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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

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).

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.

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.

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