Matching Pasture with the Nutritional Needs of Cows

By Dr. John Comerford

About 90 percent of the diet in the life cycle of beef production is from grass and forages. The typical method for managing grazing in a cow herd is to fence the perimeter of the pasture, turn them out, and hope you can make enough hay on some other ground when the grass is gone. We know there's a better way to manage this most important part of beef production - and doing so will reduce costs and improve returns to the enterprise.

Every element of production has a cost, so what does pasture cost? A report from Comerford et al. (2005) describes some of the costs and returns from different grazing systems in central Pennsylvania based on the impact of the system for calf weight gain and the sale of hay produced in the system (see Table 1).

This report indicates two important outcomes. First, the use of annual plants as pasture for beef cows is probably not cost-effective. The cost of seed and tillage will overcome any economic or production advantage compared to perennial plants. Secondly, orchardgrass appears to be the most productive and valuable forage. However, further investigation of the data will show much of the additional value from the orchardgrass monoculture was from the sale of hay, and weather conditions tended to change orchardgrass production faster than pasture with variable species, such as in the third system.

After studying these forage combinations, we concluded the most cost-effective pasture for central Pennsylvania was one-third of the acreage in tall fescue, one-third in orchardgrass, and one-third in an alfalfa/grass mixture.

I had the opportunity to work with Dr. Sinclair Mayne in Ireland a few years ago, and I recall an important concept he explained. This concept can be described as balancing the ration for a grazing cow. The basic diet for a cow from grass is determined by three things: the energy density of the sward, the density of the sward and the biting rate.

Energy density of the sward

The energy in grass is the most important nutrient that is captured. In the 2005 Comerford et al. study, forages were measured twice weekly for three years during the grazing season. The results indicated that among the dozen or so forage varieties evaluated, there was little difference in energy value at any given time or within any given year. Secondly, at no time were protein levels less than the requirement for the cows. These results provide incredible flexibility for pasture species that can be adapted to specific soil and environmental issues in this region.

The energy density of the plant is also affected by maturity and fertility. These management issues are the keys to increasing nutrient intake with each bite. Table 2 indicates how important plant maturity is for energy density. These results clearly indicate that the appearance of seed heads in cool-season grasses is a signal that nutritional value is diminishing in the grass.

Sward density

If there are more leaves of grass in a square foot of pasture, it follows that a cow will be able to get more grass with each bite. Sward density is a function of proper seeding, optimum fertility and proper grazing management.

In the Missouri Grazing Manual (1999), Dr. Jim Gerrish explained the need for proper grazing management to increase sward density. In summary, when cows are allowed to continuously graze a pasture, they will selectively eat the most desirable plants. Before the plant has a chance to regrow stem, leaf and root, it will be eaten again, potentially causing permanent damage to the plant.

When cattle are allowed to eat the plant only until there is sufficient leaf left to capture sunlight and renew root and leaf growth, the plant will actually grow faster. Secondly, if grazing is initiated prior to plant maturity and seed head appearance, the nutritional value of each bite is increased. Therefore, Gerrish concluded that rotating cattle through pastures when most grasses are between 3 and 10 inches tall and providing a proper rest period between grazings to allow for plant renewal and growth will increase grass growth, nutritional value and total dry matter production for the year.

Biting rate

The final step in obtaining optimum nutrition from grass is to get the cattle to eat it. Biting rate can be influenced by animal health, air temperature and humidity.

The first return from grass intake is to provide maintenance energy. The basic calculation for megacalories (Mcal) for animal maintenance in a thermal-neutral environment is usually .077 times animal weight raised to the .75 power. This value represents the amount of energy required to keep the animal alive at a resting state. It would be slightly higher in animals with a larger gut weight relative to total weight, such as dairy breeds.

For a typical 700-pound steer, the requirement would be 40 Mcal of energy, or about 55 pounds of orchardgrass, to keep the animal's body working. Additional grass intake would be partitioned into muscle, bone and fat development. In bred, mature brood cows with a calf by their side when there is little muscle and bone growth, the additional feed is partitioned into fat, fetal growth and lactation.

Variations in animal weight gain will be expected as plants change during the growing season and there is a different biting rate. The results in Table 3 outline how weight gain of grazing steers would be expected to change based on the actual energy values for the forage.

Grass and forages are the key components for nutrition of beef cattle. A higher level of nutrient density and greater forage availability can increase stocking rates and potential performance of the cattle while reducing costs.

Dr. John Comerford is an associate professor of and extension beef speacialist at Pennsylvania State University.