There are three important factors that affect how grasses respond to grazing: 1) frequency, 2) intensity and 3) season. Range ecologists and physiologists have found that as grazing increases, grass productivity typically declines. Frequency can be more important than intensity. A plant that is harvested often has more photosynthetic tissue removed and little opportunity for regrowth. These plants may enter a period when soil moisture, temperature and growth stage limit regrowth and little leaf area remains for photosynthesis. Thus, their ability to replenish reserves or produce additional new tillers is restricted.
If a plant experiences several defoliations, reserve levels and forage production might decline further. A plant that is grazed intensely during early spring and given a deferment during the remainder of the growing season may produce additional growth and be more vigorous than a plant that receives less intense defoliations throughout the growing season.
Grasses can produce large amounts of nutritious leaf growth during spring months. Leaves continue to age and die; therefore, a portion of the leaves can be harvested through livestock grazing with little effect on the plant. However, enough photosynthetic material must remain for production of carbohydrates to meet growth and respiration demands of the plant. If grazing removes too much leaf material, growth rate is slowed materially, and additional reserves may be required for regrowth (Figure 3). Root growth usually is affected by heavy defoliation, which makes the plant less competitive and more vulnerable to drought, because roots may not penetrate to depths where adequate moisture exists.
Livestock grazing during the growing season can affect regrowth of grasses. When moisture no longer is available and temperatures are too high or too low for rapid growth, regrowth is reduced considerably by grazing. Therefore, grazing in this pasture should be discontinued or reduced. If defoliations continue, little leaf area may remain throughout the growing season, and plants could enter dormancy with less vigor and lower reserves. This could significantly reduce growth the following year.
Grasses can withstand greater defoliation during early and rapid growth stages than they can later in the growing season, after most growing is complete and little opportunity for regrowth exists (Figure 3). Plants produce more leaves than stemmy tissue in the spring. These leaves contain abundant supplies of energy, protein and other nutrients necessary to meet most grazing-animal requirements. Grasses can be used heavily during this period, but discontinue or reduce grazing in time to allow for regrowth of leaves for photosynthesis and carbohydrate production.
If grasses are grazed in the reproductive phase, use them less intensely than during spring growth. Little opportunity for regrowth exists during midsummer, so sufficient leafy material should remain after grazing to maintain carbohydrate levels within the plant.
Grazing during the fall and winter periods, after plant growth is complete and plants are dormant, can be much heavier than at other periods of the year. This old material is of little value to the plant, as photosynthetic capability will be low, at best. This older and dead material is low in some essential nutrients, particularly protein. Energy content, however, remains moderate to high. Removal of dead leaf material and stems during dormancy has little direct effect on the plant.
However, mechanical injury to crowns can occur through trampling. Removal of mulch and litter may cause greater temperature extremes near the soil surface. This may adversely affect growth the following year. Although fall and winter grazing has the least detrimental effect on grasses, there may still be some negative impact if grazing is heavy.
Heavy grazing throughout the growing season usually is the least desirable grazing strategy. A management strategy that incorporates rest periods and movement of animals through different pastures usually is more desirable for grass growth than season-long grazing. If you know the amounts, kinds and locations of available plants (cool- and warm-season grasses), and what grasses grazing animals prefer, you can develop a strategy that meets the needs of plants and animals.
Management plans should use the forage resource and maintain it through time. Grazing plans, however, must be flexible. Consider differences in growing conditions across years as a result of drought or wet cycles, depletion of forage supply by wildlife or insects, and other rapidly changing environmental conditions. Consider these along with the impacts of grazing livestock to determine what effects the combined impacts will have on plants.
Try to avoid rigid plans that require moving animals from one pasture to another on given dates. Other environmental factors certainly will influence grass growth and use at any point in time. Base your decision to move stock on how much the grasses are used and how much green leaf material remains, not on a predetermined date.