Effects of grazing system, season, and forage carbohydrates on glucose and insulin dynamics of the grazing horse

Abstract

Rotational grazing is a recommended practice for grazing livestock, but little is known about its benefits with respect to grazing horses. The objective of this study was to investigate the effects of continuous (CON) and rotational (ROT) grazing on forage nutrient composition and whether those concentrations influenced circulating glucose and insulin concentrations in the grazing horse. Twelve mature Standardbred mares were paired by age and weight and randomly assigned to 1 replicate of either a 1.5 ha cool-season grass CON or ROT system for a total of 3 mares in each system. Mares on CON were allowed to graze the entire system at all times, whereas mares on ROT were given access to a 0.4 ha pasture section and stress lot where they were confined during inclement weather and slow forage growth. Blood and feces from horses and forage from each system were sampled over one 24-h period in June, August, and October. Blood was assessed for plasma glucose (GLU) and serum insulin (INS), feces for pH, and forage for nutritional composition. Data were analyzed by ANOVA with repeated measures with significance set at P < 0.05. There were no treatment differences for water and ethanol soluble carbohydrates (WSC and ESC, respectively), starch, ADF, and NDF, but CP was lower in ROT compared to CON (P = 0.04). With respect to month, WSC were highest in June compared to August and October, whereas ESC were highest in June compared to only August. Starch was lower in October than in June and August. Concentrations of ADF and NDF were lowest in October compared to June and August. Crude protein was higher in October than June and August. Plasma GLU and serum INS were affected by season and time of day but not grazing system. For all horses, GLU was highest in August (105.6 ± 1.3 mg/dL), whereas INS was highest in October (0.

Effects of 27 mo of rotational vs. continuous grazing on horse and pasture condition

“…Season did have an effect on both forage nutrient content and glucose metabolism of the horses in that previous report, which is similar to other previous studies on seasonal nutrient content and horse sugar metabolism ( McIntosh, 2007 ). However, in the present study, differences in BCS and FAT became more pronounced later in the study period, while samples for the sugar metabolism companion study ( Williams et al, 2019 ) were collected only during the first full grazing season of this study (2015). As discussed above for pasture condition, evaluating horses maintained under these grazing strategies over additional years will provide useful information to determine if rotational grazing offers long-term benefits for controlling weight and preventing deterioration of metabolic health in the grazing horse.…”

“…Obesity is a primary risk factor for the development of insulin resistance and episodes of pasture-associated laminitis ( Carter et al, 2009 ). Williams et al (2019) did conclude that rotational grazing did not affect the soluble carbohydrate content of forages compared with continuously grazed forages, and therefore did not affect glucose and insulin concentrations of the horses. Season did have an effect on both forage nutrient content and glucose metabolism of the horses in that previous report, which is similar to other previous studies on seasonal nutrient content and horse sugar metabolism ( McIntosh, 2007 ).…”

“…Therefore, it is possible that the nutritional analysis of our randomly selected samples did not accurately represent the plants selected by the horses. For more details on the soluble carbohydrate differences in these systems and the effect on sugar metabolism in these horses, see a companion study, Williams et al (2019) .…”

Fecal microbiome of horses transitioning between warm-season and cool-season grass pasture within integrated rotational grazing systems

“…This previous study did not evaluate the impact of nutritional composition of the pasture forage. The nutritional profile of pasture forages varies across the grazing season [ 1 , 30 , 83 ] and even over the course of a given day [ 84 – 86 ]. Given the established influence of diet and dietary nutrients on the gut microbiota [ 26 – 28 ] it is likely that changes in pasture forage nutrients also influenced microbial composition of grazing horses evaluated by Salem et al [ 56 ].…”

Yield, nutrient composition, and horse condition in integrated crabgrass and cool-season grass rotational grazing pasture systems

“…Seasonality of NSC values in CSG forages, with lower NSC during summer months compared to spring and fall, has been previously documented ( Jensen et al, 2014 ; Williams et al, 2019 , 2020 ). As the SLUMP is the period during which horses in an integrated grazing system would spend the most time grazing CRB rather than CSG, results of this study indicate that benefits of this practice may be minimal if the nutritional management goal is to limit dietary NSC.…”