Nutrient Content of Australian Horse Pastures—Effect of Season

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. Some were irrigated, but most were not. These results were tabulated and calculations made for average, standard error, normal range, mineral ratio, and effect of month and season. Statistical analysis used Tukey-Kramer ANOVA tests set at p < 0.05%.

Table 1: Seasonal difference in pasture composition. Mean and superscripts indicating significant difference (P<0.05) from other seasons: a = summer, b= autumn, c= winter, d= spring.

Season	Summer	Autumn	Winter	Spring
No. of samples	87	81	129	138
Dry Matter, %	28.6bcd	25.0ac	21.6abd	24.4ac
DE, (Mcal/kg)	1.93cd	1.96cd	2.11ab	2.12ab
Crude Protein, %	16.1cd	18.3c	22.9abd	18.7ac
NDF, %	62.0cd	58.7c	51.3ab	55.9ab
WSC, %	5.2cd	5.3cd	7.8ab	7.8ab
Ca, %	0.54b	0.64ad	0.62a	0.54b
P, %	0.32bcd	0.38a	0.41a	0.37a
Zn, ppm	32.9	33.7	33.8d	29.9c
Cu, ppm	8.3	8.7	8	8.0
Quarter	1 (Jan, Feb, Mar)	2 (Apr, May, Jun)	3 (Jul, Aug, Sept)	4 (Oct, Nov, Dec)
Ca:P ratio	1.89ac	2.32acd	1.39ab	1.7b

There were significant seasonal differences in many nutrients as shown in Table 1. Interestingly, the winter pastures had the highest protein content whilst energy and WSC content was higher in winter and spring than summer and autumn. Not surprisingly, fibre content was highest in summer and dry matter lowest in winter. There were seasonal differences in calcium (Ca) and phosphorus (P) content, but little difference in the trace minerals zinc and copper. Seasonal changes in Ca and P content did not reflect the influence on the vital Ca:P ratio in horse pastures. The Ca:P ratio was best examined by quarter rather than season. In Q3, the mean Ca:P ratio of 1.39 was lower than in Q1 and Q2. Importantly only 11% of pastures had a Ca:P content less than 1:1 in Q1 and Q2, but 25% pastures were affected in Q3 and 16% in Q4. This situation would create problems with calcium resorption from bone in horses not receiving supplementary feed, and is more widespread than was previously recognised in late winter and early spring. Pasture analysis would be needed to detect the problem and should be performed more commonly to better define the contribution pasture can make to horse nutrition and the needs of supplementary feeding.

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