Thunderstorms and hail are a normal occurrence across the Midwest U.S. during the growing season. Hail and wind damage to alfalfa fields leads to questions regarding harvest management of the forage crops, especially related to the stage of growth when damaged and the time proximity to the next planned harvest. Hail damaged fields vary in degree of severity, ranging from some terminal bud and leaf damage to completely defoliated plants. Stands may also be lodged by accompanying wind and rain.
Alfalfa terminal bud damage.
Alfalfa damaged by hail.
Hail physically defoliates alfalfa and can damage its growing points. Alfalfa grows from the terminal growing point (top of each stem) of the plant. If these are damaged, further growth in height is stopped on that stem. If hail damages the terminal growing point of a stem, the plant must regrow as branches from buds further down the stem or from crown buds.
Yield losses will be in relation to the percentage defoliation. Data collected at University of Wisconsin Marshfield Research Agricultural Station suggest that alfalfa yield losses from hail damage on first cutting will be approximately 35 pounds of dry matter per acre for each percent of defoliation occurring with in two weeks of harvest.
Hail damage losses for later cuttings are usually less. Forage quality losses from hail also occur, since the highest quality portions of the plants are removed. However, these losses are small relative the yield loss.
Hail damage occurring earlier than two weeks before harvest generally occurs to plants still with good potential to produce new crown shoots and a new growth cycle. The resulting new growth will delay the next harvest. There is concern that hail damaged and soil contaminated forage can adversely affect fermentation during silage making. If defoliation from hail was greater than 50 percent and within two weeks of harvest and the intention is to make haylage, flail chop remaining forage back onto ground immediately, and allow the next growth cycle to come from newly emerged crown shoots.
Guidelines for managing hail damaged alfalfa.
Wind, rain and hail can lead to significant lodging of alfalfa. When harvesting a lodged alfalfa crop, research has indicated that disc mowers, especially those with angled knives, will pick up more forage than sickle bar mowers. Harvesting against the direction the forage is leaning allows more downed forage to be harvested. With both mower types, tilt the cutter bar or discs forward to increase forage picked up. When using a sicklebar mower, move the reel forward and down, and increase reel speed to help pick up downed forage.
Adapted by Stephen K. Barnhart, Iowa State Extension Agronomist, from a May 2008 newsletter article by Dr. Dan Undersander and Krishona Martinson, Uinv. of Wisc. Extension.
Stephen K. Barnhart is a professor of agronomy with extension, teaching, and research responsibilities in forage production and management. Barnhart can be contacted at (515) 294-7835 or by email firstname.lastname@example.org.