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High and dry: Drought changes forage and affects livestock

Janna J. Kincheloe and Kevin Sedivec for Progressive Forage Published on 29 March 2018
Drought impacts forages

In the Northern Great Plains, 70 to 75 percent of annual precipitation is received between April and the end of June. Rainfall during this period is critical in order to have a “normal” or “above normal” year in terms of forage production.

Historically, the western half of the Dakotas, eastern Montana and Wyoming have experienced drought for part of the year in seven out of 10 years. In general, it is considered a drought situation when cumulative plant-year precipitation is 20 to 25 percent below average.

Extreme drought this past summer, combined with high temperatures, resulted in poor conditions for pastures and rangeland production, with production losses estimated as high as 85 percent. Developing drought management plans that include options for grazing, alternative feed sources and critical action dates is highly recommended in our area.

Beef cattle producers are heavily dependent on grazed forages to supply the majority of nutrients required by livestock. Nutrient requirements are affected by a variety of factors, including sex, age, breed, stage of production and environment. Nutrient demand is greatest at the time of peak milk production, which is generally considered to occur approximately 50 to 60 days after calving.

Timing of peak lactation (and increased nutrient requirements) varies based on time of calving. Forage quality is typically highest during peak growing periods and gradually decreases as plants mature and senesce.

Cool-season forage growth and quality peaks during spring, while warm-season forages are of highest quality in the early summer. Season of calving and species composition are two factors that influence how nutrient requirements match up with variations in forage quality. For the remainder of this discussion regarding effects of drought on livestock requirements, a spring-calving cow herd on cool-season- dominated rangeland is assumed.

From a livestock nutrition perspective, forage quality is defined primarily by intake and digestibility. As digestibility of forage increases, passage rates through the ruminant digestive tract increase, and cattle are able to increase intake. Drought can have a significant impact on forage quality and availability, making it difficult to achieve livestock production goals without supplementation.

Impacts of drought and livestock management response will vary significantly depending on timing and severity of drought. Forage quality may be increased or decreased depending on the stage of forage maturity and precipitation. Moderate moisture stress typically causes a reduction in stem growth, which can increase the leaf-to-stem ratio and forage quality. However, severe stress will reduce overall forage quality as plants senesce.

A drought occurring early in the spring will have the greatest impact on forage production since 80 percent of growth is completed by July 1 for cool-season grasses and July 15 for warm-season grasses. If forage availability is limiting, energy will likely be deficient.

This could negatively impact conception rates, particularly for thin and young cows. Evaluating cow body condition score using a scale of 1 (extremely thin) to 9 (obese) is one of the best ways to get an indication of energy status.

During a drought, it is important to condition score cows to determine if early weaning should be considered or if supplemental feed is necessary. Weaning calves early will reduce nutrient requirements and help maintain reproductive performance in times of nutritional stress. If cows start to lose condition even with adequate grass in the pasture, then forage quality may be an issue.

If forage losses are severe, it may be necessary for producers to consider reducing the number of animals to match forage demand with supply. Other options are to utilize alternative forage sources such as fall-seeded annuals, small grains or cover crops. If the drought ends by summer, producers would likely observe only a slight increase in total production, but quality should increase as plants green up.

A drought that begins in late summer is more likely to impact forage quality rather than quantity, primarily because adequate moisture early in the growing season allows grass to grow normally and reach maturity. This results in a mature plant community that is brown and dry with no new leaf tissue. Even if forage supply is adequate, it is likely to be deficient in protein and energy (See Figures 1 and 2).

Moist summer

Dry summer

In this situation, supplementation may be used to correct nutrient imbalances.

Other potential nutritional concerns with drought-stressed forages are deficiencies in important minerals such as calcium, phosphorus and potassium, and vitamins such as A and E. The presence of nitrate-accumulating broadleaf plants such as kochia, pigweed and Russian thistle may also increase the risk of nitrate toxicity during drought. Sampling and analyzing standing forage is the best way to determine what deficiencies or toxicities may exist.

Drought that begins in the spring and extends throughout the summer presents the greatest challenge to livestock producers because of impacts on both forage quantity and quality. In this situation, alternative feed sources or reductions in herd size are necessary. Additionally, drought extending into fall will impact growth of cool-season leaves, which represents the starting growth for the next year’s production.

Warm-season grasses are not affected as they go through a true dormancy, while cool-season grasses grow all winter long. Therefore, a fall drought can lead to delayed spring growth, which results in the need to delay turnout the following spring.

Past management and amount of litter/standing vegetation will have the greatest impact on production potential during a drought. If plants are overgrazed and litter is minimal, heat from the sun contacts the soil at a greater intensity, creating high losses of water due to evaporation.

This heat makes the plant respire a greater amount of water through evapotranspiration to keep cool. With droughty conditions and a lack of water, plants must senesce early to survive, thus creating more impacts on forage production.

Grazing management practices that allow for recovery and create some litter – but not too much – help plants maintain high vigor, leaf growth and cover. Proper grazing management will also help ensure optimal livestock performance in times of limited forage quantity and quality.

Understanding that potential impacts of drought vary depending on species composition, plant stage of maturity and livestock requirements is important in developing practices that minimize financial and production losses.  end mark

PHOTO: Preparations for drought require understanding the potential impacts on forages, not just in quantity but also in quality, and changing management practices to minimize losses. Photo by Getty Images.

Kevin Sedivec is an extension and rangeland specialist at North Dakota State University and also interim director for North Dakota State University’s Central Grasslands Research Extension Center.

Janna J. Kincheloe
  • Janna J. Kincheloe

  • NDSU Area Extension Specialist, Livestock Systems
  • Hettinger Research Extension Center
  • Email Janna J. Kincheloe

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