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Hail damage to alfalfa

Craig Sheaffer, Extension forage agronomist, Tyler Rice and Troy Salzer, Extension educators

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.

Fig. 1. Number of reports and days with 1.5 inch hail in
Minnesota, 2007-2022. Source. K. Blumenfeld, Climatologist,
MN State climatology Office.
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 (Figure 3).
Fig 2. A damaged alfalfa stand with the top 1/3 of the plant removed by hail. The remaining plant stems have a significant amount of leaf loss.
Fig 3. A hail damaged alfalfa stem with regrowth from an axillary bud on the stem. This stem will have a significant lower yield potential because regrowth will not compensate for the loss in yield from the original stem.

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.

Table 1. The percentage of leaf, protein, cell walls (NDF), cell wall digestibility (NDFD), lignin, and ash in undamaged whole alfalfa stems and damaged broken stems resulting from hail damage. Alfalfa foliage deposited on the soil surface (ground) is also shown
Fraction Leaf Protein NDF NDFD Lignin Ash
% % % % % %
Whole (undamaged) 56 23 40 51 5.5 14
Broken 35 17 50 44 7.8 10
Ground 85 30 20 56 4.1 15

Hail effects on forage yield

Fig 4. Fraction (%) of alfalfa yield at 4-inch height
increments harvested at early bud and the percentage
of leaves and stems at each height increment. For alfalfa
at the early bud stage, most dry matter yield and leaf
percentage is in the top 8 inches or half of the plant.
The bottom half of the plant is greater than 65% stem.
Leaf loss normally occurs from the bottom of stems due to
leaf diseases.
As with forage quality, the impact of hail on forage yield is related to the proportion of the stem loss. Loss of the top portions of alfalfa plants will result in significant dry matter and leaf loss (Figure 4 and 5). This impact is greater for less mature, bud stage alfalfa. Loss of the top 8 inches of the bud and flowering stage alfalfa results in about a 60% and 30% yield reduction, respectively. Leaves have significantly greater forage quality than stems. Because the leaf percentage is overall greater in the bud stage alfalfa than the flowering alfalfa, we can expect greater effect of hail damage on the forage quality of less mature alfalfa.
Fig. 5. Fraction (%) of yield at 4 inch height increments for alfalfa harvested at flowering and the percentage of leaves and stems at each height increment. For alfalfa at flowering, most dry matter and leaf yield is in the top 12 inches of the plant. As occurred at the early bud stage, the bottom 8 inches consisted of mostly stem due to leaf senescence.

Managing hail damaged alfalfa

 The University of Wisconsin (Undersander, 2008) developed specific guidelines related to harvest management of hail damaged alfalfa. A portion of those guidelines are described here. For a complete description see:
  • If alfalfa is within two weeks of harvest, has 50% of more of the stems damaged and is lodged:
    •  Wait 3 to 4 days to allow lodged stems to recover and harvest.
  • If alfalfa is within two weeks of harvest and greater than 50% of terminal buds damaged:
    • Harvest immediately because little additional growth will occur from terminal meristems. Waiting for crown shoot development will just reduce yield at the following harvest.
  • If alfalfa is within two weeks of harvest and but less than 50% of terminal buds damaged: 
    •  Allow stands to mature to normal harvest schedule and harvest. Yield will be reduced but undamaged buds will continue to grow and produce additional yield. Plants may also regrow from buds on stems.
  • If alfalfa is not within two weeks of harvest (stand generally 12 inches or less tall): 
    • Wait for stand to regrow from new shoots and harvest when forage at normal harvest height and quality. 

Hail effects on other forages 

There is a lack of information on hail effects on other forage plants. However, we can infer that hail effects to legumes and grasses with upright stems will likely be most dramatic at the the top portions of the stems. Since legume leaves are more fragile than grass leaves, we would expect less damage to grass leaves than legume leaves. In addition, grasses and legumes that do not have upright stem growth during summer and fall, will likely have less loss of yield. Of course, the size of hay will have a significant effect on damage.

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