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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 8, No. 3

Impact of stocking rate and grazing management
on pasture composition

Introduction: Selecting the proper stocking rate for the forage resource is one of the most critical decisions that a grazing manager must make. We have completed three years of a four year study evaluating effects of continuous and rotational grazing at four stocking rates with yearling steers. The number of animal, forage, and soil parameters which stocking rate affects is staggering. In this issue of the Forage Systems Update, we will report on the effects of stocking rate on species composition of the pastures. A full report on the project can be found at our website at the following location: http://aes.missouri.edu/fsrc/research/index.stm.

Research approach: The pastures consist of endophyte-free tall fescue with associated orchardgrass and Kentucky bluegrass in lesser amounts. 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. Eight pastures have been in established tall fescue for 16 years and eight were no-till seeded to tall fescue in the spring of 1994. In the spring of 1994, the entire area was overseeded with red clover and birdsfoot trefoil. Blocking is on the basis of the age of fescue stand. Within each block, pastures were randomly assigned a stocking rate and then spilt to either continuous or rotational stocking treatments. 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. The rotational grazing cells consist of 12 equal sized paddocks. Usually the first grazing cycle consisted of daily moves followed by two-day moves for the remainder of the season, allowing 22 days of rest. In 1996 and 1998 at least one cycle of three-day moves was used. Grazing began in early to mid-April each year and ended around September 10. Botanical composition of the pastures was made in each year in late April- early May, early July, and again in late September. Species composition was measured based on a step-point procedure with 1200 points per pasture at each sampling date.

Results and Discussion: Several trends in species composition change have emerged during the course of the project. Grass cover has declined in all treatments, with the greatest change occurring during the 1996 grazing season (Figures 1 & 2). Grass density stabilized or increased slightly in 1997, but declined again in 1998. Excessively wet conditions in spring and early summer of 1996 and 1998 may have produced more severe trampling damage resulting in stand reduction. The apparent high percentage of grass present at the beginning of data collection in spring of 1996 may also be an artifact of the previous year's management. In 1995 all of the pastures were harvested as hay in June and then were allowed to accumulate growth until mid-September. This type of management usually results in reduction of the legume component and an increase in grass cover. Grass cover declined more severely with increasing stocking rate while legume cover increased with increasing stocking rate.

Figure 1. Change in percent grass cover in pastures continuously grazed at four stocking rates from April, 1996 to October, 1998.

Figure 2. Figure 1. Change in percent grass cover in pastures rotationally grazed at four stocking rates from April, 1996 to October, 1998.

Figure 3. Change in percent legume cover in continouosly grazed pastures from April, 1996 to October, 1998.

Figure 4. Change in percent legume cover in rotationally grazed pastures from April, 1996 to October, 1998.

Legume composition had initially increased in all treatments, except 300 pound stocking rate, but declined in continuously grazed pastures in 1998. (Figures 3 & 4). We believe the positive legume response to increasing stocking rate is related to removal of the grass canopy and reducing interspecies competition for light and water. The trends were similar for both continuously and rotationally grazed pastures in 1996 and early 1997, but the legume component did decline in most continuously grazed pastures during the latter part of the 1997 grazing season and throughout 1998. The greater legume component in the higher stocking rate treatments and in the rotational grazing treatments is a likely contributor to the higher forage quality measured in those treatments. Part of the observed decline in grass composition is also likely to be simple replacement of grass plants with legume plants.

Individual grass components have responded similarly to both stocking rate and grazing method. Because of similarity in responses, data is presented only for the 300 and 1200 pound stocking rates as this is where the greatest differences occur (Figures 5,6,7 & 8). The tall fescue component has remained fairly constant in all treatments averaging between 30 and 35 percent. Most of the decline in grass stand density has come as reduction in the Kentucky bluegrass component and, to a lesser extent, orchardgrass. The decrease in bluegrass for the low stocking rates would be expected due to the tall forage residual remaining throughout the grazing season. Bluegrass frequently is a pasture increaser at high stocking rates so the decline in bluegrass is surprising and not easily explained. One possible explanation is that legumes may be more likely to establish in the more closely grazed bluegrass patches and bluegrass may be being replaced by legumes in the higher stocking rate treatments.

Figure 5. Change in individual grass components in continuously grazed pasture at 300 pound/acre stocking rate.

Figure 6. Change in individual grass components in rotationally grazed pasture at 300 pound/acre stocking rate.

Figure 7. Change in individual grass components in continuously grazed pasture at 1200 pound/acre stocking rate.

Figure 8. Change in individual grass components in rotationtally grazed pasture at 1200 pound/acre stocking rate.

Figure 9. Change in individual legume species in continuously grazed pastures at 300 lb/acre stocking rate.

Figure 10. Change in individual legume species in rotationally grazed pastures at 300 lb/acre stocking rate.

Legume content of both continuously and rotationally grazed pastures at the 300 pound/acre stocking rate have remained similar through the three years of the project (Figures 9 & 10). All treatments exhibit a pattern of highest legume presence at the midsummer observation date with similar, but lower, presence in both spring and fall. Birdsfoot trefoil is particularly noticeable in this characteristic at the 300 pound stocking rate. Trefoil is slower to begin growth in the spring and with the heavy grass canopy at the low stocking rate it is not visible to the casual observer. As with bluegrass, the decline in white clover in the 1200 pound stocking rate pastures is somewhat surprising, particularly given the high spring rainfall in 1996 and 1998 (Figures 11 & 12). Even at the highest stocking rate, red clover has become the predominant legume. The lack of reseeding opportunity for continuously grazed 1200 pound stocking rate explains the loss of red clover.

Our observations over the year indicate that both red clover and birdsfoot trefoil require a 40 to 45 day rest period to ensure that natural reseeding will occur. The only time during the course of the study when this much rest has been allowed has been in the fall after grazing ended in early September. The fall regrowth period is adequate for red clover to flower and set seed for stand maintenance. Birdsfoot trefoil has peak flowering near the longest days of the year and does not flower adequately late in the season for stand regeneration. One advantage of trefoil compared to red clover is that it flowers much closer to the ground and will frequently set some seed even if rest periods are shorter than 40 days, particularly at low to moderate stocking rates.

Figure 11. Change in individual legume species in continuously grazed pastures at the 1200 lb/acre stocking rate.

Figure 12. Change in individual legume species in rotationally grazed pastures at the 1200 lb/acre stocking rate.

As the grass component of the pasture has declined, legume component and summer annual grasses have increased. The total ground cover of desirable forage has actually changed very little over the first three years of the project, particularly at the 300 and 600 pound/acre stocking rates. At the lowest stocking rate, 300 pounds/acre, total forage cover have decreased slightly as the amount of dead residue covering the soil surface has increased. At the same time, the amount of bare ground has increased in the highest stocking rate pastures has increased somewhat, particularly in the continuously grazed pastures.

Practical application of results: This study gives us a couple of good pointers on how to improve pastures and maintain desired grass legume mixtures. One of the first lessons to be learned is that undergrazing can be as unhealthy for pastures as overgrazing. The visible decadence and lack of legumes in the 300 pound/acre stocking rate pasture is testimony to the fact. The pastures were much more diverse and vigorous in 1996 at the beginning of the study than they are presently.

On the opposite end of the spectrum are the 1200 pound/acre stocking rate pastures. Most of you have probably been told that if you graze your pasture too hard you will lose the legumes. While this appears to be true in the continuously grazed pastures in this study, the high stocking rate rotationally grazed pastures contain the highest percentage legumes of any pastures in the study, with red clover as the primary clover, not white clover as many would have predicted.

Using a high stocking rate for a few years to condition a pasture, or land coming out of CRP, can quickly increase the legume presence in the pasture. Moderating stocking rate after the legume component has been restored may be appropriate in some cases. In this study, rest periods were not long enough to allow trefoil to significantly reseed itself. That management was an artifact of the research design not a "best management practice". Within a grazing season, high early season stocking rates with short rest periods followed by reduced stocking rate with somewhat longer rest periods after mid-July may well produce the best balance of grasses and legumes in the pasture.

One other observation of note is that this is one more study which has illustrated the significance of volunteer summer annual grasses in predominately cool-season pastures. Although the data was not presented here, summer annual grasses were frequently the component that helped maintain the near constant total forage cover at the 600 and 900 pound/acre stocking rates. Crabgrass and barnyardgrass are the most significant species and seem to be the most preferred by grazing livestock.

We will complete the fourth and final year of this project in fall of 1999. A final project report will again be posted on our website by mid-winter.