Introduction: In almost all biological systems, time is a key factor that affects the ebb and flow of relationships. The length of time grazing animals occupy a particular paddock affects several aspects of the plant-animal interface. The authors have previously reported change in available forage, change in quality of forage, daily intake of grazing animals, milk production, and forage utilization all to be affected by the duration of the grazing period. If we can identify key relationships along the time continuum of a grazing period, we should be able to make more sound recommendations to grazing managers. Based on both previously reported data and new findings, we have identified what we consider to be important fundamental relationships for decision making in rotational stocking systems.
Materials and Methods: The data presented or referenced in this paper have been collected over a ten-year period and represents six separate replicated grazing studies each lasting two to four years. The pastures have included very diverse mixed cool-season grass-legume pastures, N-fertilized stockpiled tall fescue (Festuca arundinacea) pastures, and sown big bluestem (Andropogon gerardii) pastures. Detailed descriptions of research procedures for pasture and animal management can be found in the referenced papers. Forage intake for cows grazing in the big bluestem project was determined by dosing with chromium oxide and subsequent fecal collection. Intake in the stockpile study was calculated by three indirect methods and in the cow-calf study intake was calculated based on difference between pre- and post-grazing forage mass. Although different stocking rates may have been used for certain treatments within a project, all regression analysis has been performed within a given stocking rate to avoid confounding of stocking rate with duration of the grazing period.
Results and Discussion: Our research has shown that both forage availability and quality decline through the duration of a grazing period (Gerrish et al., 1994; Morrow et al., 1991,1994). This decline would be expected anytime that the forage consumption rate exceeds plant growth rate. During the summer months in north Missouri, forage demand would likely exceed pasture growth rate anytime stock density exceeded two animal units/acre (2200 lb animal liveweight/acre). In rotational stocking systems, stock density and grazing pressure typically increase as number of pasture subdivisions increase. As the optimum length of the grazing cycle during the growing season is determined by the required rest period, the length of the grazing period will therefore become shorter as level of subdivision increases.
Based on daily measurements of sward parameters, we have found the decline in forage quantity and quality to be near linear when the grazing period does not exceed three days (Morrow et al., 1991). When the grazing period is four to ten days in length, the decline is best expressed as a quadratic function with the steepest decline occurring during the first three to four days and declining very slowly thereafter. Voluntary forage intake, on the other hand, is near constant for the first two or three days of the grazing period and then declines linearly over the next several days (Figure 1) (Davis, et al., 1995; Morrow, et al.1994). Our results suggest that intake becomes limited when forage availability falls below approximately 1800 lb/acre due to both reduction in potential bite size and reduced quality in the lower canopy.
Estimated dry matter intake of beef cows grazing mixed cool-season grass-legume pastures was affected by both the duration of the grazing period as well as herbage on offer at the beginning of the grazing period (Figure 2). Forage availability at the beginning of the grazing period was greater for paddocks having longer grazing periods but all paddocks were grazed to similar residual levels resulting in somewhat greater temporal utilization but significantly reduced animal intake. To achieve 2.5% daily intake, the daily forage allowance needed to exceed 6% of the animal’s liveweight on a daily basis. While most reports indicate forage dry matter intake peaking around 3% of animal liveweight, several of our studies have shown intake continuing to increase above this level with very favorable sward conditions.
If a target level for intake is set at 2.5 % of liveweight, then forage availability must be maintained above approximately 1800 lb/acre to maintain the target intake for grazing periods less than four days (Figure 3). The base level for the surface is set at an intake level of 2.5% of liveweight. Any point on the surface that appears flat represents a forage availability and length of grazing period combination which results in less than 2.5% intake for the grazing period. As the length of the grazing periods extends beyond four days, the required forage availability to maintain target intake increases. The increase in required forage availability to maintain intake appears to be linked to the declining forage quality in the lower portions of the canopy.
Maintaining forage availability above 1800 lb for a given period of days will depend upon the grazing pressure or stock density in the paddock. The response surface depicted in Figure 3 is based on a stock density of 12,000 to 15,000 lb animal liveweight per acre. If stock density is increased, then herbage on offer will fall below the target level in fewer days while if stock density is decreased the target level can be maintained for more days. At a given stock density, the appropriate number of grazing days for a given herbage mass can be determined from such a response surface.
The implication of Figure 3 is that target post-grazing forage residual must be raised as length of the grazing period is increased if a target livestock performance level is to be maintained. Recommendations for a single target residual regardless of frequency of rotation or length of grazing period will likely result in lower than expected animal performance. A confounding factor to consider is that, on the average, as length of grazing period increases, the length of the rest period also increases in fixed paddock systems. Forage quality is, therefore, likely to be lower in paddocks grazed for longer periods. This would also lead to reduced intake, particularly during the latter days of the grazing period. If paddock number is reduced and rest period is thereby shortened, forage quality would be maintained at a higher level and the negative effect of length of grazing period on intake would be lessened, but our results still indicate reduced intake from longer grazing periods when compared to shorter grazing periods with comparable rest periods.
For animals at maintenance, such as dry stock and mature bulls, a lower target intake level would be appropriate and residual forage level can be lowered and the effect of depressed intake with longer grazing periods would be less critical. Our data suggests that when using leader:follower grazing systems combining producing livestock and dry stock, the length of grazing period for the lead grazers should not exceed 4 days, regardless of available forage at turn-in.
As we gain a better understanding of how grazing management strategies affect livestock responses in a whole-system context, we can increase the efficiency of the production system and maintain better control of the plant and soil resource. Providing grazing managers with better tools for making management decisions should be one of our highest priorities in grassland research and education as we move into the 21st century.
Davis, B.W., F.A. Martz, J.R. Gerrish, P.R. Peterson, and R.L. Belyea. 1995. A comparison of stockpiled tall fescue forage intake: Forage availability vs animal performance indices. In Proc. Amer. Forage Grassl. Coun. Vol 4:219-222. Lexington KY March 12-14.
Gerrish, J.R., F.A. Martz, R.E. Morrow, and G.B. Garner. 1990. Milk production of Polled Hereford cows on four pasture systems. In Proc. Amer. Forage Grassl. Coun. pp 251-255. Blacksburg VA, June 6-9.
Gerrish, J.R., P.R. Peterson, F.A. Martz, and R.E. Morrow. 1994. Impact of grazing management on production and persistence of big bluestem pastures. In Proc. Amer. Forage Grassl. Coun. Vol 3:299-303. Lancaster, PA. March 6-10.
Morrow, R.E., V.G. Schulz-Tate, J.R. Gerrish, and C.A. Roberts. 1991. Implications of daily quality changes in rotationally grazed pastures for beef cattle. In Proc. Amer. Forage Grassl. Coun. pp 145-149. Columbia, MO. April 1-4.
Morrow, R.E., D. Quinlan, M. Kerley, J.R. Gerrish, and F.A. Martz. 1994. Influence of grazing system on intake when cows graze big bluestem pastures. In Proc. Amer. Forage Grassl. Coun. Vol 3:229-232. Lancaster, PA. March 6-10.
Length of Beginning Ending Dry Matter Temporal Grazing Period Forage Forage Intake Utilization ------------------------------------------------------------------------- (days) ------ (lb/acre)------ (lb/hd/day) (%) 2 1904 1112 43.9 42 3 2165 1141 36.9 47 4 2231 1233 28.4 45 5 2521 1401 24.8 44 6 2511 1345 21.5 46 7 2984 1427 24.7 52
Figure 2. Estimated daily forage dry matter intake of beef cows grazing big bluestem pastures with a daily or weekly rotation.
Figure 3. Effect of length of grazing period and mean forage availability on voluntary forage dry matter intake of beef cows grazing mixed cool-season grass-legume pastures.
1 University of Missouri – Forage Systems Research Center, 21262 Genoa Road, Linneus, MO 64653-922