Hail damage in some regions this week ranged from a small smattering to 50-100% damage. Plants injured in the late vegetative or early flowering stages seldom die. A well established root system and the ability to develop secondary flower clusters help the plant recover. When buds and flowers are lost due to injury, the plant recovers rapidly by developing of flowers that normally would have been aborted. The plant also develops flowering branches from growth buds lower down on the plant, replacing to a degree, the lost buds, flowers and pods. Seed yield loss will depend on the per cent of leaves and branches lost. Click here and scroll down to Table 7 for details.
At the rosette stage, yield loss will be mainly related to lost leaf area. A general rule of thumb is that lost yield will equal about a quarter of the percentage of leaf area lost. Based on this, if 40% of petals are lost, the yield loss will be 10%.
Weed control in hailed canola. Hail that hits canola will hit the weeds, too, and if canola plants were ahead of the weeds before the hail, canola will likely be able to keep ahead of the weeds as it recovers from hail. Also remember that hailed canola is under stress and will be more susceptible to injury from the herbicide application. Finally, if canola is regrowing and immediately sending up buds, spraying herbicide could cause abortion of these buds. A plant that is already weakened and set back by hail may not be able to compensate for these losses.
New weeds that emerge after the hail are unlikely to impact crop yield. It might be better in this case to save your money until preharvest timing.
If recovering plants are still within the label staging restrictions (sending up new leaves not buds) and have yet to be treated then a statement regarding a negative impact on yield and therefore a recommendation to spray may have merit. Consider a grassy herbicide alone if weed pressure dictates.
Fertilizer top up for hailed canola? Fertilizer top dressing right after a hail has been shown in some anecdotal cases to improve canola recovery after a hail, but third party research into this practice is limited. As noted above, consider the stage of canola and its chance of recovery before investing in a fertilizer top up.
If your fertilizer rate was in line with yield potential before the hail, fertilizer top up after hail may not provide an economic yield benefit but it may provide a stress relief factor. Late fertilizer can also extend maturity, which increases the risk of fall frost reducing yield and quality. Remember, the yield response from a nitrogen application after the 5-leaf stage is reduced and any benefit may be offset by a delay in maturity.
Micronutrient or hormone treatments. Some growers are putting boron with herbicides and fungicides thinking it will boost yield for hailed, late or thin canola crops. Growers considering this treatment should leave a check strip to see whether any benefit shows up on the yield monitor. Note that the check strip should have the herbicide or fungicide treatment without boron. (The check strip should not be “no treatment” because if a benefit is observed in the treated strips, the grower won’t know if it resulted from the micronutrient or from the herbicide or fungicide.)
In the case of boron, for example, do soil tests show low boron levels? Does canola show signs of deficiency? Leaf symptoms from boron deficiency include leaf convex cupping, yellow chlorosis, red coloration or brownish or even whitish interveinal coloration. Root symptoms also are evident (small, thick roots). Boron symptoms would be worse in high pH soils.