This presentation will review the existing multi-species literature on nutrient cycling and how it is affected by the horse’s diet and rotational grazing.
Grazed pastures, particularly rotationally grazed pastures, recycle nutrients faster than ungrazed pastures. Nutrients on pasture land enter through animal waste, and waste feed or fertilizer; they leave through removal of forage, leaching/runoff, or animal product/waste removal. Taking away the animal component removes about half of the inputs needed to recycle the nutrients. Dietary nitrogen (N), phosphorus (P) and potassium (K) are required for basic maintenance of horses; however, not all of what is consumed is used by the animal, therefore the dietary concentrations of these nutrients will impact the nutrient cycling. Digestibility of N, P and K in horses is approximately 80, 25 and 75 %, respectively. What does not get digested will end up excreted back into the soil.
For example, in one study eight Standardbred mares were divided into two groups and received diets of grass hay and grain. The high P (HP) group received 142 g/d of NaH2PO4, formulated to provide 4.5-times the dietary P requirement, or 65 g phosphorus/d. The low P (LP) group received 28 g of phosphorus/d in the basal diet. Data showed that horses receiving the HP diet excreted higher P and water extractable P in the manure than those fed the LP diet (Table 1; Westendorf and Williams, 2015). The same goes for N, where one study used a treatment group that was supplemented with 700 g/d of soybean meal top dressed on 500 g of sweet feed per day (TRT; 1042 g protein/d DM total), while the control group received the sweet feed meals without the soybean meal (CON; 703 g protein/d total). Both groups were also fed 8 kg/d of a grass hay mix (562 g protein /d DM), water and salt ad libitum. Horses fed the TRT diet excreted more N and NH3 than horses fed the CON diet (Figure 1; Williams et al., 2011).
More intensive grazing also creates an increased rate of nutrient cycling due to the added animal inputs on the land. Even though no horse related studies have been performed on this topic studies in cattle have found that plant-available N levels doubled when cattle were rotationally grazed with five grazings per season instead of three (Baron et al., 2002). Kenny (2016) looked at horses grazed under either a continuous or rotational grazing system (see Pictures 1 and 2, Left to Right, respectively) and found no differences in system after one year of grazing, however, the author concludes that more time on the system could have generated differences.
Other factors that affect the rate of nutrient cycling include amount of legumes in the pasture, distribution of manure on pastures (i.e. relation to water, shelters and fencing), and use or rates of fertilizer.
More equine specific studies need to be performed looking at how grazing systems and equine diets affect nutrient cycling and how horse farm owners can utilize this to best manage their farm for optimal nutrient utilization.
Carey A. Williams, Equine Extension Specialist, Rutgers, the State University of New Jersey, Department of Animal Science
Baron, V. S., E. Mapfumo, A. C. Dick, M. A. Naeth, E. K. Okine, and D. S. Chanasyk. 2002. Grazing intensity impacts on pasture carbon and nitrogen flow. J. Range Manage. 55:525-541.
Kenny, L. B. 2016. The Effects of Rotational and Continuous Grazing on Horses, Pasture Condition, and Soil Properties. Master thesis, Rutgers, the State University of New Jersey, New Brunswick, NJ.
Westendorf, M. L., and C. A. Williams. 2015. Effects of excess dietary phosphorus on fecal phosphorus excretion and water extractable phosphorus in horses. J. Equine Vet. Sci. 35:495-498. doi:10.1016/j.jevs.2015.01.020
Williams, C. A., C. Urban, and M. L. Westendorf. 2011. Dietary protein affects nitrogen and ammonia excretion in horses. J. Equine Vet. Sci. 31:305-306.
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