In the current economy, one thing that has become abundantly clear is that things are going to change. One of the changes
occurring rapidly is the increased interest in allowing the animal (cattle) to harvest the forage growing in the pasture.
Historically, the cattle industry (both beef and dairy) has relied on forages harvested by man being transported to the cow
to provide a large portion of nutrients required for production. When this forage was not adequate to meet the demands of
production, cheap grain was located and added to the diet to meet any deficiencies that might occur. However with increasing
fuel, fertilizer, and grain prices accompanied with decreases in prices received for beef and dairy products, profit margins
are becoming narrower for all operations. As a result, many producers are attempting to minimize these expenses by maximizing
the amount of forage harvested by the cow. This form of pasture management is commonly referred to as rotational grazing,
intensive grazing, controlled grazing, management intensive grazing, or rotational stocking. Unless we received the training
prior to veterinary school, we (veterinarians) are poorly equipped to deal with some of the issues and questions that commonly
occur. This paper is designed to serve as a resource for some of those common questions and issues.
Stocking Density or Stocking Rate
One of the most common questions asked by producers is how many cows can I put on this pasture. The correct answer is
"It depends". Factors to consider include stage of production, season of year, type of pasture, weather conditions, nutrient
requirements, and many others. Stocking rate is defined as the number of animals or live-weight present on grazing unit for entire season or year. Stocking density is
defined as number of animal units present within a pasture of known area for a specified period of time. For example we assume
100 cows (1200 pounds each) on a 100 acre pasture with continuous grazing, the stocking rate is 1 cow per acre or 1200 pounds
per acre. However if the pasture is divided into 10 equal segments and all 100 cows are in one 10 acre pasture, the stocking density is 10 cows per acre or 120,000 pounds per acre. Although an unlikely example for extended periods of time, it is important
to make this distinction when giving recommendations to producers. According to Jim Gerrish, stocking rate should be thought
of in seasonal terms while stocking density should be thought of in immediate terms.
Other definitions important to consider when discussing grazing include:
Rotational length (grazing cycle) is the time required to make one complete cycle of the pasture in a grazing unit.
Grazing period is defined as the time that a group of animals occupy a pasture.
Rest period is defined as period between the end of one grazing and the start of the next grazing on the same pasture. Rest period can
also be defined as grazing cycle minus the grazing period.
Forage Production and Utilization
Forage production is influenced by many factors including temperature, moisture, sunlight, availability of nutrients in soil,
and degree to which plants are defoliated (either by grazing or harvesting). When a large proportion of leaf area is removed,
regrowth is usually slow due to a low rate of photosynthesis, and the need to use carbohydrate stores to satisfy both metabolic
functions and initiate leaf growth. This is where the art of grazing takes place. It is important that enough residual leaves
are left on the plant to allow photosynthesis to occur and regrow the plant (usually 3 to 4 inches depending on plant species
and season of year). However if excess growth is left on the plant, the resulting leaves will serve as canopy shading the
lower levels of leaves with resulting decrease in photosynthesis and plant growth.
Leaf area index (LAI) and tiller density are greatly affected by grazing management. LAI is defined as surface area of leaf
tissue per unit area of ground. Pasture yield will increase as leaf area increases until leaves intercept about 90% of light.
Unfortunately most leaves begin to die after 30 to 60 days (depending on plant species), and these dead leaves are less palatable
to cattle and have lower nutritional value in most instances.