Forage quality and blood metabolites of horses grazing alfalfa, cool-season perennial grass, and teff

Abstract

The impact of forage species on blood metabolites concentrations of grazing horses (Equus caballus L.) is unknown. However, these differences can be crucial as plasma amino acid (AA) concentrations as well as the glucose and insulin response of grazing horses can be indicators of nutritional status or metabolic health. As a result, the objectives of these studies were to determine the impact of different forage species on plasma AA concentrations, protein synthesis, as well as the glucose and insulin response across seasons. Research was conducted in May (spring), July (summer), September (fall), and late October (late-fall) in St. Paul, MN in 2016. However, the data collected was divided into three different studies (1) July samples taken during the first 4 hours were used to evaluate the forage and plasma AA concentrations (2) samples collected in July and September were included in the glucose and insulin response analysis of the regular grazing season and (3) May and October samples were used to analyze the glucose and insulin response during the extended grazing season. Forage treatments included alfalfa (Medicago sativa L.), a mixed perennial cool-season grass (CSG) and teff (Eragrostis tef [Zucc.] Trotter), however, not all forage species were grazed every season. Alfalfa and CSG were grazed in May while CSG and teff were grazed during the October, with all three species grazed in July and September. During these times, forages were grazed by six horses (24 ± 2 yr) randomly assigned to a forage in either a Latin-square or cross-over design. Jugular catheters were inserted 1 h prior to the start of grazing and horses had access to pasture starting at 08:00 h for either 4 or 8 h depending on the season. Jugular venous blood samples were collected from each horse prior to being turned out (0 h) and then at 2 hour intervals following turnout. Plasma and serum samples were collected and analyzed for AA, glucose, and insulin. Corresponding forage samples were taken by hand harvest. Equine muscle satellite cell cultures were treated with sera from grazing horses to assess de novo protein synthesis. Seasons were analyzed separately and data were analyzed using the MIXED procedure in SAS with P ≤ 0.05. When evaluating forage, AA were generally lowest in teff and highest in CSG (P ≤ 0.05). Significant differences in threonine concentration in the plasma were observed; there was no effect on de novo protein synthesis of cultured equine myotubes treated with plasma obtained from the grazing horses (P ≥ 0.20). As a result, although there were significant differences in forage AA content only plasma threonine concentration was different at 4 h with no effect on protein synthesis of cultured equine satellite cells. When evaluating the glucose and insulin response during the regular grazing season, teff generally had lower (P ≤ 0.05) equine digestible energy (DE), crude protein (CP) and nonstructural carbohydrates (NSC) compared to the other forages. Differences in peak insulin were observed between horses grazing CSG and teff during the fall grazing (P ≤ 0.05). Additionally, when evaluating the extended grazing season, teff had lower NSC than CSG in the late-fall (P ≤ 0.05) with subsequently lower average glucose, average insulin, and peak insulin in horses grazing teff compared to CSG (P ≤ 0.05).These results suggest grazing teff could lower the glucose and insulin response of some horses, specifically in the fall and late-fall, and may provide an alternative forage for horses with metabolic concerns,