Chris Teutsch, Extension Forage Specialist
Southern Piedmont AREC
It has been a tough summer for pastures in many areas of Virginia. Dry weather and high temperatures have limited forage growth causing many pastures to be grazed closer than they should have been. Moisture is a primary factor limiting forage growth in Virginia. In most cases it is not total annual precipitation, but rather the seasonal distribution of rainfall. High temperatures coupled with short-term drought stress commonly limits cool-season grass growth during the summer months. Elevated temperatures and the lack of dependable rainfall during July and August clearly illustrate the need to incorporate drought resistant warm-season grasses into our forage systems. However, it is important to note that even warm-season grasses require some rainfall to remain productive.
Plant Growth and Drought Stress
Shoot growth slowed. Plant leaves grow by increasing cell numbers through division and then expanding these cells using water in much the same way as air is used to blow up a balloon. When water is limiting cell expansion is reduced.
Photosynthesis continues. Initially photosyythesis continues, which results in an increase in sugars and carbohydrates in the plant. This increase in sugars and carbohydrates (energy) helps the plant to adjust to the drought stress, maintain the growing point during dormancy, and regrow after dormancy.
Grazing closely reduces the plants ability to adapt to drought stress. So it is important to leave at least 2-4 inches of leaf area on pastures at all times.
Root system increases growth. The roots of the plant expand to explore a larger soil volume for water.
Stomata (holes) in the leaves call stomata close to reduce water loss. This not only reduces water loss, but also limits carbon dioxide from entering the leaf.
Photosynthesis is reduced. Since carbon dioxide is required for photosynthesis, this process can no longer continue after the stomata have closed.
Leaf temperatures increase. Water evaporating from the stomata cools the plant leaf. After the stomata close, leaf temperatures increase and the leaf will eventually overheat and die.
Leaves die. The loss of leaf tissue is a survival mechanism of the plant, which allows all excess water to be shifted to maintain the growing point of the plant. Although the plant appears dead, it is really in a state of dormancy and will initiate growth when conditions are favorable.
Plants dormant but still require energy. During dormancy the plant still consumes a small amount of energy to maintain the growing point.
Plants need energy for regrowth. After rain, the plant will break dormancy and energy will be required for growth.
Managing Pastures Drought Stressed Pastures
In most cases drought alone rarely kills well managed pasture grasses. However, drought coupled with others stresses can weaken, thin, and even kill pasture stands. These stresses include poor fertility, overgrazing (prior and during drought), and elevated pest pressure. Although droughts can not be predicted or prevented, they can be prepared for. The best management strategy is to continuously prepare for drought. This is accomplished by using good pasture management. The following suggestions will help to maintain healthy pastures that will grow longer into a drought and recover faster after rain comes.
Maintain correct soil pH. Soil pH for grass-clover pastures should be maintained between 6.2 and 6.5. Lime pastures more frequently rather than waiting and applying large quantities of lime at once. Use dolomitic lime when magnesium levels are low.
Maintain phosphorus and potassium. Phosphorus and potassium levels should be maintained in the high range. Phosphorus is important in element in the compound ATP which is the energy currency in the plant. It is also plays a major role in root growth and survival of newly established seedlings. Potassium is required in relatively large quantities and substantial amounts are removed from fields where is when hay is cut. Potassium is involved in stand persistence especially for legumes. Proper levels of this element increases winter hardiness and disease resistance.
Rotationally graze pastures. Energy is required for pasture regrowth after grazing. The source of this energy is photosynthesis taking place in residual leaf area and carbohydrates stored in the plant. Rotational grazing allows producers (rather than the animal) to regulate the amount of leaf area retained after grazing and the amount of time pastures are rested between grazing events. Most pastures should not be grazed closer than 2-4 inches, but 4-6 inches would be better. Rest periods are important since they allow the plant time to recharge the carbohydrates that were utilized for regrowth after grazing. Rest period length will vary with season and weather conditions. In general shorter rest periods are required in the spring and fall when plants are actively growing while longer periods are required in the summer months when plant growth is slowed by high temperatures and moisture stress. Rotationally grazed pastures recover from drought faster than continuously grazed pastures.
Maintain stubble. Maintaining 4-6" of stubble helps to shade the soil surface and prevent water evaporation. Stubble also shades grass crowns aiding in the survival of species such as orchardgrass and endophyte free tall fescue.
Maintain healthy root systems. Roots not only anchor plants to the soil, but also provide a means to absorb water and nutrients required for growth. Roots are out of sight and often out of the mind of many producers, but their importance to the overall health of the pasture should not be underestimated. Close and frequent grazing reduces the size and depth of the root system making the plant less tolerant to drought. Rotational grazing helps to maintain a healthy root system that can sustain plant growth longer going into a drought and speed growth after rain finally comes.
Feed hay when pastures are not growing. Pastures can easily be damaged by overgrazing during a drought. A better alternative is to restrict animals to a single paddock and feed hay. This will isolate the damage to one paddock and will allow for rapid recovery of the other paddocks when rain finally comes. Resist the temptation to open all the gates and let the animals roam wherever they want.
It is easy to say don't graze drought stressed pastures, but in many cases there may no way to avoid it. Most healthy pastures should be able to withstand some severe grazing (removing most of the leaf area, but not repeatedly) during a drought. However, pastures that are overgrazed (closely grazed and never allowed to rest) will be much more susceptible to injury from grazing during drought. In addition, not all plant species respond to drought and grazing in a similar manner. Below is a brief description of the responses of common forages species to drought and grazing.
Alfalfa. Alfalfa possesses a deep taproot making it one of our most drought tolerant legumes. During periods of severe drought and high temperatures, alfalfa will go dormant but is generally not damaged. During these periods alfalfa will bloom at a short height, and can be grazed off without injuring the stand.
Red Clover. Red clover also possesses a taproot, but it is much shallower than alfalfa. Drought stress can injure established stands of red clover, shorting stand life. Hot and dry conditions are especially damaging to newly established seedlings.
White Clover. White clover is a relatively shallow rooted legume. Production during drought is low, but plants usually persist and regrow from either stolons or hard seed.
Sericea lespedeza. Sericea is a deep-rooted warm-season legume that has good drought tolerance. It thrives on acid soils with low fertility where other legumes will not persist.
Orchardgrass. Orchardgrass is a strong perennial grass with fair drought tolerance. This grass will persist during hot and dry conditions if it is not overgrazed. Orchardgrass will not tolerate close and frequent grazing and therefore works best in rotationally grazed systems. Orchardgrass is not as well adapted to southern and eastern Virginia and will not persist under poor management.
Tall Fescue. In Virginia, endophyte infected tall fescue is the best adapted cool-season grass and will in most cases survive even severe drought. It is more tolerant of mismanagement that orchardgrass, but also responds well to rotational grazing. The endophyte imparts grazing and drought tolerance to tall fescue, thus endophyte free varieties are not as tolerant to drought stress, but can survive with optimal management.
Kentucky Bluegrass. Bluegrass is a sod forming perennial cool-season grass that tolerates close and frequent grazing and is best adapted to the higher elevations and areas west of the Blue Ridge Mountains. This grass possesses a relatively shallow root system and is not drought tolerant. Bluegrass routinely goes dormant during the summer months when temperatures are high and moisture is limiting. However, bluegrass normally resumes growth in the fall when soil moisture is abundant and temperatures are lower.
Bermudagrass. Bermudagrass is a sod forming perennial warm-season grass that is best adapted to the Southern Piedmont and Coastal Plains Regions of Virginia. This grass tolerates close and frequent grazing and possesses excellent drought tolerance. As previously noted, even bermudagrass requires some water to remain productive. An advantage of bermudagrass is that it produces about twice as much dry matter per unit of water used. It also responds well to smaller amounts of water supplied by summer thunderstorms compared with cool-season grasses.
Reviving Drought Stressed Pastures
Virginia's drought stressed pastures often look worse than they really are. This is especially true for pastures that were well managed prior to drought. In many cases pastures can be revived without reseeding. They key element of course is rain. The response of pastures to every input or management practice will be dependent on moisture. In many cases, pastures simply need to be rested and fertilized. Adjust the soil pH and bring phosphorus and potassium to the high level and apply a small amount of nitrogen (40-50 lb/A) in November or early December. This late nitrogen application will not produce a great deal of fall growth, but it will stimulate tiller production and root growth. Spring growth from these stands will be vigorous and thin areas will thicken faster.
Pasture legumes such as red and white clover are important components of pastures and in many cases could use thickening up even before drought. Pasture sod suppressed by drought and overgrazing provide a perfect opportunity for interseeding clover and alfalfa. Legumes can be either drilled in the fall or spring or frost seeded in late winter. Frost seeding works best with red and white clover. Alfalfa is better established using a no-till drill. More information on interseeding pastures is available at your local extension office.
Stockpiling for Winter Grazing in Drought Years
Stockpiling tall fescue is one the cheapest and best ways to provide winter grazing for livestock in Virginia. In good years, tall fescue pastures top-dressed with 60-80 lb nitrogen/A in mid August can produce 1-2 ton/A hay equivalent. The question in drought years is does this recommendation work for dried up, overgrazed pastures. No pasture will respond to nitrogen until it rains. In addition, pastures that have been overgrazed have the least potential for fall growth. Applications of nitrogen for stockpiling should target pastures that have not been overgrazed/overgrazed the least. The next question is when and how much nitrogen to apply. Ideally nitrogen for stockpiling should be applied in mid August at a rate of 60 to 80 lb/A. In a drought year there are several approaches to stockpiling. The first is to apply nitrogen in mid August at normal rates and then pray for rain. The second is to delay applications until rain looks like a sure thing. This option requires more planning since nitrogen needs to be applied prior to the impending rain. As the application date becomes later decrease the amount of nitrogen since the grass will have less time to grow before frost and cool temperatures set in.
In drought years, winterfeed is often tight, so maximizing the utilization of stockpiled grass is essential. Strip grazing stockpiled fescue can increase utilization by 30 to 40%. Allocate only enough pasture for 2-3 days of grazing. This is easily accomplished by using a forward temporary electric fence. No back fence is required since plants are dormant. During wet periods feed hay in a sacrifice area to avoid wasting stockpiled grass and damaging pasture sod.
Interseeding Cool-Season Annuals into Drought Stressed Pastures
Drilling cool-season annuals such as small grains and annual ryegrass in drought stressed/dormant pastures seems like a logical way of producing quick fall pasture. In many cases it does not work as well as expected. This is due to the fact that the ground is very dry and when the rain finally comes the seed not only starts to germinate and grow, but so does the dormant sod. An established fescue sod has an extensive root system that competes well for limited moisture. On the other hand, newly established seedlings have a very small root system and are at a serious disadvantage when competing for water with an established fescue sod. In some cases interseeding cool-season annuals into dormant sods can be cost effective. In this situation sods are normally in very poor conditions and there are simply not enough remaining plants to actively compete with the cool-season annuals. The best place for cool-season annuals is on cropland that has already been harvested. In general production on these areas will be greater due to the absence of any significant competition.
Drought rarely kills well managed pasture plants. In most cases, drought stressed pastures are in better condition than they appear. Most pastures can be revived with rain, rest, and fertilization. Weakened sods provide a prime opportunity for incorporating legumes in established pastureland. With a little tender loving care this year's drought stressed pastures will be next year's profit. Remember grass is one of Virginia's most valuable resources!
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