have had multiple direct and indirect effects on alfalfa forage yield and quality. On most soils there is adequate water for good forage growth, but untimely rainfall has had some detrimental effects. These include rain damage, windrow disease, and damage to alfalfa plants by insects and leaf disease.

Problem 1: Rain damage

Rained on hay and windrow disease.

This year’s frequent rainfall events presented very narrow windows for harvest of alfalfa and other forage crops. It was often impossible to harvest alfalfa as hay when 3-4 drying days were required. Rainfall at any time after cutting results in losses of forage yield and quality. These losses are greatest for partially dried forages due to leaf loss. Average yield losses range from 20 to 30%, but can be as high as 100% if no hay is harvested. Losses in forage digestibility and protein be as high as 25% per inch of rain.

Another issue is loss of quality with delays in harvest to avoid rain. First harvest alfalfa forage crude protein declines about 0.25% per day with digestibility declining about 0.43% per day. Relative feed value (RFV) declines about 3 units per day.

Solution

Designate fields for production of high- and low-quality forage. For high quality forage production at bud stages, harvest as haylage or baleage to remove forage in 1-2 days. For lower quality forage, delay the harvests until later maturities when longer periods of rain-free weather occur.

Problem 2: Windrow disease

Loss of alfalfa stand due to windrow
disease.

Rain delays in windrow pick-up can result in strips of uneven regrowth in fields or alfalfa. Windrows of unharvested forage interfere with regrowth, damage stands and reduce subsequent yields. Windrow disease can cause weakening of plants and stand loss because plants that are not smothered will mature faster while plants temporarily smothered due to delayed windrow pickup mature slower. Some plants may be flowering while other may be vegetative at harvest.

Solution

Remove windrows from the field as soon as possible. If windrow removal is delayed and uneven regrowth occurs in the field, wait to cut until stunted plants begin to bloom so you can avoid weakening them even more. This will result in overall low-quality hay from a field that may be best suited to livestock with lower nutrient requirement but will allow the stunted plants to recover.

Problem 3: Hopperburn

Hopperburn in alfalfa.

Potato leafhopper (PLH) is a consistent threat to alfalfa yield and quality at second and third harvests in Minnesota. This year because of delays with timely harvest and spraying, potato leafhopper effects are more dramatic in some areas. PLH migrate from the southern United States on wind currents and start arriving in the upper Midwest in mid to late May; large populations develop and affect the second and third cutting of alfalfa as well as new seedings. PLH damage plants by injecting saliva and removing nutrients, and disrupting phloem function. The resulting symptoms are a distinct yellow to red V-shape on leaf tips and plant stunting.

Solution

Early cutting and insecticide application based on PLH populations are the most effective approaches for leafhopper control. PLH numbers should be determined by in-field scouting following harvest to identify critical PLH numbers. Don’t wait for visible symptoms to occur because insecticide application might not have any benefit. For stands within a week of harvest, harvest as soon as possible. Glandular-haired alfalfa varieties with potato leafhopper resistance have been developed that provide significant protection against PLH damage; however, because the level of resistance varies among varieties, insecticide spraying may still be necessary.

Another approach is the use of alfalfa/grass mixtures. Grasses are not utilized as host plants by the PLH, and it has been shown that mixed stands of some grass species with alfalfa can reduce PLH population buildup.

Problem 4: Leaf diseases

Humid conditions and delayed harvests of alfalfa have led to significant alfalfa disease issues. These include common leafspot, Leptosphaerulina leaf spot, spring black stem and leaf spot, and Stemphyllium leaf spot. Leaf diseases primary affect alfalfa yield and quality by causing loss of leaves lower in the alfalfa canopy.

Leptosphaerulina leaf spot or Lepto leaf spot.

Common leaf spot in alfalfa.

Solution

Harvesting alfalfa at late vegetative to bud stages (a 28 to 30-day schedule) will minimize the alfalfa leaf loss as symptoms of many foliar diseases do not occur until 3 to 4 weeks of regrowth. Fungicide treatments also sometimes have value in reducing leaf diseases but are generally not cost effective. Unfortunately, varieties are not available with resistance to leaf diseases.

Information sources

Hodgson, E. 2014. Managing potato leafhoppers in alfalfa. Iowa State University Extension. https://crops.extension.iastate.edu/cropnews/2014/06/managing-potato-leafhoppers-alfalfa

Hanson, A. 2019. Scouting for insects in alfalfa. MN Crop News.
https://blog-crop-news.extension.umn.edu/2019/06/scouting-for-insects-in-alfalfa.html

Hanson, A. 2023. Updated alfalfa weevil management recommendations and challenges for 2023. https://blog-crop-news.extension.umn.edu/2023/05/updated-alfalfa-weevil-management.html

Martinson, K. et al. 2023 Harvest moisture level and bale wrapping affects hay quality for horses. https://extension.umn.edu/horse-nutrition/harvest-moisture-level-and-bale-wrapping-affects-hay-quality

Potter, B. 2024. Alfalfa insects-what to look for and how to scout. https://extension.umn.edu/forage-pest-management/alfalfa-insects-what-look-and-how-scout#potato-leafhopper-1119160

Smith, et al. 2015. Using Fungicides on Alfalfa for Dairy Production in Wisconsin. https://cdn.shopify.com/s/files/1/0145/8808/4272/files/A4090.pdf

Stordahl, J. 2023. Wrapping hay. https://extension.umn.edu/forages/forage-harvest-and-storage

Undersander, D. et al. Relative forage quality. University of Wisconsin Extension. https://cropsandsoils.extension.wisc.edu/articles/relative-forage-quality/