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Superintendent
David Davis
21262 Genoa Road
Linneus, MO 64653
Phone: 660 895-5121
FAX: 660 895=5122
Email: DavisDK@missouri.edu

Forage Systems Update
Vol 7, No. 2

Why is Stocking Rate So Important ?

Jim Gerrish

Research methods: The pastures consist of endophyte-free tall fescue with associated orchardgrass and Kentucky bluegrass in lesser amounts. In the spring of 1994, the entire area was overseeded with red clover and birdsfoot trefoil. Sixteen 10 acre pastures are being used in the study to provide two blocks of each treatment in a randomized complete block design with split plot assignment of treatments. Within each block, pastures were randomly assigned a stocking rate and then spilt to either continuous or rotational stocking treatments. The rotational grazing cells consist of 12 equal sized paddocks. Initial stocking rates of 300, 600, 900, or 1200 lb liveweight per acre as yearling steers with a starting weight of approximately 550 lb/head were utilized. As the steers gained weight through the season, the effective stocking rate increased. Effective stocking rates at the end of the season were 419, 797, 1141, and 1461, respectively.

In 1996 grazing began on April 20 and ended August 31 for all treatments, while in 1997 grazing began on April 3 for the 300 and 600 lb/acre stocking rates, April 10 for the 900 lb/acre groups, and April 17 for the 1200 lb/acre groups and continued for all treatments until September 10. Cattle were weighed on two successive days at the beginning and end of the study and were weighed unshrunk at 21 day intervals in between.

Forage availability was measured before and after each grazing period in four of the twelve paddocks in each rotational grazing treatment and was measured at bi-weekly intervals at four locations in each of the continuously grazed pastures. Species composition, stand density, and soil bulk density were measured in each pasture in April, July, and October.

Results and Discussion: In 1997 grazing conditions were very good April through early July after which below normal precipitation occurred. The dry period extended from about June 20 through August 20 with less than 3 inches rain falling during that time period. Fortunately mean daily temperature averaged 2-5^oF below normal for the same time period reducing the effects of dry conditions on forage supply. A two-day grazing period with 22-day rest period was used through most of the 1997 season for all rotationally grazed pastures. In 1996, a three day grazing period with 33 day rest was used for most of the season. The legume component in most pastures was higher in 1997 compared to 1996.

Animal performance: Steer ADG declined near linearly with increasing stocking rate but was largely unaffected by grazing method (Fig 1). In 1997 ADG was approximately .3 lb/hd/day higher than in 1996, probably reflecting the better grazing conditions existing in the early part of 1997 compared to the excessively wet weather of May-June 1996. The lack of response of ADG to grazing method may reflect the high quality of the continuously grazed pastures at the outset of this study. These pastures were well established and had been under rotational management for six years prior to the initiation of this study. The legume component in these pastures is well above what is typically seen in pastures in this region that have been under long term continuous grazing.

As we think about efficiency of production we must consider input costs both from the animal and land resources. While individual performance is almost always favored by low stocking rates which allow the animal to selectively graze and achieve large bite size, the efficiency of use of the grassland resource may be quite low and so we must also consider production per acre. Gain per acre: Production per acre will tend to follow an opposite trend from individual animal performance, with higher stocking rates achieving higher levels of gain per acre. While stocking rate can be increased to the extent that all animals lose weight and gain per acre is nil, in this study we tried to consider only stocking rates in the range that we believed would be economically relevant and we did in fact achieve the highest gain per acre at the 1200 lb/A stocking rate (Figure 2).

As a measure of pasture use efficiency, we can compare temporal utilization rates (TUR), that is the percent of standing forage harvested in each grazing period. The TUR for the 300 lb stocking rate was 20 - 25 % and 40-50% for the 1200 lb stocking rate, indicating that the investment in land, pasture establishment and maintenance, and fence and water development was not being efficiently utilized at the lower stocking rate.

There are three measures of efficiency that we might discuss relative to this study: animal efficiency, land use efficiency, and economic efficiency. Each measure would predict a different optimal stocking rate low, high, and intermediate, respectively. Dr. Kevin Moore, our cooperating Ag Economist, is presently working on the data and will present economic information at our annual pasture day on June 23.

Species composition: The composition of these pastures have already shown some marked changes over the first two years of the study. Total grass ground cover has decreased in all pastures with the greatest decrease coming at the highest stocking rate (Figure 3.) Continuously grazed pastures followed a similar trend as shown for rotationally grazed pastures, but the degree of stand reduction was slightly greater at the higher stocking rates for continuously grazed pastures.

Legume composition followed exactly the opposite trend with an increase in legume cover over the first two years and the greatest increase occurring at the highest stocking rate. At first this may seem contradictory to the warnings that overgrazing will eliminate legumes in your pasture. Legumes must have access to sunlight to be able to compete with companion grasses. At the 300 lb stocking rate, the grass was never grazed below a 6-8 inch level, thus shading legumes. At the high stocking rate, the grass canopy was constantly kept short, allowing legumes sunlight. In the long term, the legume component probably cannot withstand the highest grazing pressure and will begin to decline.

Soil Bulk Density: Over the first two years of the study we have measured significant increase in soil bulk density with a stepwise increase as stocking rate increases. This will affect water infiltration and runoff.