Beside the obvious but complicated impact of being partially severed, livestock also impact grass growth and regrowth by trampling, fouling, selecting or rejecting certain plants and pugging the soil. All these interactions should be considered in determining forage yield and efficient management.
Grasses and other types of forage are consumed by all classes of domestic animals and many classes of wildlife although the various animals do not consume grasses in the same way or in the same amount. Dog foods contain some grains; poultry rations contain some grain to supply vitamins, minerals, and proteins; swine consume some grasses but horses, goats, sheep, and dairy and beef cattle may get all their feed from forage. The differences in how much and what is consumed result from the different digestive systems and abilities to handle food containing celluloses and lignins. Horses, cattle, and sheep have advantages for utilization of forages over single-stomach animals. It is also important to look at how different animals chew their food and interact with a pasture to really understand how to best utilize the growth and regrowth of grasses. It is beneficial to look at the the chewing habits of the main forage livestock, as well as the effects of trampling of grass plants by livestock, and the effects of pasture pugging and fouling by urine and manure.
When cows eat grass, their tongue sweeps out in an arc, wraps around the plant parts, then pulls them between the teeth on the lower jaw and a pad on the upper jaw. The cow swings its head so its teeth can sever the grass. It then grinds the food and mixes it with saliva before swallowing. The lips, teeth, and jaws of a cow make it difficult to get closer than 2 inches (5 centimeters) from the soil. They eat most efficiently when the grass is about 6 inches (15 centimeters) tall. At that height, cows can snip the grass and don't have to pull it into their mouths. This allows them to concentrate on arranging the feed into a bolus for swallowing. Any time the feed is longer or shorter than about 6 inches, cows have to work harder for each mouthful of food. When eating efficiently, cows can take about 80 bites a minute, 8 hours a day with about 12 hours for rumination. That often adds up to more than 130 pounds (59 kilograms) of food each day. Cows will not work overtime to eat more food...even if they have very little grass to eat. Pasture height, quality, and density should be considered.
Sheep do not extend their tongues. Rather their split lips move away from the teeth on the lower jaw, bringing in feed and cutting it across a dental pad on the upper jaw. Sheep take smaller bites than cows and can be more selective. They also can sever plants closer to the ground.
Horses have teeth on the upper and lower jaws. A set of incisors are in the front of the mouth for biting and severing and a set of molars used for grinding are in the back. A horse's bit fits between the two sets of teeth. Horses have a short tongue and a sensitive, strong, upper lip that brings in food for the front incisors to bite. Horses are selective grazers, yet spend less time eating than cows or sheep. Of course, weight gain, milk or wool production are not desired. These main livestock types are basically grazers, eating grasses, legumes, and other leafy plants.
Goats are browsers, consuming woodier plants. Goats have a pad on the upper jaw like sheep and cows but their upper lips are very mobile and their tongues are great for selecting and grabbing.
Trampling: To a young grass seedling the size and grazing habits of the livestock on a pasture can mean total destruction. Every pasture will experience the loss of plant material because the plants could not survive the traffic of livestock. Certain species are more sensitive to trampling and the stage of maturity also influences the effect of heavy traffic. The size of the livestock also enters into the picture. Trampling, or treading as it is termed in some references, damages pastures of all soil types, soil moisture levels, plant species, or livestock species. Forage yields are reduced most when animals are allowed to graze plants on wet soils. Trampling is more adverse on clay soils than sandy. Shorter forage bears more damage than tall forages, and trampling promotes more prostrate than erect growth. And, of course, trampling packs the soil which reduces the moisture infiltration into the soil.
The above generalities can lead a manager to consider what species will best tolerate livestock traffic. Then grass growth and regrowth concepts can be applied to the specific grazing system. Perennial ryegrass and white clover mixtures are more tolerant of trampling than orchardgrass or red clover.
Pugging: The weight of livestock animals has its impact on the soil and the feed growing in that soil. The indentations of the animals' hooves into soil, especially damp or wet soil are called pugs. A field is dramatically altered by pugging. The pug holes are areas where seedlings and regrowth have been smashed backed underground. Also buried are weed seeds that benefit from the planting. A pasture loses production of the pugged area.
Fouling: The urine and manure recycled into the soil has many benefits but this recycling is random and, therefore, has different effects on the different plants within the field. The growth and regrowth of plants is directly influenced by fertilization. Some plants will receive too much urine or manure while others are not directly treated with one or either.
Cattle defecate 11-12 times daily and urinate 8-11 times daily, with more dung being excreted at night than during the day. But dung pats cover less than 9.7 ft2 (0.9 m2) and urine patches 3.2-4.3 ft2 (0.3-0.4 m2). Many have learned that excreta is beneficial to a pasture but a small percentage of the acreage is affected especially since excreta is often concentrated near water or salt sources and shade spots. (The problem is worse at low stocking rates than at high stocking rates on intensively managed pastures.) Only 10-15% of a pasture benefits from excrement. Urine deposits will provide concentrated N and K which stimulate grass growth. But the utilization of the possible N and K from the urine is influenced by moisture and temperature. Much of the nutrients are lost through leaching and volatilization (vaporization). Dung, however, provides P, Ca, and Mg and is often buried into the soil within 4-5 days by dung beetles. Without the beetles, dung deposits may remain for 3-12 months in the tropics. Livestock will quickly graze urine spots but will avoid the odor of dung deposits and eventually the less palatable grass plants that have matured there.
Although most of the pasture is not greatly affected by excrement, managers can use this knowledge in determining traffic lanes, water locations, salt availability, day/night paddocks, and fertilizer needs.