Hail Damage to Alfalfa

Thursday, July 6, 2023

Thunderstorm season is upon us! Thunderstorms and hail are a normal occurrence in Minnesota during the growing season. Hail sizes can vary in size from BB to softball with the most common being pea (1/4 inch) to baseball size. Severity of hail damage to alfalfa is affected by hail stone mass and wind speed with larger hail stones causing the greatest damage.

During the 16-year period, 2007 to 2022, we averaged about 99 damaging hail reports somewhere in the state each year, with an average of 21 days per year (Figure 1; K Blumenfeld, MN. State Climatologist). Of course, these events did not always damage alfalfa. This number likely understates the incidence of hail as not all events are observed and reported.

Hail and severe winds typically damage the leaves, stem tips, and growing points which are the softest portion of the alfalfa plant. Defoliated, more lignified stem bases often will remain (Figure 2). Normally, alfalfa grows from an apical meristem at the tip of the stems. This meristem is responsible for stem elongation and development of leaves and flowers. Hail damage to the tip of the plant will stop stem elongation, and regrowth can only occur from axillary buds located on the stem or buds located on the crown

Hail effects on forage quality

Hail has a significant negative affect on forage quality. Following a thunderstorm-hail event with hail size about one inch, we photographed (Figs. 2 & 3) and sampled damaged alfalfa which was at early bud stage, and observed hail damage to the top 1/3 of the stem. A few stems that escaped hail damage still consisted of 56% leaves while damaged stems had only 35% leaves (Table 1).

Not surprising, 85% of the plant material recovered from the ground was leaves, the highest quality fraction of the forage! The remainder of the plant material on the ground was likely stem tips. Loss of leaves caused significant reduction in protein content (6%) and increase of structural component of the plant described in the cell wall (NDF) and lignin. As a result, overall cell wall digestibility of broken stems subject to hail damage was reduced. Presumably because of the wind and rain effects on splashing of soil, ash content of all fractions was increased beyond the 8% normal level.

Source: umn.edu