Richard Taylor, Extension Agronomist; rtaylor@udel.edu
Over the past few weeks, a number of hail storms have impacted corn, soybeans, and small grains on Delmarva. In some of the wheat fields I’ve visited, the field sometimes appears to have heads remaining although when closely examined little grain actually remains while other fields appear to have been previously combined with a stripper head leaving the stem upright but little to no grain on the head. Barley fields have faired even worse with some flattened so badly that they appeared to have been previously combined with the straw baled off even though this was not the case. In most cases, these fields will be total losses; and if the rains continue, double-cropping to soybeans may not be possible. If any of the fields are successfully planted to soybeans, be aware that volunteer barley or wheat will be a major problem.
The real dilemma growers face is whether to replant corn and soybean fields that have been hard hit with hail. For corn, the impact of hail damage really depends on the type and severity of damage and the growth stage of the crop. Since all our corn is still in the pre-tassel stage, I’ll concentrate on that aspect. Keep in mind that if the damage is restricted to leaf shredding, grain yield reductions are not directly proportional to the amount of leaf area loss since there is an increase in dry matter production in the remaining leaf area and movement of dry matter from other plant parts later to the developing ear. Very young corn if able to grow through the damaged top will produce a fully functioning plant since many of the lowest leaves are lost to shading and leaf aging before the plant even tassels or silks.
Leaves beginning with the first rounded tip leaf are counted as each collar region becomes visible. Little yield loss occurs before about the 7th leaf stage (although many of the corn hail damage estimate charts actually count the upper most leaves as long as about 50% of the leaf is exposed from the whorl and the leaf tip points below a horizontal line). At this stage, which should equate to five to six leaves with collars visible, a one percent yield loss can occur if 40 to 45% of the leaf area is missing or dead but even at 100% leaf loss the yield loss will only be about nine percent. As another example at the 10 leaf stage which is about the size of the largest corn I saw with damage, yield loss is one percent at 25% leaf loss, six percent at 50% leaf loss, nine percent at 75% leaf loss, and 16% at 100% leaf loss (see photo 1).
Photo 1. Hail damaged corn with an estimated leaf loss of >50%.
An added complication occurs when the growing point is above ground and the hail damages the stem below the growing point (Photos 2 and 3). If the hail impacts the stem causing minor injury (Photo 2), the leaf loss tables from charts assessing hail damage to corn can be useful by themselves but if the hail either breaks the stem below the growing point (Photo 3) or causes such severe injury that disease issues become important, another factor must be considered. This added factor is the impact of stand reductions on yield losses. Many of the available fact sheets on assessing hail damage include this type of information and the two charts must be combined to estimate yield losses.
Photo 2. Hail damaged corn with injury to the stem from the impact of the hail but without clipping the stem (note that the plant was bent over to allow a better photo of the injury).
Photo 3. Hail damaged corn where the stem has been clipped below the growing point.
One of the most difficult decisions to make is when to evaluate a field to see if it will grow out of the hail damage. In most cases, in 7 to 10 days after the hail you should be able to see renewed growth and estimate whether the new leaves will successfully emerge. This is especially critical on very young (less than five leaves emerged) corn that has been totally flatted or clipped by hail. In older corn where only the leaves have been shredded (Photo 1), new growth can often be identified in as little as 3 to 5 days but wait about a week to get a better assessment.
Should the corn be replanted is the question most frequently asked and the answer almost always is no unless the stand loss is nearly 100%. The reason is that replanting corn in our area this late in the growing season seldom results in enough yield to justify the expense of more seed, more fertilizer, and more labor and time to plant. With the added delay of waiting to evaluate regrowth and, this year, for the field to dry enough to replant, I doubt if any replant decision will result in making you money.
What about soybeans that have been damaged by hail? In most cases, yield loss from hail will be primarily due to stand reductions. Generally, within 5 to 10 days after the hail damage occurs you can make an assessment of the recovering stand to estimate yield loss (Photo 4). In almost all cases, unless the stand has fallen well below about 75,000 plants per acre (one plant per foot of row on 7 inch drilled rows), replanting will not be profitable. The combination of yield loss from a delay in planting of several to many weeks from the ideal time plus the ability of soybeans to compensate in yield potential for low populations will make replanting unnecessary and unprofitable. Although I did see some plants clipped off below nodes where secondary buds can develop, many plants were pock marked from the hail damage and will quickly recover (photo 5).
Photo 4. Hail damaged soybean in double-cropped barley showing bud development at the cotlyledonary nodes about 5 days after hail.
Photo 5. Note pock marked (holed) leaves from hail damage in double-cropped soybeans.