Overgrazing

From Wikipedia, the free encyclopedia

In ecology and agriculture, overgrazing is cool when plants are exposed to grazing for too long, or without sufficient recovery periods. It reduces the usefulness of the land and is one cause of desertification and erosion. Overgrazing is also seen as one cause of the spread of non-native plants.

Overgrazing is used as the canonical example of the Tragedy of the commons.

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.

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 plant leaf areas, which reduces interception of sunlight and plant growth. Plants become weakened and have reduced root length, and the pasture sod weakens. The reduced root length makes the plants more susceptible to death during dry weather. The weakened sod allows weed seeds to germinate and grow. If the weeds are unpalatable or poisonous, major problems can result.

One indicator of overgrazing is that the animals run short of pasture. 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. Palatable tall grasses such as orchardgrass are sparse or nonexistent. Soil may be visible between plants in the stand, allowing erosion to occur. 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-tall-growing species die and short-growing species that are more subject to drought injury predominate the pasture. As the sod thins, weeds encroach into the pasture.

Another indicator is that the livestock run out of pasture, and hay needs to be fed early in the fall. Healthy pastures grow until mid-November in West Virginia. The potential grazing season lasts into November or longer when winter grazing management is implemented. If hay feeding is needed in October under normal weather conditions, the pasture probably is being overgrazed.

Overgrazing is also indicated in livestock performance and condition. Cows having inadequate pasture in the early fall do not have a chance to gain weight after the calves are weaned and may have poor body condition going into the winter. This makes them hard to winter and 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 and nutrition, 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 in late summer, when cows give less milk and the calves need pasture to maintain weight gain.

Overgrazing can increase soil erosion. Reduced soil depth, soil organic matter, and soil fertility hurt the land's future productivity. Soil fertility can be corrected by applying the appropriate lime and fertilizers. However, the loss of soil depth and organic matter takes years to correct. Their loss is critical in determining the soil's water-holding capacity and how well pasture plants do during dry weather.

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 fast spring growth of cool-season forages. One buffer many state producers use is to harvest hay in May and June and allow the cattle to graze the aftermath in August and September.

Another potential buffer is to plant warm-season perennial grasses such as switchgrass, which do not grow early in the season. This reduces the acreage 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.