Scientifically Understanding your Pastures
May 13, 2011
UNDERSTANDING YOUR PASTURES
The feeding value of pasture
The feeding value of a forage can be defined as the product of available nutrients contained in the forage times the amount of forage consumed (voluntary intake). Leafiness and stage of growth are factors that will affect the feeding value of plants. High leaf content is associated with a low proportion of cell wall constituents and a high proportion of cell contents. The main effect of advancing maturity in grasses is an increased proportion of cell wall and a reduction of cell contents.
Feeding value will also vary within the same grazing season and among forage species. Among the climatic variables, light and temperature are the most important, followed by moisture, which most of us have more of than we need at this critical planting time of the year! God bless all of you South of the Mason-Dixon line.
Normally, animal production depends on the ability of the herbage to provide energy, provided that protein, minerals, vitamins and water are consumed in sufficient amounts to sustain the type of production sought.
The chemical makeup of your pastures
From a nutritional standpoint, your pastures/forages can be divided into organic acids, soluble carbohydrates, crude protein, fats, and soluble ash and hemicellulose, cellulose, lignin, cutin, and silica.
The cell contents are usually highly digestible and readily available in your cattle’s rumen. On the other hand, the availability of plant cell walls varies greatly depending on their composition and structure. Maturity or stage of growth, species and environmental factors can affect the chemical composition of your forages.
As plants mature, the proportion of their cell’s wall’s and their constituent fractions increase and the cell content fraction decreases. Don’t ask, I just tell you what I read. I had to read that and reread it a couple of times before I understood it too. An exception is nonstructural carbohydrates, which increase in stem, stem base and inflorescens. Cool season grasses will normally have a higher cell wall concentration than legumes, especially in leaves, but a lower cell wall concentration compared to warm season grasses.
Digestibility of pastures
Measurement of digestibility is one of the first important steps in evaluation of forage quality.
Digestibility is the proportion of food consumed which disappears in the digestive tract and defines the nutrient availability per unit of feed intake. Plant cell contents are almost 100% digestible.
In general, legumes are typically more digestible than grasses.
Digestibility is relatively easy to measure but is probably not the most useful measure for predicting intake. This is because some "feeds" may be poorly digested but pass through the digestive tract relatively quickly, thereby occupying space for less time than a more digestible forage with a slower rate of passage.
During periods of scarce pasture availability, such as when seasonal pastures are dormant or "recovering" from grazing, supplementing cows with good quality haylage will tend to increase total DMI. Supplementing your cattle with good quality haylage in addition to dry hay is also a great way to wean your cattle off of dry hay onto pastures before turning them out on lush spring forages and risking bloat!
Timing
Cattle have a distinct grazing pattern, which includes a major meal beginning at sunrise, and again at sunset.
They do graze overnight, but nighttime grazing represents a small percentage of the total daily grazing time and contributes minimally to the daily forage intake. Almost 85% of their total grazing time is spent during daylight and only 15% overnight. This pattern of grazing is a foraging response to an increase of digestible nutrients in the forage at this time of day/night due to the photosynthesis process in plant leaves which occurred during the day and not at night. Another important factor that can affect grazing behavior is air temperature.
