Nutrient Content of Australian Horse Pastures—Nutrient Intakes Compared to Requirements

Most Australian horses rely on pasture for some of their nutrient intake, and many are just on pasture without supplementary feed for some part of the year. To determine the nutrient intake of horses on pasture for comparison with recommended daily allowances, nutritionists need to know not only the pasture intake but also the nutrient content of the pasture. However, pasture analysis is uncommon, so nutritionists must rely on estimates of nutrient composition.

A total of 435 samples from pastures grazed by horses in all states from 2000 to 2014 were submitted to Equi-Analytical Laboratories, Ithaca, New York, for forage NIR analysis. Some pastures were sampled several times during different seasons and years, and multiple pastures were sometimes sampled from the one property. Most were improved pastures in temperate regions with an emphasis on Thoroughbred studs. Some pastures were kikuyu-dominated, and some contained significant lucerne content, but specific pasture composition details were not recorded routinely. These results were tabulated and calculations made for average, range, standard deviation, normal range (+/- 1 SD), estimated fructan content (WSC-ESC), mineral ratio, and effect of season. Statistical analysis used ANOVA with significance set at p<0.05. Average values were used to compare nutrient intakes and requirements for different classes of horse grazing pasture alone without any supplementary feed. It was assumed that horses would eat 100% NRC (2007) dry matter intakes.

Table 1. Results of average and normal range (+/- 1 SD) from nutrient analysis of 435 horse pastures

	DM %	DE Mcal/kg	CP %	ADF %	NDF %	WSC %	Fructan %	NFC %	Cr Fat %	Ash %
Ave	25.3	2.05	19.3	34.7	56.3	6.9	1.95	11.3	3.4	11.3
Lower	14.8	1.84	12.8	28.9	46.8	3.8	2.2	6.0	2.3	8.8
Upper	35.8	2.27	25.9	40.6	65.7	9.9	3.5	16.6	4.6	13.7
	Ca %	P %	Ca:P	Mg %	K %	Na %	Fe ppm	Zn ppm	Cu ppm	Mn ppm
Ave	0.60	0.38	1.8	0.27	2.97	0.25	208	33	8.3	87
Lower	0.33	0.24	0.64	0.18	1.92	0.05	65	17.5	5.4	19
Upper	0.86	0.52	2.98	0.36	4.02	0.44	350	47.8	11.2	156

The pastures samples had lower DM, DE, WSC, ESC, NFC, and Fe content, and higher Na, than the mixed-mostly-grass pasture in the Equi-Analytical Laboratory feed library, which is drawn from over 11,000 samples. A 500-kg horse at maintenance grazing the average pasture would have deficient copper and zinc intakes in all seasons and would have excess DE intake in winter and spring, allowing it to gain weight. The same horse in the “lower normal” pasture would be deficient in Ca, Na, Cu, Zn, and Mn. Using seasonal average figures, a lactating Thoroughbred mare would have a diet that was underfortified for Cu in all seasons and would have excessive DE intake in winter and spring so could gain weight. A late-pregnant mare on pasture alone would have a diet that was underfortified for Cu and Zn. A 250-kg, 6-month-old Thoroughbred weanling growing at 0.9 kg/d would have deficiencies of DE, Ca, P, Cu, and Zn grazing the average summer pasture, and DE, Cu, and Zn on the average autumn pasture. The expected growth rate could not be achieved without supplementary energy sources. A 350-kg, 12-month-old yearling growing at 0.6kg/d would be deficient in DE, Ca, Cu, and Zn on average spring pasture.

This research was published in Proceedings of the Australasian Equine Science Symposium, 2014.