Rutgers equine pasture research results: Part 2 (Animal Science Update)
(Editor’s note: Jennifer Weinert is a graduate student of Carey Williams at Rutgers University. A Wisconsin native, she graduated from UW-River Falls with an animal science degree and worked for a reining trainer before going back to River Falls to manage its 70-plus head herd of university teaching horses. She started her doctorate studies at Rutgers in 2016 and plans to graduate next summer.)
Last month, we introduced the concept of integrated rotational grazing systems that incorporate both cool-season and warm-season grasses and discussed the impact of this practice on pasture production.
In addition to differences in growth patterns, these forage types also vary in how carbohydrates are stored within the plant.
Cool-season grasses have the capacity to store large quantities of fructans and are thus high in non-structural carbohydrates.
Warm-season grasses, conversely, accumulate little to no fructan and are typically lower in NSC.
High levels of NSC are of concern for overweight horses or those with a history of metabolic dysfunction and laminitis, and current recommendations are to feed these horses a diet that is less than 10-percent NSC.
In the integrated rotational grazing study at Rutgers, researchers evaluated pasture forage nutrient composition and the impact on horse body condition.
In this study, three healthy adult Standardbred mares grazed in each system — the integrated system containing both monoculture “Quick-N-Big” crabgrass and mixed cool-season grass sections and a control system in which all sections were planted with the cool-season grasses.
Pasture nutrients were assessed in samples collected at the beginning of each rotation and horse measures were collected monthly throughout the study.
This study found that the integrated rotational grazing approach did not result in marked differences in forage nutritional composition or horse condition in comparison to the traditional cool-season control.
During the “summer slump”, there was a trend for lower WSC in the crabgrass sections vs. the cool-season sections, and there was a season-long trend for lower NSC and WSC in the integrated system.
But other nutrient fractions such as digestible energy, protein, and fiber did not differ between systems.
Furthermore, with the exception of one cool-season grass rotational section, the NSC levels measured in both forage types during the “summer slump months” fell below the 10-percent NSC threshold.
The NSC levels were elevated above 10 percent for several cool-season rotations occurring earlier in the growing season (mid-May to mid-July) and in November, but NSC levels never exceeded 15 percent. Horse condition metrics such as body weight, body condition score, and body fat did not vary by grazing system (integrated vs. control cool-season).
These measures did differ across the grazing season, however, with horses in both systems gaining weight during early-season grazing of cool-season pasture forage and then maintaining condition throughout the remainder of the grazing season. Animal and grazing management in conditions within each system may explain this lack of differences in horse condition.
Neither grazing approach resulted under-weight or obese conditions in grazing horses.
These findings thus demonstrate that the crabgrass pasture forage provided adequate nutrition to maintain acceptable body condition in healthy non-obese horses.
The changes in horse condition over time noted above may also have implications for grazing management of overweight or obese horses.
In this study, horses gained weight from their initial baseline measurements through the early grazing period when horses had unlimited access to cool-season pasture grass.
Horses then maintained their body condition throughout the “summer slump” regardless of system.
This was despite the fact that horses in the control system spent over half of this period receiving a hay diet fed at maintenance level, while horses in the crabgrass system had ad libitum access to pasture forge.
When ad libitum access is provided, intake of pasture forage by grazing horses can exceed 3 percent of body weight (BW) per day [dry-matter (DM) basis].
Thus, there is potential for pastured horses to greatly exceed dietary caloric requirements, leading to weight gain, obesity and an increased risk of metabolic dysfunction.
Horses in the integrated system maintained their weight during the “summer slump” period despite ad libitum pasture access throughout, having no differences with their counterparts in the control system that spent over half of the “summer slump” receiving a maintenance hay diet due to limited availability of cool-season pasture grass.
These results suggest that integrating warm-season crabgrass could provide some benefit for weight-control in grazing horses.
Next month, join us as we discuss the impact of establishment method (monoculture vs. interseeded) on pasture forage yield and nutrient composition.