In times when we are expecting the best production from our grass resource, it is important for the producer to understand how the AUM is defined. Such an understanding can help in fine tuning pasture management to produce the maximum returns without causing damage to the available grassland resources.

Animal Units

The AUM is the amount of forage needed by an "animal unit" (AU) grazing for one month. The animal unit in turn is defined as one mature 1 000 pound cow and her suckling calf. lt is assumed that such a cow nursing her calf will consume 26 pounds of dry matter (DM) per day as forage. That consumption, combined with a factor for tramping and waste of about 25%, results in an estimate of about 1000 pounds of dry matter (DM) from forage to supply one AU each month. This translates into 1108 lb of hay at 90% dry matter. Various other types of stock are assigned AUM equivalents based on size and consumption. For example, the fact sheet Grazing Tame Pastures Effectively (Agdex 130/53-1) suggests a mature bull is the equivalent of 1.3 AU, a yearling steer or heifer is 0.67 AU and a weaned calf is 0.5 AU. These are the equivalents suggested by the Society for Range Management.

Adjustments

Now come the problems. The unit and the equivalents do not allow for the change in frame size of our cattle in recent years. To start with, most of our crossbred beef cows with European breeding or some cows of the larger frame British breeding will probably weigh from 1200-1400 pounds. Cows in this weight range need from 23.0 to 25.6 pounds of DM, or 14 to 27% more than the 1000 pound cow. The larger calves would probably consume proportionately more as well. Therefore, cows in the 1200 to 1400 pound weight range would be equivalent to from 1. 1 1 to 1.21 of the original Animal Units.

Yearling heifers going onto pasture in central Alberta probably weigh about 700 pounds. Yearling steers can weigh quite a lot more! Going back to the Nutrient Requirements of Beef Cattle (1984), yearling steers weighing 900 pounds will need 20.3 pounds of DM to sustain a gain of 2 pounds per day. The yearling needs almost 78% of the cow-calf combined consumption. This suggests that perhaps the 0.67 given as the AU for yearling steers is a bit low.

The AU equivalent of 1.3 will provide the maintenance requirements for a 2200 pound bull, so there should be no need for adjustments on his account.

Table 1. Calculating AUM (Nutrient Requirements of Beef Cattle, 1984)
	Daily Forage Dry matter intake (lb)
Average 1000 lb cow producing 10 lb milk per day	20.2
Extra energy needed by the calf from grazing*	6.0
Total forage consumption	26.2
Waste (25%)	6.5
Total required	32.7
Total for a Month of 30.5 days	997.4

Table 2. Dry Matter Intake of Beef Cows and Suckling Calves on Pasture (Nutrient Requirements of Beef Cattle, 1984)
Average Milker1		Superior Milker2
Weight of Cow	Dry Matter intake (lb) (cow & calf)	As % of a 1000 lb Cow	Dry Matter Intake (lb) (cow & calf)	As % of a 1000 lb Cow
1000 lb	26.2
1100 lb	27.6	105.3	26.6	100.4
1200 lb	29.0	110.7	28.1	107.3
1300 lb	30.3	115.6	29.6	113.0
1400 I b	31.6	120.6	1.0	118.3

Summary

The main purpose of this discussion is to point out that a producer with 1300 pound cows with suckling calves should reduce the estimate of pasture - carrying capacity by 16% compared to the requirement for a 1 000 pound cow with suckling calf.

For example, a pasture of 640 acres of rough fescue prairie in range zone 5 has a carrying capacity of 1.8 acres per AUM. In 5 months each AUM needs 9 acres. The pasture will carry 71 cows weighing 1000 lb with their calves.

However, only 59 average milking cows weighing 1400 lb or 61 similar cows weighing 1300 lb could be grazed on this pasture. Any more would be overstocking at our current estimates of forage production. Similarly, using 0.67 as the equivalent animal unit for yearlings may lead to an overestimate of about 10% when calculating the carrying capacity of a pasture for yearling steers for the whole summer grazing period.

Table 3. Yearling Steers and Heifers Forage Dry Matter Intake for Various Rates of Gain and Frame Size (Nutrient Requirements of Beef Cattle, 1984)
Sex	Weight Size	Frame Size	Daily Rate of Gain	Dry Matter intake	Percent of 1000 lb Cow & Calf.
Heifer	700 lb	Medium	1.5	15.5	59.2
		Large	1.5	16.6	63.4
	800 lb	Medium	1.5	17.2	65.6
		Large	1.5	18.3	69.8
Steer	700 lb	Medium	1.5	16.5	63.0
			2.0	16.8	64.1
		Large	1.5	17.2	65.6
			2.0	17.8	67.9
	800 lb	Medium	1.5	18.2	69.5
			2.0	18.6	71.0
		Large	1.5	19.0	72.5
			2.0	19.6	74.8
	900 lb	Medium	1.5	19.9	76.0
			2.0	20.3	77.5
		Large	1.5	20.8	79.4
			2.0	21.4	81.7

Table 4. Herd sires forage dry matter intake (2 years and older) (nutrient requirements of beef cattle, 1984)
Sire Weight (lb)	Daily Gain (lb)	Dry Matter intake	Percent of 1000 lb Cow & Calf
1800	0	28.9	110.3
	0.5	30.9	117.9
1900	0	30.1	114.9
	0.5	32.2	122.9
2000	0	31.3	119.5
	0.5	33.5	127.9
2100	0	32.5	124.0
	0.5	34.7	132.4
2200	0	33.6	128.2
2300	0	34.7	132.4

May 2001