Horses grazing continuously within a single pasture often graze selectively resulting in under- and over-grazed areas. The net result is inefficient use of forage and/or eventually loss of ground cover. This practice contributes negatively to pasture health and the environment. Rotational grazing can alleviate this problem by forcing horses to be less selective due to constraints on space and time allowed for grazing. It is generally accepted that grazing cells should be sized to provide enough forage for no more than 7 d in order to prevent selective grazing. However, little information is available to definitively confirm this maximum residence time. If the residence time could be increased to greater than 7 d by increasing the size of the grazing cell without the occurrence of selective grazing then labor inputs associated with reconstructing fences and moving horses could be reduced. A reduction in labor might also contribute to an increased acceptance of this practice among horse owners and managers. Therefore a study was designed to compare effect of increasing residence time by increasing grazing cell size on the level of grazing uniformity.
What did we do?
A predominately tall fescue pasture (approximately 1.5 ha; Lolium arundinaceum Schreb cultivar Max-Q; Pennington Seed, Madison, GA) was divided into four equal sub-plots (approximately 0.37 ha). Eight mature geldings (approximately 500 kg; 9.75 ± 4.4 yr) were paired and randomly assigned to one of two grazing regimes within subplots as follows to determine the effect of residence time and grazing cell size on pasture characteristics reflecting uniformity of grazing: 1) single large grazing cell (SLGC) where horses had access to the entire 0.37 ha subplot for 21-d, or 2) multiple small grazing cells (MSGC) where horses had access to approximately one-third (0.123 ha) of the 0.37 ha subplot for 7 d and were then moved to the next adjacent one-third of the subplot every 7-d for a total of 21-d. Subplot size was estimated to contain enough DM to support DM intake of 2.4% of BW/d for 21 d assuming a grazing efficiency of 0.7. Pasture herbage mass, sward height, compressed sward height and percent ground cover were determined on d-0 and d-21within each sub-plot. The percent compressed sward height below 5 cm within each subplot was used as an estimate of “over-grazed” area. Response variables were analyzed as a repeated measures design for treatment, time and treatment x time interactions. A P-value of 0.05 was considered significant; whereas a P-value of 0.1 was considered a tendency.
What have we learned?
Pasture herbage mass, sward height, compressed sward height and percent ground cover were not affected by treatment or treatment time interactions. Pasture herbage mass tended to decrease over time (P = 0.08). Sward height and compressed sward height decreased over time (P < 0.05). Percentage of compressed sward height below 5 cm tended to increase at a greater rate within MSGC as compared to SLGC (P = 0.07). Results of this study suggest that sizing grazing cells for longer residence times is feasible and that sizing grazing cells for a shorter residence time requires more management to insure overgrazing does not occur.
Although the results of this study suggests that two horses can graze a 0.37 ha area containing enough dry matter to facilitate 2.4% of BW intake (assuming a grazing efficiency of 0.7); it is unknown how increasing the stocking rate (and related grazing cell size) will affect uniformity of grazing. Future experiments will investigate this question.
Paul D. Siciliano, Professor, Dept. of Animal Science, North Carolina State University Paul_Siciliano@ncsu.edu
Jennifer Gill, Department of Animal Science, North Carolina State University
Bott, R.C., Greene, E.A., Koch, K., Martinson, K.L., Siciliano, P.D., Williams, C., Trottier, N.L., Burke, A., Swinker, A. 2013. Production and environmental implications of equine grazing. J. Equine Vet. Sci. 33(12):1031-1043.
This project was supported by the North Carolina Agricultural Research Service.
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