Figuring Out AU's per Acres Grazed
May 27, 2010
STOCKING RATE & GRAZING STRATEGIES
Traditionally, pastures have been stocked on the basis of animals per acre per season. This approach presents problems because of variation in animal age and weight. Weights of different kinds of beef cattle have changed dramatically in the last 15 years because of genetic improvement and/or crossbreeding. Bigger cows wean heavier calves, and those bigger cows and calves also consume more forage. Because of this, pasture conditions have declined where cattle numbers or days of grazing per pasture have not been reduced.
Animal unit (AU) equivalents for beef cattle are easily estimated by dividing the average shrunk weight of the class or herd of animals by 1,000. Animal unit equivalents for cattle can be based on their average weight for the grazing season or adjusted at monthly intervals. Cows with an average weight of 1,200 lb. would be equal to 1.2 AU. Some of our calves begin foraging as early as two weeks of age, but what they consume isn’t usable by their rumen (or underdeveloped gut) until they are about two months old. By the time the calves are four months old, they spend as much time grazing as the older cows. It is generally recommended that the average calf weight be added to the average cow weight to calculate AU equivalents for pairs when the average age of your calf is three months.
Yearling cattle with an average weight of 550 lb. and a seasonal gain of 220 lb. would be 0.66 AU [(550 + 110)/1,000] for the season. About 60% to 70% of the total summer gain in growing cattle generally occurs in the first half of the summer grazing season. In the Northeast, that period would be from now until Aug. 1. Optimum stocking rates for economic returns to land, labor and management in yearling farm operations increase as the difference between buying and selling prices decreases. According to a report I read last year, the most profitable stocking rate for yearling cattle during a two-year period near Cheyenne, Wyo., was 60% to 80% above Soil Conservation Service (SCS) recommendations, but the increase in net profit over SCS recommendations was small and it was projected that range condition and forage production could not be maintained at the higher rate.
Animal performance in response to a given stocking rate is variable from year to year because of differences in vegetative/forage availability. Forage production on semi-arid rangelands is influenced primarily by precipitation. When we were considering the option of ranching in northeast Montana, we found that that area (1.5 hours north of Billings) only receives approximately 15" of annual precipitation. Our stocking rate would have been approximately 4.25 acres per cow/calf pair. Here in northeast Pennsylvania, however, the average stocking rate is approximately 1.5 acres per cow/calf pair due to the ample amount of precipitation and abundant forage diversity. Even though stocking rates based on AU months (AUM) or AU days (AUD) per acre are practical units for grazing management, it must be remembered that cattle graze forage, not acres. Consequently, stocking rates often need to be varied from pasture to pasture and from year to year to provide adequate amounts of forage for livestock.
When it comes right down to it, every farm/feedlot/rangeland/pasture is different -- sometimes even within the same farm or ranch, due to topography, elevation and overall size. If you need additional assistance with figuring out the optimal stocking rate for your herd or herds, contact your local FSA or University Extension office. Most Extension services still have grazing specialists on staff or at least a retired specialist in your area who would love to assist you and your farm/ranch with making sure your cattle can perform and gain as efficiently as possible.
Understanding why domestic livestock select certain plant species or certain plants within a given species is not well understood. Plant-animal interactions are complex. You can’t just turn a herd of cattle out on something green and expect them to perform at a desired level if you don’t know what they're consuming. Cattle grazing patterns are dependent on factors such as available forage allowance, season of use, species available/consumed, site preferences and length of the growing season.
When pastures are grazed two or more times, initially ungrazed forages are less likely to be grazed in following grazing periods. Preferred plant species are consistently grazed more intensively and frequently than less-preferred species regardless of the grazing schedule or stocking rate. Grazing schedules and stocking rates may have little effect on the height at which forages are consumed. Intensity and frequency of grazing tend to be greatest when forage growth is most rapid. When stocking rate and livestock distribution are properly controlled, season-long continuous grazing improves or maintains desirable range and meets livestock nutritional needs better than most specialized rest-rotation systems. Continuous grazing can be especially critical for the special growth requirements of replacement heifers that must reach puberty at 12 to 13 months of age if they are to calve as two-year-olds. A different pasture could be used for replacement heifers each year. The potential for livestock distribution problems to occur under season-long continuous grazing increases as the diversity of plant species, plant communities and topography increase within the pasture or paddock. Seasonal suitability of forage resources should always be considered when developing grazing strategies.