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September 25, 2000


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

Jim Gerrish
Research Assistant Professor,
University of Missouri - Forage Systems Research Center,
21262 Genoa Road, Linneus MO, 64653
(GerrishJ@missouri.edu)

Abstract


Nutrient intake by grazing animals is affected by both forage availability and forage quality. Yearling steers were rotationally stocked from April to September for four years at four stocking rates (300, 600, 900, 1200 pound liveweight/acre) on cool-season grass-legume pastures to determine the effects of stocking rate on forage availability and quality through the season. Lower stocking rate resulted in higher forage availability but lower quality forage while higher stocking rates produced less available forage but higher quality forage. Peak forage availability at highest stocking rate occurred 58 days earlier than for the lowest stocking rate. While forage availability declined through the season, net energy content of the forage remained nearly constant through the season. The combined effect of available forage and net energy content resulted in significantly less net energy available per acre for the 1200 pound/acre stocking rate compared to all other stocking rates. These results indicate that forage availability is more likely to limit steer performance on pasture than is forage quality. Steer performance after mid -summer was less than predicted based on available net energy per acre. Factors other than forage availability or quality evidently limit steer performance after mid-summer.

Introduction: Both forage availability and forage quality may limit animal performance in grazing situations. In many research trials, only one of these parameters may be measured. Failing to take both into account often leads to over-prediction of expected animal performance or erroneous conclusions about what factor is limiting animal performance. In other cases both are measured but are not combined to provide a meaningful relationship to animal requirements and performance. Knowing both forage production and nutritive value of the forage allows the researcher or grazing manager to determine the nutrient supply and demand balance.

Net energy demand of most classes of livestock under given production and environmental scenarios has been determined. Net energy content of forage can be calculated from forage analysis and forage availability or accumulation rate can be measured. With this information, supply and demand relationships can be determined and appropriate stock policy and stocking rate decisions can be made. Our objective was to compare yearling steer net energy demand per acre with forage net energy available per acre and determine what factors most likely limit gain of grazing yearlings.

Materials and Methods: This project determined net energy accumulation rates for cool-season grass-legume pastures rotationally stocked at four stocking rates. Energy demand of yearling steers grazing these pastures was determined based on net energy requirements for maintenance and growth.

Pastures consisted of endophyte-free tall fescue (Festuca arundinacea Schreb.), orchardgrass (Dactylis glomerata L.), and Kentucky bluegrass (Poa pratensis L.) overseeded with red clover (Trifolium pratense L.) and birdsfoot trefoil (Lotus corniculatus L.) at the beginning of the study in 1995. Within each block, pastures were randomly assigned a stocking rate and then split to either continuous or rotational stocking treatments. The rotational grazing cells consist of 12 equal sized paddocks. Target stocking rates were 300, 600, 900, or 1200 lb liveweight per acre at turn-out as yearling steers weighing approximately 575 lb/head. Sixteen 10 acre pastures were used in the study to provide two replications of each treatment in a randomized complete block design with split plot assignment of treatments. Only rotational grazing data is presented in this paper due to limited space.

The study was conducted from 1996 through 1999 with grazing beginning in early to mid-April and ending around September 10. First grazing cycle consisted of daily rotation through the 12 paddocks and subsequent cycles usually consisted of 2-day grazing periods with 22 day rest periods. All stocking rates were managed on the same rotation frequency.

In each rotationally grazed pasture, four paddocks were sampled in each grazing cycle to determine forage availability. Nine 3.2 ft2 quadrats were clipped both pre- and post-grazing in each paddocks. Bulk wet weight was measured and a 150 g (+/-) subsample was oven dried to determine forage dry matter. Oven dried samples were retained from each clipping for forage analysis. All forage samples were analyzed by NIRS to determine crude protein, ADF, and NDF levels. Each year approximately 100 samples were analyzed through standard wet chemistry procedures to determine the same parameters and calibrate the NIRS data. Net energy for maintenance (NEm) was calculated from ADF value using the relationship:

Forage NEm = 1.04-(0.0104 X ADF)

Availability of NEm per acre was calculated as:

Forage NEm/acre = Herbage mass (lb/acre) X NEm (Mcal/lb)

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

NEm=.077 X ((liveweight/2.205).75

NEg=.0493 X ((liveweight/2.205).75) X ((ADG/2.205)1.097)

Animal demand per acre was calculated as:

Required NE/acre = NE/head X Head/acre

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

Jim Gerrish
Research Assistant Professor,
University of Missouri - Forage Systems Research Center,
21262 Genoa Road, Linneus MO, 64653
(
GerrishJ@missouri.edu)

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.


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