Impact Of Stocking Rate And Grazing Management System On Profit And Pasture Condition
Steer performance at the three lower stocking rates was similar to previous years but was significantly lower than the four-year mean at the highest stocking rate. Stocking rate continued to have a significant effect on steer performance while grazing method was significant only at the highest stocking rate, with rotationally grazed cattle gaining more rapidly than continuously grazed cattle. Because of the removal of the one group of cattle from the continuously grazed-high stocking rate pasture, gain per acre was significantly greater for rotational grazing at the highest stocking rate in 2000. Otherwise stocking rate had a significant effect while grazing method did not.
Rotational grazing in 2000 resulted in higher green forage availability with less dead material accumulation than continuous grazing at all stocking rates. Differences in green forage availability became more pronounced as the summer progressed. Fall regrowth was also significantly greater for rotational grazing at all stocking rates while stocking rate effect was also significant.
Legume content which had declined dramatically in 1999 due to the extremely dry conditions rebounded in 2000 in almost all pastures. Legume content at the end of 2000 was significantly greater in rotationally grazed pastures at all stocking rates compared to continuous grazing.
Introduction: This research has compared continuous grazing to a flexible 12-paddock management-intensive system across four stocking rates for each system. Objectives include measurement of animal performance, pasture productivity and quality, species composition of the plant community, impact of grazing treatments on the soil bulk density and nutrient redistribution, and economic analysis of the systems. Year 2000 was the fifth and final year of the grazing study. Final measurements of species composition and nutrient distribution will be made in 2001. The same grazing treatments were applied to the same fields for all five years of the study to assess longer term effects of stocking rate and grazing management.
- Determine and model the relationships among forage supply, stocking rate, gain per acre, and average daily gain of yearling cattle either continuously grazing or rotationally grazing a mixed cool season grass-legume pasture.
- Measure the impact of stocking rate and method on pasture plant community.
- Monitor soil bulk density, soil fertility, and nutrient redistribution in each pasture system.
- Assess the impact of stocking rate and method on profitability in a stocker operation.
Research approach: Pasture grasses were predominantly endophyte-free tall fescue, orchardgrass, and Kentucky bluegrass, with lesser amounts of timothy, red top, and reed canarygrass. Several pastures contained a small remnant population of big bluestem and switchgrass. In the spring of 1994, the entire area was overseeded with red clover and birdsfoot trefoil. Annual lespedeza and white clover were also seasonally present in most fields. Sixteen 10 acre pastures were 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 had been in established tall fescue for 16 years and eight were no-till seeded to tall fescue in the spring of 1994. Blocking was 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. Rotational grazing cells consisted of 12 equal sized paddocks constructed with temporary fence. Stocking rates of 300, 600, 900, or 1200 lb liveweight per acre at turnout were utilized with grazing by yearling steers with a starting weight of approximately 550 lb/head. Steers were stratified by herd origin and weight and then assigned to treatments randomly from each by group.
Beginning and ending dates for each year and each stocking rate are given in Table 1. All treatments began grazing on the same date in 1996. In all subsequent years, initiation of grazing was staggered to reflect the relative amounts of forage residual resulting from the different stocking rates. Termination of grazing was on the same date for all treatments within each year with the exception of one block of continuous grazing at 1200 lb/acre stocking in 2000 which was terminated on August 2. At the beginning of the study, a criteria was set for early removal of livestock from any treatment. If the test animals lost weight during two consecutive 21-day periods, that treatment group would be removed from the study. Cattle were weighed unshrunk on two successive days at the beginning and end of the study and were weighed unshrunk at 21 day intervals in between. While various groups of cattle exhibited weight loss for a single 21-day period on several occasions through the study period, only the one group in 2000 exhibited weight loss in two successive periods. Cattle were dewormed at the beginning of each grazing season. They were not implanted or fed any supplemental feed. Salt blocks were available to the cattle at most times.
|Table 1. Beginning and ending dates for grazing for each year and stocking rate combination.|
*Except one block of continuous grazing which was terminated on August 3, 2000.
General Description of Sampling System
Within each rotationally grazed pasture, 12 paddocks were created using portable electric fences and water tanks. Fence lines were always set on the same line and water tanks were usually placed within the 25 ft radius of the water valve. In most pastures, each of these paddocks was a strip approximately 350 to 450 ft long and 90 to 115 ft wide. In two pastures, paddock shape was different due to location of existing permanent fences and water course. All sampling was on an individual paddock basis. While baseline data and ending data was collected from all paddocks and transects, during the 5 grazing seasons, only 4 monitor paddocks were sampled for forage availability during the grazing season due to labor constraints. Paddocks 1 and 12 were not used as monitor paddocks in any pasture due to pre-existing soil and plant conditions occurring at these locations. With the exclusion of paddocks 1 and 12, the four monitor paddocks within each pasture were randomly selected from the remaining ten paddocks. Within each rotational pasture treatment, paddocks were used as additional replications for plant and soil data in the statistical analysis. In the continuously grazed pastures, the sampling stratification followed what would be the same logical paddock subdivision system used in the rotation pastures. The basic difference in sampling method was that, within a rotation pasture, samples were collected from the defined paddock area while in the continuous pastures the samples were collected along a transect line.
Herbage mass and Available forage:
Forage availability was determined before and after each grazing period in four randomly selected paddocks within each rotational grazing pasture. The same 4 paddocks were used throughout the five-year period. Within each paddock, a stratified sampling system was used to divide each paddock into 3 zones with Zone A representing the 1/3 nearest to water, zone B the middle third, and zone C the one third most distant from water. Within each zone, 3 randomly placed .3 m2 quadrats were clipped to ½ in. stubble height with hand-held manual grass shears. Prior to clipping, mean sward height was measured using a yard stick within the area to be clipped. The clippings from all three quadrats within a zone were combined in a single bag and labeled with the date, pasture ID, paddock number, zone letter, and whether it was a pre-grazing or post-grazing sample. Post-grazing samples were clipped only in 1996 and 1998. In all other years post-grazing residual was predicted based on the height:yield relationships developed in 1996 and 1998. Data for forage availability presented in this report is the mean of pre- and post-grazing measurements.
In continuously grazed paddocks, nine 0.3m2 quadrats were clipped along four transect lines on a bi-weekly basis to monitor forage availability. As with continuous grazing, three quadrats were combined in a single bag and labeled as A, B, or C. Mean sward height was determined at each quadrat site as described for rotational paddocks. Sward height measurements were made along the eight other transects in each continuously grazed pasture at bi-weekly intervals. Forage dry matter availability at the height-only transects was predicted from the height:yield relationship
Clipped samples were brought to the lab and individually weighed. After wet weight is recorded, all three sample bags from a paddock were combined, thoroughly mixed, and a 150 to 200 g subsample collected. The sample was oven dried at 130o F, ground to pass a 1mm sieve, and was analyzed for nutritive composition. After the paddock was grazed and cattle moved to next paddock residual sward height was measured and residual forage availability was calculated using height:yield relationships developed for each grazing system during the 1996 grazing season.
At each quadrat site, a visual estimate was made of the percentage of dead forage present in the sward. When the samples from each set of three bags was combined and thoroughly mixed prior to sub-sampling, a second visual estimate of dead material was made. The mean of those two estimates is presented as percent dead forage in the report and was used in calculating green forage dry matter yield for each sample. Regression analysis was used to determine the consistency of the two methods of estimating percent dead material and the r-square value was greater than 0.8.
In 2000 no clipped yield estimates were made. Mean sward height measurements were made at the beginning and end of grazing on all 12 paddocks in rotational grazing pastures in all 12 transects in continuous grazing pastures. Forage availability was then predicted based on the height:yield relationships developed in the previous years.
Species composition and stand density data were collected near initiation of grazing each year (April 15 – May 1), midway through the grazing season (July 10-July 25), and after termination of grazing (Oct 1). Within each rotational grazing pasture, data was collected from all paddocks. Species frequency was determined through a step-point sampling system with 50 points per transect and 2 transects per paddock. In continuously grazed pastures, two point-step transects with 50 points each was made approximately 10 ft off the primary transect line on either side. The point-step transect was offset from the primary transect to avoid sites that may have been harvested as clipped quadrats. Percent of species frequency was based on total number of hits for any given species.
Prediction equations for forage dry matter yield were developed using linear regression of herbage mass and green forage dry matter yield as dependent variable and mean sward height as independent variable. First and second order equations were calculated. In most cases, fit improvement using the quadratic equation was minor and first order equations were used for simplicity. Broad equations across year, month, grazing method, and stocking rate provided poor fit. Analysis compared equations developed across year, month, and stocking rate within grazing method provided better prediction for rotational stocking but were not significant for continuous stocking. Adequate predictive capability for bulk forage yield for rotational stocking was found by developing equations within stocking method and month, across years and stocking rate. No significant relationships between bulk yield and mean sward height were found for continuous stocking. When dead material in the sward was subtracted from the bulk yield, mean sward height provided acceptable prediction of green forage dry matter yield for both rotational and continuous stocking within grazing method and month. Separate sets of equations were used for pre-grazing and post-grazing predictions.
Soil bulk density:
Soil bulk density was measured in early April, mid-July, and mid-October of each year except April, 1998, when soil conditions were too wet for sampling and October, 1999, when limited labor availability restricted data collection . Within each rotationally grazed paddock or along each transect in continuously grazed pastures, four 3 in. X 3 in. soil cores were extracted, oven dried, and weighed to determine dry soil bulk density during each sampling period.
Soil fertility and nutrient redistribution:
Prior to initiation of grazing in 1996, soil samples were collected from each paddock and transect within the study. Samples were extracted from the surface three inches and the 3 to 6 in. Each sample consisted of 20 cores (.75 x 3 in.). Samples were analyzed for organic matter, soil pH, neutralizable acidity, Bray P1 phosphorus, exchangeable K, calcium, and magnesium. In spring 2001, a final set of samples will be collected from the same sites and change in nutrient status occurring during the five years of the study will be determined.
Overall treatment effects were determined using the SAS general linear models (GLM) procedure with stocking rate as main plot and grazing method as sub-plot with appropriate error terms for each treatment. Where analysis across years indicated a significant year effect, subsequent analysis was done within years. Time sequence data including plant community and soil bulk density were analyzed as repeated measures across years.
All agricultural enterprises are at the mercy of the weather, as are all agricultural research projects. The five years this study encompassed provided a wide range in environmental conditions including the wettest single month recorded at FSRC in 25 years (May, email@example.com inches), the second driest July-August period in 25 years (1999 @ 2.65 inches), and monthly mean high temperatures from 8o F below normal (May, 1997) to near normal and monthly mean low temperatures as much as 10o F below normal (September, 1996). All in all, typical Missouri weather. Two years were more than 20% below normal precipitation, one was more than 20% above normal precipitation, and two were very near normal, although distribution through the season was extremely variable. Overall, the five-year period was characterized by above normal spring precipitation and below normal late summer precipitation (Table 2)
|Table 2. Monthly precipitation for the study period with five-year study period mean and 25-year norm for the location.|
Mean monthly high temperatures during the study period tended to be below the 30-year norms for the area, with a 2 to 9 degree year to year variance (Figure 1). In most cases, high temperature did not appear to be a factor in reducing summer pasture growth. Mean night time low temperatures also tended to be 2 to 5 degrees below the long term means for the area. Of the thirty months included in the five grazing seasons, only four had mean day-time high temperatures were above the 30-year norm for the location. The below-normal temperatures probably helped alleviate the effects of the drought that commonly occurred during the late summer periods.
|Figure 1. Mean monthly high temperatures for April through September 1996-2000 were generally below the 30-year norm based on 1951-1981 data.|
Results and Discussion
Steer performance: As in the first four years of the study, steer ADG in 2000 declined linearly with increasing stocking rate but was largely unaffected by grazing method (Figure 2). There has been no statistically significant difference in season ADG due to grazing method in any year, although differences have been noted in individual weigh periods. Five-year mean ADG is shown in Figure 3 and shows the consistency of the performance trend across all years. In each year, there has been about a one tenth pound/day numeric advantage to rotational grazing at the 900 and 1200 pound/acre stocking rates. In four out of five years, the same numeric advantage has been present at the 300 pound/acre stocking rate. The consistently superior performance of steers continuously grazing at 600 pound stocking rate is not consistent with the usual theoretical linear model of declining animal performance with increasing stocking rate. The response at this stocking rate may be an effect of the characteristics of the two specific pastures where the 600 pound stocking rate was located rather than a true effect of the treatment. By random assignment, that particular treatment fell on two superior sites based on soil type and past performance.
The similar ADG across grazing method may reflect the high condition of the continuously grazed pastures at the outset of this study. Pastures were well established and had been under rotational management for six years prior to this study. Legume component in these pastures was well above what is typically seen in pastures in this region that have been under long term continuous grazing. However, legume composition of continuously grazed pastures declined during the 1998 season and were quite low through 1999 but rebounded somewhat in 2000. Another factor to consider in assessing animal performance in a study of this nature is that the steers were weighed every 21 days creating a stress on the animals that is typically not present in a commercial production system. While every attempt was made to handle the cattle in a relatively gentle manner, research at University of Hawaii has shown depressed performance by cattle put through a handling chute to last in excess of six weeks (Smith, B. 1998. A guide to low stress animal handling. The Graziers Hui, Kamuela, HI, p5). Handling stress may be an overriding factor in determining steer performance.
|Figure 2. Steer average daily gain on continuously and rotationally grazed pastures at four stocking rates in 2000.|
|Figure 3. Four year mean steer average daily gain for continuously and rotationally grazed pastures at four stocking rates.|
The shade issue must also be considered in light of the treatments and the possible confounding effect noted in the weather discussion. Recent research at University of Kentucky has shown a benefit to yearling cattle allowed access to shade on high-endophyte pastures. Pastures used in this study were mixed grass-legume pastures with low endophyte fescue. At the beginning of this study we did not consider shade to be an issue and no provision was made to provide equal shade opportunity for the different grazing treatments. There is no way of knowing whether or not the lack of shade played a role in this study but it is a possible confounding factor that we should be aware of. Steers in all of the continuously grazed pasture had access to shade every day of the grazing season, while cattle in five of eight of the rotationally grazed pastures had shade access only on two days out of each 24 day grazing cycle. In two of the remaining rotationally grazed pastures, shade was available perhaps 50% of the time while the final pasture had shade available until solar noon on almost all days.
|Figure 4. Change in steer liveweight through the 2000 grazing season for steers grazing at four stocking rates on continuously grazed pastures.|
For most of the 2000 season, little difference existed in rate of gain due to grazing management (Figures 4 & 5). Steers in all treatments gained steadily for the first 100 days of the season and then rate of gain in all treatments slowed significantly during the hot period in July-August. Continuously grazed steers at 1200 lb/acre stocking rate in one block lost weight from June 31 until August 3, resulting in their removal from the study. During most of the month of July, we experienced difficulties with the pump and water distribution system and cattle were frequently without water for periods from 4 to 24 hours. Weights for eleven out of the sixteen treatment groups on July 12 indicated weight loss. During the next 21-day period all herds except the one continuously grazed 1200 lb/acre had positive weight change.
In 1996, 1998, 1999, and 2000, rotationally grazed steers gained more than continuously grazed steers in the last 6 weeks of the grazing season, but not in 1997. One of the claims that has been made for rotational grazing is that it will maintain ADG later in the summer than continuous grazing. While that trend was apparent in this study, the measured increased rate of gain by itself would not be adequate justification for implementing a rotational grazing system. Rate of gain declined much more rapidly at higher stocking rates earlier in the grazing season (Figures 6 and 7).
There are several negative effects associated with maintaining cattle on pasture after cattle growth rate significantly slows. Two major factors affecting profitability of the grazing enterprise are very little additional salable gain being produced from the forage resource and delayed marketing in late summer or early fall usually results in lower prices received. The land and pasture cost for the steer gaining 2.5 lb/hd/day in May and June is essentially the same for a steer gaining only 1 lb/hd/day in August. Obviously, the pound produced in June is much more profitable than the pound produced in August. The seasonal price for cattle typically steadily declines from mid-July through early winter. Maintaining cattle on pasture past midsummer usually results in lower price per pound. Combined with higher cost of production for late summer gains, the producer gains very little from late summer grazing of stockers.
|Figure 5. Change in steer liveweight through the 2000 grazing season for steers grazing at four stocking rates on rotationally grazed pastures.|
|Figure 6. Five-year pattern of net liveweight accumulation for continuously grazed steers at four stocking rates.|
From an environmental perspective, late summer grazing often results in very low forage residuals remaining after grazing resulting in loss of plant species diversity, vegetative ground cover, and potential increase in water runoff and soil erosion. These factors will be discussed later in this report.
|Figure 7. Five-year pattern of net liveweight accumulation for rotationally grazed steers at four stocking rates.|
We have identified dates by which 85% of the seasonal gain was achieved for the different treatments in this study (Table 3). Most of the profit in a stocker operation will be made on the early season gain. Heavier stocking rates require earlier removal of cattle from the system, but this strategy would allow a longer recovery period for the pasture. This approach is termed early-intensive double stocking and is used widely in the Kansas Flint Hills as a management practice to maintain health and vigor of native tallgrass range in that environment. This may be a useful management tool to maintain pasture ground cover, particularly in more fragile south Missouri environments.
|Table 3. Date by which 85% of seasonal liveweight gain was achieved with continuous and rotational grazing at four stocking rates (5-year mean).|
|Continuous||July 23||July 14||July 4||June 21|
|Rotation||July 18||July 13||July 20||July 11|
Gain Per Acre
Steer gain/acre in 2000 followed a very different pattern than had been observed in previous years (Figure 8). Higher ADG at lower stocking rates in 2000 compared to previous years, particularly with continuous grazing, produced higher gain/acre than previous years. Lower ADG at higher stocking rates combined with a shorter grazing season for a continuously grazed pasture at the highest stocking rate resulted in significantly less beef production per acre at 1200 lb/acre stocking rate for continuous grazing compared to rotational grazing. This is the only year in the study that the highest stocking rate did not produce maximum gain/acre (Figure 9). Gain per acre increased linearly with increasing stocking rate for rotationally grazed pastures during the five year study period while gain/acre responded asymptotically to stocking rate in continuously grazed pastures. The non-linearity with continuous grazing was largely due to the change in pattern in 2000.
|Figure 8. Steer gain per acre for continuously and rotationally grazed pastures at four stocking rates in 1999.|
|Figure 9. Five-year mean steer gain per acre for continuously and rotationally grazed pastures at four stocking rates.|
Forage height:yield relationships: For a number of years, we have been working on developing regression-based prediction equations for predicting forage dry matter yield from mean sward height. The sward stick that is used in all grazing schools in Missouri uses the sward height:yield relationship to predict forage availability. All of the data used to develop the table on the sward stick was derived from rotationally grazed pastures or small plots that were periodically harvested. This project was the first time that we have attempted to use height:yield relationships in predicting forage availability in continuously grazed pastures. We have found it to be much more difficult to develop predictive relationships for bulk forage yield in continuously grazed pastures compared to rotationally grazed pastures (Table 4).
|Table 4. Predictive equations of bulk forage dry matter yield from mean sward height, r2values, and coefficients of variation for rotationally and continuously grazed pastures for each month across four years of data collection.|
|April||1136 + 329 (ht)||.25||26.7||1013 + 324(ht)||.29||31.6|
|May||1470 + 190 (ht)||.38||21.9||NS|
|June||1006 + 444 (ht) .||56||24.7||1587 + 189 (ht)||.17||41.8|
|July||1074 + 305 (ht)||.63||17.7||1842 + 167 (ht)||.13||48.2|
|August||577 + 566 (ht)||.72||19.9||2116 + 149 (ht)||.05||58.7|
|September||536 + 224 (ht)||.74||15.0||571 + 461 (ht)||.53||44.3|
|October||567 + 220 (ht)||.62||21.7||929 + 196 (ht)||.24||41.6|
|All months||1718 + 141 (ht)||.27||40.3||1661 + 165 (ht)||.13||48.7|
Equations developed for prediction across all months for all years were imprecise with relatively low r2 values, high C.V.’s, and very broad confidence intervals. When equations were developed within month but across year, the statistical fit improved dramatically for rotationally grazed pastures, but not for continuously grazed pastures. Sward variance at time of data collection appears to be the key factor. Whereas all measurements for rotational grazing pastures were made following the rest period, continuously grazed pastures were sampled every two weeks during the grazing season. Spot grazing in continuously grazed pastures resulted in a higher level of within pasture variance than occurred in rotationally grazed pastures, as evidenced by the C.V.’s in Table 4. Accumulated dead material in the sward was greater in continuously grazed pastures. This factor would also lead to less precision in predicting bulk yield. The regrowth which was measured in the rotationally grazed paddocks tended to be much more uniform and less within pasture variance was present at each sampling date.
Regression equations were also calculated for the height:yield relationship by month within each stocking rate level for continuously grazed pastures. Significant relationships were not found for five of seven months at 300 lb/acre, for four of seven months for 900 and 1200 lb/acre stocking rates, and for two of seven months for 600 lb/acre stocking rate. Even at 600 lb/acre stocking rate where the best fits were found, the highest r2 was only 0.43 and the values for the other four significant months were under 0.3. The same analysis for rotationally grazed pastures produced significant relationships for every month at all four stocking rates with several individual month X stocking rate combinations having r2 values greater than 0.8.
Visual estimates of dead material in the sward were made at every quadrat that was clipped throughout the first four years of the study. Subtracting the dead material from the bulk yield provides an estimate of green forage present in the sward. Prediction equations for green forage dry matter yield from mean sward height were also developed (Table 5). Mean sward height was a significantly better predictor of green forage yield than bulk forage yield, particularly later in the season. As green forage is what grazing animals preferably select for grazing, green forage yield is actually a better parameter to use when describing the suitability of a particular pasture for grazing.
|Table 5. Predictive equations of green forage dry matter yield from mean sward height, r2 values, and coefficients of variation for rotationally and continuously grazed pastures for each month across four years of data collection.|
|April||-125 + 503 (ht)||.72||10.6||830 + 190 (ht)||.28||22.6|
|May||704 + 162 (ht)||.52||15.9||1326 + 264 (ht)||.26||44.1|
|June||1220 + 217 (ht)||.58||19.2||513 + 188 (ht)||.45||27.4|
|July||373 + 267 (ht)||.83||14.6||442 + 106 (ht)||.42||37.2|
|August||329 + 336 (ht)||.73||16.1||489 + 220 (ht)||.60||35.3|
|September||398 + 304 (ht)||.69||22.5||242 + 218 (ht)||.41||43.3|
|October||871 + 179 (ht)||.66||12.1||-75 + 429 (ht)||.65||34.1|
|All months||971 + 153 (ht)||.27||33.4||1092 + 133 (ht)||.11||60.8|
Quadratic equations improved the statistical fit of the model in almost all scenarios, but the improvement was fairly minor with only .04 to .08 improvement in r2 values. For purposes of simplicity when dealing with producers, we recommend using the simple linear equations shown here.
No quadrats were clipped on any treatments in 2000 and all forage availability estimates for the final year were predicted from the height:yield equations in Table 5. Because of the low accuracy of the equations for bulk yield in continuously grazed pastures, only green forage dry matter yield is presented for 2000 (Figures 10, 11, 12,& 13). From June through September, rotational grazing resulted in significantly more green forage available/acre at all stocking rates compared to continuous grazing. Accumulated dead material in continuously grazed pastures resulting from uncontrolled growth during spring reduced the percentage of green forage available. It is difficult to explain the lack of animal response to rotational grazing management in this study in light of such prominent differences in forage availability and potential quality.
|Figure 10. Green forage dry matter yield in 2000 by month for continuously and rotationally grazed pastures at 300 lb liveweight/acre stocking rate.|
|Figure 11. Green forage dry matter yield in 2000 by month for continuously and rotationally grazed pastures at 600 lb liveweight/acre stocking rate.|
|Figure 12. Green forage dry matter yield in 2000 by month for continuously and rotationally grazed pastures at 900 lb liveweight/acre stocking rate.|
|Figure 13. Green forage dry matter yield in 2000 by month for continuously and rotationally grazed pastures at 1200 lb liveweight/acre stocking rate.|
Over the five-year course of the study, rotational grazing consistently maintained a higher level of green forage availability at every stocking rate. At the lowest stocking rate, bulk herbage mass tended to be slightly higher for continuously grazed pastures compared to rotational grazing, while green forage dry matter yield was significantly greater for rotational grazing (Figures 14 & 15). At both 600 and 900 lb liveweight/acre stocking rates, bulk herbage mass were similar for continuous and rotational grazing while green forage yield was significantly greater for rotational grazing (Figures 16, 17, 18, & 19). At 1200 lb liveweight/acre stocking rate, bulk herbage mass was greater late in the season for rotational grazing and green forage dry matter yield was significantly greater for rotational grazing throughout most of the grazing season (Figures 20 & 21).
|Figure 14. Five-year mean bulk forage yield for continuously and rotationally grazed pastures at 300 lb liveweight/acre stocking rate.|
|Figure 15. Five-year mean green forage yield for continuously and rotationally grazed pastures at 300 lb liveweight/acre stocking rate.|
|Figure 16. Five-year mean bulk forage yield for continuously and rotationally grazed pastures at 600 lb liveweight/acre stocking rate.|
|Figure 17. Five-year mean green forage yield for continuously and rotationally grazed pastures at 600 lb liveweight/acre stocking rate.|
|Figure 18. Five-year mean bulk forage yield for continuously and rotationally grazed pastures at 900 lb liveweight/acre stocking rate.|
|Figure 19. Five-year mean green forage yield for continuously and rotationally grazed pastures at 900 lb liveweight/acre stocking rate.|
|Figure 20. Five-year mean bulk forage yield for continuously and rotationally grazed pastures at 1200 lb liveweight/acre stocking rate.|
|Figure 14. Five-year mean bulk green yield for continuously and rotationally grazed pastures at 1200 lb liveweight/acre stocking rate.|
Green forage yield is a much greater determinant of voluntary forage intake by grazing animals than is total herbage mass. Low stocking rates tend to result in significantly higher percentage of dead material in the sward. At all stocking rates, continuously grazed pastures tended to have more dead material present in the sward early and late in the grazing season while levels were similar between continuous and rotational grazing in the middle part of the grazing season (Figures 22, 23, 24, & 25). The corresponding pattern of higher yield of green forage early and late in the grazing season for rotationally grazed pastures and similar levels at midseason was apparent in the preceding figures illustrating forage yield.
|Figure 22. Four-year mean percent dead material in sward of continuously and rotationally grazed pastures at 300 lb liveweight/acre stocking rate.|
|Figure 23. Four-year mean percent dead material in sward of continuously and rotationally grazed pastures at 600 lb liveweight/acre stocking rate.|
|Figure 24. Four-year mean percent dead material in sward of continuously and rotationally grazed pastures at 900 lb liveweight/acre stocking rate.|
|Figure 25. Four-year mean percent dead material in sward of continuously and rotationally grazed pastures at 1200 lb liveweight/acre stocking rate.|
Pasture regrowth from the end of grazing around September 10 through mid-October was significantly greater for all stocking rates on rotationally grazed pastures compared to continuously grazed pastures (Figure 26). Fall regrowth was not measured in 1996, but was in each subsequent year with the overall four-year effect being similar to what occurred in 2000 (Figure 27). There was year-to-year variance in fall regrowth response. Stocking rate effect was significant in every year, but had the least impact following the extremely dry conditions of 1999. Grazing method had the greatest effect following the same dry year. Grazing method was not a significant effect in 1997, but was in the following three years. Late summer moisture conditions were very good in 1997 which may have allowed all pastures to regrow optimally.
|Figure 26. Green forage yield measured five weeks after grazing ended was significantly greater for rotationally grazed pastures at all stocking rates in 2000.|
|Figure 27. Green forage yield measured five weeks after grazing ended tended to be greater for rotationally grazed pastures over five years.|
Because no quadrats were clipped to measure forage availability in 2000, forage quality was also not measured. The data presented are the four-year means for 1996-1999. As a main effect stocking rate affected both crude protein and net energy content of the forage in all four years. Both crude protein and net energy content increased as stocking rate increased (Figures 28-31). This response is highly correlated with the lower amounts of dead material present in the sward at higher stocking rates and in rotationally grazed pastures. The tendency for higher legume content in the higher stocking rate pastures would also likely contribute to higher crude protein level at higher stocking rates.
Grazing method did not affect crude protein level of the forage in all years, but affected net energy content in all years with rotationally grazed pastures consistently having significantly higher net energy levels compared to continuously grazed pastures. As a general rule, net energy is considered to be a greater limiting factor to animal production from cool season forages compared to crude protein content.
|Figure 28. Four-year mean crude protein content of continuously and rotationally grazed pastures at 300 lb-liveweight/acre stocking rate.|
|Figure 29. Four-year mean crude protein content of continuously and rotationally grazed pastures at 600 lb-liveweight/acre stocking rate.|
|Figure 30. Four-year mean crude protein content of continuously and rotationally grazed pastures at 900 lb-liveweight/acre stocking rate.|
|Figure 31. Four-year mean crude protein content of continuously and rotationally grazed pastures at 1200 lb-liveweight/acre stocking rate.|
|Figure 32. Four-year mean net energy for maintenance content of forage in continuously grazed pastures at four stocking rates.|
|Figure 33. Four-year mean net energy for maintenance content of forage in rotationally grazed pastures at four stocking rates.|
|Figure 34. Monthly net energy for maintenance levels in continuously and rotationally grazed pastures.|
|Figure 35. Monthly net energy for gain levels in continuously and rotationally grazed pastures.|
Species composition of most pastures varies both within season from season to season depending on weather conditions and grazing practices. A single observation of species composition during a single season can often be misleading. The five years of observation in this study may allow some conclusions to be drawn regarding the effects of both stocking rate and grazing management on species composition. Longer term evaluation of plant community would be needed to confirm the apparent trends.
|Figure 36. Variance in total forage cover in continuously grazed pastures over the five-year course of the study.|
|Figure 37. Variance in total forage cover in rotationally grazed pastures over the five-year course of the study.|
While total forage cover varied seasonally (Figures 36 & 37), regression analysis identified significant trends across the five-year study period. Total forage cover was significantly affected by both stocking rate and grazing management (Figures 38 & 39). Total forage cover declined at the two lower stocking rates in both grazing management systems while the highest stocking rate maintained total forage cover over the five years of the study.
|Figure 38. Trends in total forage cover in continuously grazed pastures.|
|Figure 39. Trends in total forage cover in rotationally grazed pastures.|
Stand decline in the lowest stocking rate can be attributed to the accumulation of dead material in the sward and large patches of forage being smothered under the excess residue. The amount of dead residue present in low stocking rate pastures was significantly greater at low stocking rates at almost every observation date (Figures 40 & 41).
|Figure 40. Dead material accumulation in continuously grazed pastures.|
|Figure 41. Dead material accumulation in rotationally grazed pastures.|
Total grass cover fluctuated from year to year but was very similar at the beginning and end of the study (Figures 42 & 43). Most of the reduction in grass cover occurred when legume cover reached its peak. The shift between grass and legume components is what allowed total forage cover to remain near constant at the higher stocking rates. Legume content of the pastures fluctuated much more widely than did the grasses (Figures 44 & 45), possibly due to greater sensitivity to weather conditions and the cyclic nature of red clover in natural reseeding situations.
|Figure 42. Variance in total grass cover in continuously grazed pastures.|
|Figure 43. Variance in total grass cover in rotationally grazed pastures.|
|Figure 44. Variance in legume composition of continuously grazed pastures at four stocking rates.|
|Figure 45. Variance in legume composition of continuously grazed pastures at four stocking rate.|
Increasing stocking rate resulted in increased legume cover in both continuous and rotationally grazed pastures. Excessive grass competition at the lowest stocking rate resulted in a rapid decline in legume presence early in the study. Extremely dry conditions in 1999 brought legume content to its lowest level, but rapid recovery came in 2000 with favorable rainfall during the summer months. The earlier pattern of greater legume presence with increasing stocking rate was evident during the legume recovery in the final year of the study. Even with the annual variance in legume composition, there was a significant difference between continuous and rotational grazing in maintenance of legumes in the sward at all stocking rates except the lowest. When the trend lines are evaluated, there was only a slight decline in legume composition over the five-year period in rotationally grazed pastures while continuously grazed pastures exhibited a much more pronounced decline in legumes. The slopes of the regression lines for legume decline in continuous and rotational pastures were similar for 300 lb stocking rate but were significantly different at the three higher rates
|Figure 46. Change in total legume cover of continuously and rotationally grazed pastures at 300 lb liveweight/acre stocking rate.|
|Figure 47. Change in total legume cover of continuously and rotationally grazed pastures at 600 lb liveweight/acre stocking rate.|
|Figure 48. Change in total legume cover of continuously and rotationally grazed pastures at 900 lb liveweight/acre stocking rate.|
|Figure 49. Change in total legume cover of continuously and rotationally grazed pastures at 1200 lb liveweight/acre stocking rate.|
|Figure 50. Change in bare ground exposed in continuously grazed pastures at four stocking rates.|
|Figure 51. Change in bare ground exposed in rotationally grazed pastures at four stocking rates.|
Continuous grazing resulted in more exposed bare ground at the end of the five years compared to the beginning of the study, except at the 300 pound stocking rate. Accumulated dead material at the lowest stocking rate kept the soil well protected. Rotational grazing better protected the soil surface while maintaining less than ten percent bare ground at all stocking rates. Less exposed soil should enhance rainfall infiltration and reduce risk of runoff
This five-year comparison of rotational and continuous grazing at four stocking rates produced many of the expected results. Almost all of the measured pasture parameters responded positively to rotational grazing across the range of stocking rates. Forage availability and forage quality were both significantly greater with rotational grazing. In spite of improved forage conditions, steer average daily gain was not improved with rotational grazing. Although steer ADG was not significantly different, steer ADG on rotationally grazed pastures was numerically greater than continuously grazed pastures at three of four stocking rates every year. If that trend were real, the improvement in ADG would be acceptable to many producers. Some reasons why differences in ADG were not any greater when prediction models based on forage supply and quality predicted greater differences include: 1) Cattle were weighed every 21 days and handling stress may have inhibited performance. 2) Continuously grazed cattle had full time access to shade while rotationally grazed cattle had little opportunity. 3) All pastures were managed on the same calendar rotation frequency and no attempt was made to actually balance animal demand and forage supply as would be done in a commercial operation.
One of the concerns of this study was whether rotational grazing would help maintain groundcover and maintain plant species diversity. At all stocking rates, total forage cover and legume content was better maintained by rotational grazing. While bare ground exposed increased significantly across the five year study period in continuously grazed pastures, it remained near constant in rotationally grazed pastures. It is important to note that the continuously grazed pastures used in this study began the five years in excellent condition. Most continuously grazed pastures in Missouri have been grazed in that manner for many years and do not carry as much ground cover as these pastures did. Soil characteristics under long term continuously grazed pastures are not likely to be as favorable as under these pastures.
Other than reiterating the many good things that rotational grazing can do for pasture, one of the principle findings of this study is that the vast majority of gain achieved by stocker operations occurs by mid-July. Leaving yearling cattle on mixed cool-season grass-legume pastures beyond mid-July is a lose-lose situation. Producers costs continue to accrue while the value of the animal declines. Pastures are grazed even more closely, reducing ground cover and leaving many species weakened going into the winter. A concerted outreach effort should be carried out to make cattle producers more aware of the benefits of early removal of stocker cattle from summer pastures. Removing cattle earlier is a clear win-win situation. Producers can reduce pasture costs per head and market cattle before the annual price cycle decline begins. Pastures are allowed to rest and greater ground cover and biodiversity can be maintained. Economic gain can also be realized from grazing the forage which is allowed to stockpile from midsummer until the end of the growing season.
2000 Progress Report, (Project # 9411595-1), July 1, 1995 – December 31, 2000