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Managing Crops During a Severe Drought: Q and A

Drought and high temperatures during the summer months can affect many commodities grown by Pennsylvania field croppers. This article contains a list of questions and information for growers to utilize when experiencing such conditions.

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Updated:

July 17, 2024

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Drought stressed corn exhibiting leaf curling. J. Brackenrich, Penn State Extension.

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Corn

How does drought affect corn?

Corn growth will slow or stop when temperatures increase, and soil moisture decreases. This effect may be more pronounced in shaley or drier soils. Although we can do little to change the weather, we can make management decisions now to help minimize losses and salvage as much from a bad situation as possible. Some key questions that silage and grain producers should consider are listed below.

What should be done now?

Several things should be initiated immediately. The condition and yield potential of the crop should be assessed. Dairy and livestock producers may want to collect a forage sample to determine moisture, forage quality, and nitrate levels. Grain farmers may wish to identify ways to market their crop as silage. All producers should check with their crop insurance or FSA representative before harvesting the crop.

What is the yield potential of the field?

This will be variable depending on the severity and timing of the drought. Stress during the sixth to eighth leaf stage (V6-V8) can result in fewer kernel rows. Stress from the eighth to the seventeenth leaf stage (V8-V17) can result in fewer kernels per row. If this stress continues, it can result in delayed silking, reduced silk elongation, and poor pollination. For a more visual assessment, if corn fields are losing leaves and leaves are not unrolling at night, the yield potential will likely be low. For fields with a good stand and exhibiting leaf rolling only during the day, there may still be good yield potential with drought-breaking rain. Table 1 shows the percentage of loss per day, that plants can experience if the drought stress lasts longer than four consecutive days.

Table 1. Example corn yield loss estimates when stress occurs for four or more consecutive days. Adapted from Classen and Shaw, 1970; Rhoads and Bennet, 1990; and Shaw, 1988. From Influence of Drought on Corn and Soybean, Iowa State University.
Corn Development Stage	Estimated Yield Loss per Day of Stress (% loss/day)
Early vegetative (VE - V12)	1-3
Late vegetative (V12 to VT)	2-5
Pollination to Blister (R2)	3 – 9
Milk (R3)	3 – 6
Dough (R4)	3 – 5
Dent (R5)	2 – 4
Maturity (R6)	0

When should you decide to salvage the crop and harvest for silage?

When leaves cease to unroll at night and the tops start to brown out, the plants will probably not recover. As the browning of the crop continues, the plant will use stored carbohydrates in the leaves and stalk to sustain itself, further lowering the forage quality. Delaying harvest will reduce yield, quality, and the potential to plant a second crop. The moisture content may be higher than desired, so a dry feedstuff like chopped ear corn or hay may need to be added to adjust the silage moisture for good fermentation. Using a silage inoculum can also aid in the ensiling process of wetter-than-desired forages. Harvest should likely be delayed if the forage is extremely wet (greater than 75-80%). No definitive guidelines are available, but if half the leaves are dead or dying, they would be a good candidate for evaluation for silage harvest.

What is the yield potential of this type of crop?

A rough estimate of wet (70% moisture) silage yield is about 1 ton per foot of height of corn without ears or poorly pollinated ears. This estimate may be high on very short (1-3 foot tall) crops.

What kind of silage will this crop produce?

This crop will likely be higher in protein than normal silage but lower in energy. A ballpark estimate of silage quality might be an NEL of 0.60- 0.64 and a 9-12 percent crude protein. Some preharvest forage testing may be appropriate to assess the quality, the level of nitrates, and the potential use of the forage. Fields with some recovery potential could produce good-quality silage if it were to rain. The ear-to-stover ratio will be good in drought-stressed crops that recover, and the fiber digestibility is usually high.

Are nitrates a concern in this kind of crop?

Yes. The potential for high nitrate levels is greatest in young plants, especially in the stalks and in heavily manured or fertilized fields. The potential is generally greatest for 3-4 days following a drought-ending rain but can be a problem anytime. High nitrate levels can contribute to animal feed problems and deadly silo gas. Producers should be especially cautious when filling silos with these crops. Since nitrates accumulate in the lower stalk of the plant, risk can be reduced by leaving 8 to 12 inches of stubble in the field. This will reduce yield but also reduce the risk of nitrate-related poisonings. Because the nitrate potential can be reduced through ensiling; grazing and green chopping drought-stressed corn is less desirable as a harvesting alternative. If green chopping is the preferred method of harvest and feeding, chop and feed only what livestock can consume in a short period (approximately 2 hours). Leaving green chopped corn in bunks and wagons can worsen nitrate-related issues.

Is it worth harvesting these crops?

In some fields, it may be a tossup. The variable costs, such as fuel, labor, and repairs, associated with chopping a light corn crop are in the $15 to $25/acre range, so if producers can harvest at least one ton of silage per acre valued at perhaps $20/ton they will break even. Achieving this yield may require corn that is about two feet tall.

What crops are a potential replant following corn?

This will depend on the herbicide program used for corn. Generally, sorghum-sudan grass may be the most viable option if planting can be achieved by early August. Small grains or soybeans are also alternatives in some situations, but the dry weather may make atrazine carryover high, damaging the more sensitive crops like oats and soybeans. Check herbicide labels for replanting restrictions before making decisions or purchasing seeds. If corn fields are unsatisfactory for planting fall forage crops, producers may want to consider no-tilling into small grain stubble fields- although these soils may be problematic until it rains again.

Soybeans

Soybean plants showing signs of drought stress — Leaves beginning to curl upward is a sign of moderate drought stress in soybean plants. R. Spelman, Penn State Extension

How does drought affect soybeans?

During periods of drought, soybean leaves will curl, much like corn. They will also turn leaves upside down, showing a silverish color. Top growth will also slow down during a drought period. Plant carbohydrates will be diverted into the root system to increase the plant's ability to uptake water and nutrients. Compared to corn, soybeans can more easily recover from drought stress due to overlapping developmental stages.

Why are my soybeans flowering when they are so small?

Soybeans may begin to flower earlier than expected in drought conditions. This occurs as a survival mechanism, as stress may trigger the plant to produce seeds (reproduce) early to ensure the survival of the next generation.

How does drought affect soybean pod fill?

Drought stress may result in the abortion of flowers and pods. However, since soybeans have overlapping developmental stages, flowering will resume once favorable conditions return. Therefore, soybeans can more easily recover from drought stress than corn. If drought stress occurs during reproductive stages R3 - R4, when the upper pod is between 3/16"- 3/4", the number of pods will be reduced, and if drought occurs during pod fill (R5 – R6), the number of seeds and seed size will be reduced. However, soybean size will not be affected if rain occurs after pod fill. Generally, drought can reduce pods by up to 20% in severe conditions.

Can drought-stricken soybeans be harvested as a forage?

If yield and bean quality are severely affected by drought, soybean plants can be used as a forage crop. First, consider some of the herbicide restrictions. Unfortunately, with few exceptions, most soybean herbicides do not allow feeding the soybean plant as forage.

Plants should be allowed to mature as much as possible before harvesting if soybeans can be used as a forage. Some pod or bean development enhances the feeding value of plants harvested as hay or silage. Soybean forages are high in calcium and should be avoided as the primary forage for dry cows. It is important to ensile before plant moisture drops below 60-65%. Mix soybeans with other forages, preferably during ensiling, to enhance their palatability if possible. Stems are not very palatable; if animals have the opportunity, they will sort them out. Chopping hay and feeding it in a total mixed ration (TMR) will help prevent sorting and stretch forage supplies.

Are there other effects of drought on soybeans?

During drought conditions, microbial fixation of nitrogen will be slowed. It will resume when soil moisture returns and begin supplying nutrients back to the soybean. This can affect the growth and development of soybeans and should also be considered when accounting for nitrogen credits during the following crop.

Additionally, hot and dry conditions can favor outbreaks of certain insect pests, including two-spotted spider mites, which pose the greatest threat. These conditions reduce the generation time of the pest and suppress fungi, which generally control mites. Therefore, soybean fields should be monitored closely for the development of the pest over periods of drought. Applications of insecticides containing Dimethoate, bifenthrin, and abamectin chemistries may be used for control of spider mites if pest pressure is high and rain and cool temps are not expected. Consult the Agronomy Guide for more information on suggested insecticides.

Forages

Drought stressed hay field — Drought stressed mixed legume/grass hay field with about 6" of regrowth after 55 days. Since harvest, this field received 2.25" of rain. J. Brackenrich, Penn State Extension

What happens to alfalfa during a drought?

Alfalfa is commonly referred to as a drought-tolerant plant. During the onset of drought conditions, alfalfa will stop using carbohydrates for stem and leaf production and store those carbohydrates in the roots. This provides high levels of root carbohydrates for long-term survival if drought conditions persist, and the leaves become photosynthetically inactive. However, alfalfa's ability to survive a drought does not mean that alfalfa will not show drought-related symptoms. Water-stressed alfalfa will experience decreased stem elongation and, in some cases, mature more rapidly. Leaf production is less affected by water stress than stem elongation. This results in higher forage quality of water-stressed plants than their unstressed counterparts.

Why is my alfalfa blooming when it is so short?

As discussed earlier with soybeans, alfalfa will flower at a shorter height during stressed and drought conditions. This is a response to the plant feeling pressure to reproduce before a premature death.

Should I mow alfalfa?

The primary criterion influencing the decision to harvest drought-stressed alfalfa should be based on the cost of harvesting and forage height. Based on research from the University of Wisconsin-Madison, flowering established alfalfa over 10" can be harvested. If alfalfa is less than 10", do not harvest and allow new bud growth to grow through the standing residue when better conditions return. New or weak stands should not be harvested during drought conditions (1).

What options do I have for emergency forages?

The first thing to consider is upcoming moisture. Even supplemental forages like warm or cool-season annuals need moisture to germinate and develop. If they are planted during drought conditions, they will likely sit in the ground and suffer from diseases, poor germination, disease, or bird/mammal damage.

Once adequate moisture is on the horizon, different options depend on the operation's goals. Consider brassicas, summer annuals, or small grains. Summer annual grasses like sorghum, sudangrass, sorghum-sudan hybrids, and millet are options for summer maturing grasses. Small grains (Oats and Rye, use taller varieties) should be planted as soon as possible and harvested for silage in the milk to soft dough stage. Generally, the milk stage is less desirable than the early dough stage because it is less palatable, and studies indicate that animal performance may be reduced. Moisture levels between 60 – 70% are best for ensiling small grain silage. Small grain silage below 60% moisture is difficult to pack, and excessive heating and nutrient loss may occur. In addition, making sure that the theoretical length of the cut is less than 3/8 inch is important with small grains because some stems are hollow and filled with air. Minimizing the length of the chopped material will help minimize silage heating and maintain forage quality.

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