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Purpose
Pasture dry matter intake of many horses (e.g., mature idle horses) exceeds that necessary to provide daily energy requirements creating an inefficiency. One strategy for regulating pasture intake is to restrict the herbage mass (HM) available for grazing by “pre-grazing” with horses having higher nutrient requirements (e.g., work, growth, lactation), or an entirely different species (e.g., cattle, sheep or goats) using a “leader-follower” rotational grazing system. Another strategy for regulating pasture intake is to restrict the time allowed for grazing. Both methods have the potential to improve the efficiency of pasture use by preventing over-consumption.
What did we do?
Two experiments were conducted to evaluate the effectiveness of regulating pasture intake by: 1) restricting HM available for grazing, or 2) restricting time allowed for grazing. In the first experiment six mature geldings were assigned to a HIGH (n=3) or LOW (n=3) density HM pasture (0.37 ha) for a 7 d. Treatments were reversed and carried out for an additional 7 d. The LOW pasture HM was achieved by mowing to a predetermined sward height that yielded a target HM. Mowing was used to achieve the target HM, instead of “leader-follower” rotational grazing, in order to accurately obtain the desired target HM. Herbage mass of each grazing cell was estimated using a weighted falling plate meter according to Vibart et al. (1). Body weight (BW) was measured on d-0 and 7 and changes in BW were used to reflect differences in DM intake between treatments. Mean HM available at the start of grazing was 876 and 2180 ± 76 kg DM/ha, for LOW and HIGH, respectively (Treatment P < .001), and corresponds to approximately 11 and 27 kg DM•d^-1•hd^-1 available for grazing, for LOW and HIGH, respectively, assuming a grazing efficiency of 70%. Herbage mass density decreased from d-1 to 7 (Treatment x Day; P < 0.001) by 148 and 771 ± 105 kg DM/ha for LOW and HIGH, respectively. The magnitude of BW change tended (P = .06) to be greater for LOW (-11.5 ± 3.9 kg) than HIGH (3.3 ± 3.9). The tendency for BW loss in LOW was likely a function of decreased intake leading to decreased gut fill, as opposed to a body tissue loss, given the estimated initial HM for LOW was more than adequate to meet energy requirements of all 3 horses over the 7-d period (i.e., approximately 11 kg DM•d^-1•hd^-1) (3). The greater HM reduction in HIGH, as compared to LOW, suggests horses in HIGH consumed more forage than required to meet maintenance energy requirements (e.g., potentially 14 kg DM/d), and! represen ts inefficient use of pasture.
A second experiment using eight mature geldings maintained in a single pasture (1.5 ha) and containing approximately 3,000 kg DM/ha was conducted to determine the effect of restricting time available for grazing on pasture DM intake. Horses were randomly assigned to either continuous grazing (CG; n=4) or restricted grazing (RG; n=4) for 14 d. Horses in the RG group were muzzled to prevent grazing from 1600 to 800 the following day, but otherwise allowed to graze freely. Body weight was measured on d-0, 7 and 14. Differences in body weight between treatments were used as an indicator of differences in pasture DM intake. Body weight was not different between treatments on d-0, however BW increased from d-0 to 7 for CG (22 ± 6.6 kg; P < .01), and decreased over the same period for RG (-19 ± 6.6 kg; P < .01). The gain in BW along with the initial 3,000 kg DM/ha available for grazing (approximately 28 kg DM•d-1•hd-1) suggests CG consumed DM well above that required for meeting maintenance energy requirements; whereas the loss of BW in RG suggests reduced DM intake as compared to CG. A longer term study is necessary to determine if BW change observed for RG stabilizes or continues on a downward trajectory, indicating restriction was too severe.
1. Vibart RE, White-Bennet SL, Green JT, Washburn SP. Visual assessment versus compressed sward heights as predictors of forage biomass in cool-season pastures. J Dairy Sci. 2004;87:36.
2. Walker GA. Common Statistical Methods for Clinical Research. Vol. 2nd. Cary, NC: SAS Institute, Inc; 2002.
3. NRC. Nutrient Requirements of Horses: Sixth Revised Edition. Washington, D.C.: The National Academies Press; 2007. 360 p.
What have we learned?
The results of both experiments suggest that: 1) Mature idle horses, continuously grazing abundant pasture, consume more DM than is necessary to meet daily energy requirements representing inefficiency, 2) restriction of either herbage mass available for grazing, or time available for grazing can be developed as tools to regulate pasture DM intake of grazing horses, and ultimately enhance efficiency of pasture use.
Future Plans
Future plans include designing experiments to refine both restriction of herbage mass available for grazing, and time available for grazing as practical methods for improving the efficiency of feeding horses on pasture.
Corresponding author, title, and affiliation
Paul D. Siciliano, Professor, North Carolina State University
Corresponding author email
Other authors
Morghan A. Bowman, Graduate Research Assistant, North Carolina State University
Additional information
Glunk, E.C., Pratt-Phillips, SE and Siciliano, P.D. 2013. Effect of restricted pasture access on pasture dry matter intake rate, dietary energy intake and fecal pH in horses. J. of Equine Vet. Sci. 33(6):421-426.
Dowler, L.E., Siciliano, P.D., Pratt-Phillips, S.E., and Poore, M. 2012. Determination of pasture dry matter intake rates in different seasons and their application in grazing management. J. Equine Vet. Sci. 32(2):85-92.
Siciliano, P.D. and S. Schmitt. 2012. Effect of restricted grazing on hindgut pH and fluid balance. J. Equine Vet. Sci. 32(9):558-561.
Acknowledgements
This project was supported by the North Carolina Agricultural Research Service.
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