Overgrazing

Overgrazing occurs when plants are exposed to livestock grazing for extended periods of time, or without sufficient recovery periods. It reduces the usefulness of the land and is one cause of desertification and erosion. Overgrazing is also is a cause of acid grass spread of non-native plants.

Sustainable grassland production is based on grass management, animal management, and livestock marketing. Grazing management is the foundation of grassland-based livestock production since it affects both animal and plant health and productivity.


Process

Overgrazing can occur under continuous or rotational grazing. It can be caused by having too many animals on the farm or by not properly controlling their grazing activity. Overgrazing reduces palatable plant leaf areas, which reduces interception of sunlight and plant growth. Plants become weakened and have reduced root length, and potentially the pasture sod can be weakened although in many locales overgrazing results in an increased sod vigour dominated by unpalatable grasses. The reduced root length makes the plants more susceptible to death during dry weather. A weakened sod allows weed seeds to germinate and grow. If the weeds are unpalatable or poisonous, major ecological and ranching occur.

Indicators

One indicator of overgrazing is that the animals run short of pasture. In some regions of the United States under continuous grazing overgrazed pastures are predominated by short-grass species such as bluegrass and will be less than 2-3 inches tall in the grazed areas. In other parts of the world overgrazed pasture is typically taller than sustainably grazed pasture, with grass heights typically over 1 metre and dominated by unpalatable species such as Aristida or Imperata. In all cases palatable tall grasses such as orchard grass are sparse or non-existent. In such cases of overgrazing soil may be visible between plants in the stand, allowing erosion to occur, though in many circumstances overgrazed pastures have a greater sward cover than sustainably grazed pastures. Under rotational grazing, overgrazed plants do not have enough time to grow to the proper height between grazing events. The animals are turned into a paddock before the plants have restored carbohydrate reserves and grown back roots lost after the last defoliation (see table). The result is the same as under continuous grazing: in some parts of the United States tall-growing species die and short-growing species that are more subject to drought injury predominate the pasture, while in most other parts of the world tall, drought tolerant, unpalatable species such as Imperata or Aristida come to dominate. As the sod thins, weeds encroach into the pasture in some parts of the United States, whereas in most other parts of the worlds overgrazing can promote thick swards of native unpalatable grasses that hamper the spread of weeds.

Another indicator of overgrazing in some parts of North America is that livestock run out of pasture, and hay needs to be fed early in the fall. In contrast most areas of the world do not experience the same climatic regime as the continental USA and hay feeding is rarely conducted.

Overgrazing is also indicated in livestock performance and condition. Cows having inadequate pasture immediately following weaning may have poor body condition the following season. This may reduce the health and vigor of cows and calves at calving. Also, cows in poor body condition do not cycle as soon after calving, which can result in delayed breeding. This can result in a long calving season. With good cow genetics, nutrition, ideal seasons and controlled breeding 55% to 75% of the calves should come in the first 21 days of the calving season. Poor weaning weights on calves can be caused by insufficient pasture, when cows give less milk and the calves need pasture to maintain weight gain.

Ecological impacts

Overgrazing typically increases soil erosion. Reduction in soil depth, soil organic matter and soil fertility impair the land's future productivity. Soil fertility can sometimes be mitigated by applying the appropriate lime and fertilizers. However, the loss of soil depth and organic matter takes centuries to correct. Their loss is critical in determining the soil's water-holding capacity and how well pasture plants do during dry weather.

Continental, Lower 48, United States of America Specific Prevention

To prevent overgrazing, match the forage supplement to the herd's requirement. This means that a buffer needs to be in the system to adjust for the fastest growth of forages.

Another potential buffer is to plant warm-season perennial grasses such as switchgrass, which do not grow early in the season. This reduces the area that the livestock can use early in the season, making it easier for them to keep up with the cool-season grasses. The animals then use the warm-season grasses during the heat of the summer, and the cool-season grasses recover for fall grazing.

The grazing guidelines in the table are for rotationally grazed, cool-season forages. When using continuous grazing, manage pasture height at one-half the recommended turn-in height for rotational grazing to optimize plant health. The growth habit of some forage species, such as alfalfa, does not permit their survival under continuous grazing. When managing for legumes in the stand, it is beneficial to use rotational grazing and graze the stand close and then give adequate rest to stimulate the legumes' growth.

Proper grazing management keeps pastures healthy and productive. This ensures that the livestock using the pastures are also healthy and productive. To learn more about evaluating pasture condition and animal body condition, contact your county Extension agent.