Availability of NEm per acre was calculated as:

Net energy demand for yearling steers was determined by the following equations:

Animal demand per acre was calculated as:

Results and Discussion: Forage availability was significantly affected by stocking rate with date of peak forage availability also being determined by stocking rate (Figure 1). Increased animal demand of higher stocking rates resulted in lower mean forage availability throughout the season. While peak forage availability occurred on July 29 for 300 pound/acre stocking rate, the same event occurred on June 8 for 1200 pound/acre stocking rate. By the end of the grazing season, forage availability on 1200SR was approximately 35% of 300SR.

The NEm content of forage samples was very good throughout the season with mean levels remaining above .65 Mcal/pound for all treatments (Figure 2). This data represents whole plant samples so animal selection would be expected to result in even higher level of dietary energy intake. The highest stocking rate produced the highest NEm forage while lowest stocking rate produced the lowest NEm forage. Crude protein content, while not reported in this paper, responded to stocking rate similarly.

These two pieces of data suggest that animal performance was more limited by declining forage availability as compared to forage quality. The observed NEm levels should be adequate to produce average daily gain in excess of 1.75 pounds/day if intake were not limited. However, by mid-July all treatments were gaining less than 1.0 pound/day. It may be that reduction in ADG at this time was due more to environmental stress than forage conditions. Nighttime low temperatures were frequently above 80°F with humidity in excess of 70%. These conditions are not conducive to steers achieving high pasture intake.

Net energy available per acre was greatest throughout the season for the lowest stocking rate and least for the highest stocking rate (Figure 3). Available net energy declined rapidly after mid-June for 1200SR. During a typical two-day grazing period, NE consumption by the steers would exceed 50% of the available NE. Forage utilization in excess of 50% will usually result in depressed animal intake.

Daily net energy demand declined through the season for all treatments with more rapid decline occurring as stocking rate increased (Figure 4). The decline was due to decreased rate of gain as the season progressed. It is very difficult to say whether gain decreased due to lack of energy intake or other environmental factors. For all but the highest stocking rate, both forage availability and net energy content were at levels which would not normally be expected to restrict voluntary intake.

Steer average daily gain did not meet expectations based on available forage and net energy content of the forage. It appears that factors other than the measured forage parameters determine intake level and steer performance. High nighttime temperatures and relative humidity may limit grazing time and total daily intake. Maintaining high quality pastures with adequate availability will help maintain summer rate of gain on yearling steers but other factors such as proper environmental adaptation of livestock, parasite management, and minimizing stresses may be equally important.

Figure 1. Forage availability declined through the season as stocking rate increased.

Figure 2. Higher stocking rates produced higher net energy forage with little variance through the seasons

Figure 3. Net energy available per acre varied through the season and was significantly affected by stocking rate.

Figure 4. Daily net energy demand per acre declined through the season at all stocking rates due to decling average daily gain.

SEASONAL NET ENERGY AVAILABILITY IN
ROTATIONALLY STOCKED PASTURES AT FOUR STOCKING RATES

Stocking rate refers to the number of animals or the total liveweight of animals assigned to a grazing unit for and extended period of time. As stocking rate increases, the animal demand for nutrients from every acre in the grazing unit increases. Nutrient density is greater in less mature forage than in older plant material. Increased grazing pressure causes the animals to consume more plant material and stimulates regrowth, thus high grazing pressure increases nutrient density of the forage while lower grazing pressure decreases forage quality but leaves more forage available per acre. The challenge facing grazing managers is whether a lesser amount of high quality forage or a greater amount of lower quality forage is more conducive to animal growth. This study shows that even though high grazing pressure produces the highest quality forage, the amount available to the grazing animal is inadequate to meet their needs for both body maintenance and growth. A moderate stocking rate produces the best combination of forage quality and quantity.