Bridging more of the ‘summer slump’ forage gap
(Writer’s note: This article was guest written by my graduate student Jennifer Weinert. She is a Wisconsin native who grew up on a dairy farm, but gravitated more toward the horses than cows. She is a graduate of the University of Wisconsin-River Falls with an animal science degree, then went out to work for a reining trainer before going back to River Falls to manage its 70-plus-head herd of university teaching horses. She started her Ph.D. at Rutgers University in September 2016.)
Last month, we discussed the implementation of integrated warm- and cool-season grass rotational grazing systems as a means of bridging the “summer slump” forage gap and increasing season-long pasture production. While this pasture management approach may offer both economic and environmental benefits for equine operations, incorporating warm-season grass varieties as a source of pasture forage may also have implications in promoting optimal health in the grazing horse.
According to the World Health Organization in 2016, nearly 40 percent of adult humans world-wide are overweight, while 11 percent of men and 15 percent of women are considered obese.
Interestingly, the scope of the obesity problem is similar in the equine population, with surveys estimating the prevalence of obesity at 10-19 percent with a greater 30-50 percent of horses classified as overweight (Body Condition Score [BCS] ≥ 7).
Just as the obesity epidemic has been associated with the development and/or exacerbation of modern chronic diseases (such as Type 2 Diabetes) in humans, obesity in horses has been linked to a number of conditions with negative health implications including Equine Metabolic Syndrome, insulin resistance, laminitis, hyperlipemia, lipomas, osteochondritis dissecans, and reproductive abnormalities.
The chief concern among these is the development of metabolic dysregulation in the obese horse, which can manifest in episodes of laminitis.
The development and exacerbation of an obese state is of particular concern in the grazing horse.
Horses maintained on pasture often experiences increases in body weight, BCS, and fat mass during seasonal grazing vs non-grazing periods.
Obesity has been established as a primary risk factor for the subsequent development of insulin resistance and episodes of pasture-associated laminitis.
Additionally, intake of pasture forage has been identified as a causative factor in nearly 50 percent of reported laminitis cases.
Traditional pasture forages may accumulate relatively high concentrations (20-30 percent) of non-structural carbohydrates including sugars, starch, and fructans.
A positive correlation has been established between pasture forage NSC content and circulating insulin concentrations in the grazing horse, and there is mounting evidence that elevated insulin levels may contribute to the onset of laminitis.
Therefore, current feeding recommendations for obese, insulin resistant, and/or previously laminitis horses include restricting or even eliminating pasture access and instead providing a grass hay with less than 10- to 12-percent NSC on a limited basis.
However, maintaining horses on pasture has obvious management benefits for equine operations including: reducing supplemental feed costs as well as manure accumulations in highly-stocked dry lots; promoting the persistence of vegetative cover and thus protecting against nutrient leaching and soil erosion; providing more natural feeding conditions that meal-feeding in stalls or dry lots; and allowing for greater exercise and voluntary movement.
Thus, there is a need to develop alternative grazing solutions for obese and metabolically impaired horses.
Warm-season grasses store carbohydrates differently than their cool-season counterparts.
Warm-season grasses synthesize minimal amounts of fructans, instead relying primarily on starch as their source of stored carbohydrate.
There is a limited capacity for starch storage in the plant chloroplast, resulting in lower total NSC levels in warm-season grasses.
In grazing research trials conducted at Rutgers in 2018, average NSC levels in both tested warm-season grass varieties (“Quick N Big” crabgrass and “Wrangler” bermudagrass) were below the recommended 10- to 12-percent threshold.
Our preliminary pasture studies in 2018 also found that plasma glucose and insulin fluctuations in healthy, non-obese horses were more moderate when rotating to warm-season grass compared to cool-season grass, and glucose clearance was more rapid when horses had been adapted to warm-season grasses vs traditional cool-season grass.
These responses were more pronounced in horses maintained on “Quick N Big” crabgrass as their warm-season pasture forage.
These results suggest that utilizing warm-season grasses as a source of pasture forage may indeed offer a viable grazing strategy for obese horses and those with equine metabolic syndrome.
However, the physiological responses of overweight or metabolically impaired horses differ from those observed in healthy individuals and further investigation is necessary to determine the extent to which metabolic responses of overweight horses will be impacted by extended grazing of warm-season grasses.
Identifying grazing strategies for obese and metabolically impaired horses is necessary to ensure that this segment of the equine population can be managed in an environmentally and economically responsible manner.
Thus, in an upcoming grazing study at Rutgers (summer 2020), we will evaluate the effectiveness of “Quick N Big” crabgrass, in controlling weight and promoting metabolic health in obese horses. We look forward to sharing the results of this study with you in the coming years.
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