Every beef farmer likes to see grass getting green in the spring. Grass is the benchmark for so many important things to the beef enterprise: nutrition, reproduction and economics. If we wait until we see green grass to start planning a grazing program, we may lose some important opportunities to improve the grass and the grazing season. This month we will examine some basics of grazing management, and next month we will discuss some of the details.
My friend and colleague Dr. Sinclair Mayne in Ireland once provided some insight to grazing we should all remember. The amount of grass that gets into the cow is defined by four things:
1. The grazing time
2. The number of bites per minute
3. The density of the sward (how much grass is in each bite)
4. The tensile strength of the grass (how hard it is to bite it off)
If we remember these four elements of grazing, we can structure an entire management program around them. Our work with grass-fed beef production, for example, revealed a summary that success in that enterprise can best be achieved by having healthy cattle (grazing time and biting rate) that have access to high-quality forage on a daily basis (sward density and tensile strength).
The first step for successful grazing is not when opening the gate. Rather, it is a planning process that will allow us to best match the forage we have with the cattle we wish to feed. The enemies of good pasture management are over and undergrazing, which are the results of poor planning. Basic physiology of pasture plants tells us that plants get their energy from the sun and convert it to cattle feed. The plant uses leaves to capture sunlight, and when there are no leaves on the plants, it uses stored carbohydrates in the root. Overgrazing removes the canopy of leaves and keeps them off so more of the plant energy comes from the roots. This results in poor plant growth, loss of roots and sometimes plant death. That is why an overgrazed pasture is characterized by a very short sod and tall weeds that are less palatable to the cow. Similarly, plant diversity in the sward can be reduced when the canopy of leaves is not removed and some low-growing species are shaded from the sun.
The need for exposure to sunlight by leaves is the basis for rest period for the pasture. Necessary rest period will vary during the grazing season, with as little as 15 days in our region in the spring, and up to 40 to 45 days in midsummer for most of the cool-season grasses we graze in this area. This implies we need more cows per acre in the spring than in the summer. When the number of cows available is fixed, appropriate rest period is attained by harvesting paddocks that will not be grazed. This harvest - before seed heads appear - is essential to maintaining vegetative growth and increased pasture dry matter production for the season. Our research here at Penn State has shown the most effective harvest will be grass silage or plastic-wrapped bales for this early harvest.
There is no one answer to how much rest period there should be for any given paddock on any farm at any time of the year. Rainfall, soil type and stocking density require flexibility in any pasture program. Experience may be the best source of information.
Cattle needs for pasture are based almost entirely on animal weight. Bigger cows will eat more because they need more feed to do the same job as a smaller cow. Lactation differences from one cow to another will influence how much pasture a cow will need. The most common way of determining animal needs is by using an animal unit (AU), which is defined by 1,000 pounds of grazing animal. Table 1 is adapted from Williams and Hall (1994) and provides some benchmark AU values.
It should be noted that pasture here is defined as reasonable quality grass that is managed without overgrazing, has a similar availability every day, and has sufficient sward density to meet potential intake. An adjustment in pasture availability would have to be made if these standards are not in place. Research at Penn State (Comerford et al, 2005) indicated there are few times during the year when protein needs of a beef cow would not be met after sampling 14 forages weekly during the grazing season. Energy is the most important nutrient in the pasture for beef cows, and energy values are controlled by plant management. Plant maturity reduces feed energy because more lignin is formed. Thus, management of rest period and preventing over and undergrazing are the keys to feed quality and animal performance in the pasture.
It is estimated that in most cool-season pastures in the Northeast, the amount of feed available is 300 pounds of dry matter per inch of pasture height in a reasonably managed pasture. This is just an average, but creates a starting place for determining how to allocate cows to pastures. Table 2 uses this value to show how much land is needed to meet the needs of a grazing cow. In this case the available pasture is determined by the inches of standing forage available for grazing (750 pounds is pasture grazed between 4.5 and 7 inches of height).
The design of the pasture system to optimize the grazing resource is determined by the rest period and the number of days the paddock is expected to be grazed. The equation is: (the number of days of rest period/the number of days the paddock is grazed) + 1 = the number of paddocks needed. For example, a part-time beef producer who may only rotate cattle every five days in the spring may need only four paddocks, but this producer may need 10 paddocks - and two and a half times as many acres - in midsummer. Therefore, planning is needed to make the pasture work for the entire season.
Dr. John Comerford is associate professor of dairy and animal science at the Pennsylvania State University.