Forage is an essential part of the equine diet for health and performance. Pasture nonstructural carbohydrates (NSC) have been shown to fluctuate diurnally and seasonally throughout the year due to various factors including environmental conditions and plant stress. The intake of elevated NSC content is linked with metabolic and digestive diseases, including colic and laminitis. A yearlong grazing study was conducted at the Virginia Tech Middleburg Agricultural Research and Extension Center from October 2016 through September 2017 to investigate fluctuations in pasture NSC concentrations as well as the metabolic and digestive response of grazing horses. Twelve sporthorse mares (15 ± 3.4 yrs) were maintained on an 8.5-ha cool season mixed grass pasture with water, mineral (Buckeye Nutrition, Dalton, OH), and white salt ad libitum. Weekly pasture samples (200 g wet weight) were clipped at random 2.5 cm from the plant base at 0800 (AM) and 1600 h (PM) on d 1 and 0800 (AM) and 1300 h (PM) on d 2. Samples were weighed and dried at 70º C and submitted to a commercial laboratory (Equianalytical, Ithaca, NY) to determine NSC content (water soluble carbohydrates [WSC] + starch). Environmental conditions were measured including ambient temperature, relative humidity, solar radiation, rainfall, soil and canopy temperature, and photosynthetically active radiation (PAR). Corresponding weekly blood samples were collected at 1300 h on d 2 via jugular venipuncture into 4 mL potassium oxalate, and 7 mL EDTA vacutainer tubes and analyzed for glucose (mg/dL), insulin (μIU/mL), and L-lactate (mg/dL). Each month, fecal grab samples were collected from the midrectum to measure pH and Dlactate (µM). Additionally, monthly glucose and insulin dynamics (% ∆) were assessed via a modified oral sugar test. Pasture nutrients including DE (2.35 ± 0.12 Mcal/kg), NSC (25.45 ± 4.02% DM), WSC (19.65 ± 3.47% DM), and starch (7.25 ± 1.29% DM) were higher in the afternoon hours (PM) compared to morning measurements (AM). Pasture CP and carbohydrate fractions were higher in the spring and fall months compared to summer and winter months with NSC concentrations being highest in May (wk 19) at 25.45% DM. Pasture NSC content was correlated (P ≤ 0.05) with relative humidity (r = 0.38), solar radiation (r = 0.32), and PAR (r = 0.51) and tended (P ≤ 0.1) to have a relationship with ambient temperature (r = 0.23) and rainfall (r = 0.23). There was seasonal variation in all morphometric measures in grazing horses. BW was highest in the spring (P < 0.0001), while BCS and CNS were highest in the fall (P = 0.0021 and P < 0.0001, respectively). Metabolic responses in grazing horses also fluctuated seasonally with glucose and insulin concentrations being most elevated in the spring (P < 0.0001). There was also seasonal variation in digestive measures in grazing horses. Plasma Llactate and fecal D-lactate means differed by month (P < 0.05) with the highest concentrations in April (11.8 ± 0.91 mg/dL and 4220.4 ± 185.5 µM, respectively). Fecal pH was most acidic in April (6.52 ± 0.08). Pasture NSC content was correlated with weight (r = 0.35), glucose (r = 0.21), and insulin (0.26) in grazing horses and tended to have a relationship with CNS (r = 0.14). There was also a relationship between NSC and plasma L-lactate (r = 0.33), fecal D-lactate (r = 0.48) and pH (r = -0.27). Lastly, glucose and insulin % ∆ (P < 0.0001) were greatest during spring months, but there was no effect of fasting insulin (P < 0.2787) or fasting glucose (P < 0.2055) on glucose % ∆. These data indicate a relationship between seasonal changes in pasture NSC content and the physiological response in grazing horses. Future aims include evaluating possible seasonal fluctuations in the hindgut microbiome of grazing horses to better understand the link between the equine microbiome and nutritionally-related disturbances. Improved grazing management strategies are needed to reduce the risk of metabolic and gastrointestinal disorders in horses, which may lead to subsequent colic and pasture associated laminitis. / Doctor of Philosophy / Lush pastures are an important part of the equine diet for overall health and performance. However, there are several nutrition-related diseases that can occur when environmental conditions favor starch and sugar (nonstructural carbohydrates, NSC) accumulation in pasture grasses. Environmental conditions such as air temperature, intensity of sunlight, frost, and drought can all lead to increased accumulation of NSC in pasture grasses, especially in spring and fall months. When horses graze pastures with elevated NSC concentrations they can develop several conditions such as obesity, insulin resistance, and gastrointestinal upset. One of the most common but least understood equine diseases is pasture-associated laminitis, in which inflammation causes pain and damage to the structure of the equine hoof. The objectives of our research were to measure seasonal changes in pasture NSC concentrations as well as the metabolic and digestive response in grazing horses to better understand how the intake of pasture NSC content may lead to disturbances or disease in the horse. A yearlong grazing study was conducted at the Virginia Tech Middleburg Agricultural Research and Extension Center from October 2016 through September 2017 to investigate the relationship between pasture NSC and grazing horses. Twelve sporthorse mares were maintained on a 21-acre mixed grass pasture with water, mineral (Buckeye Nutrition, Dalton, OH), and white salt ad libitum. Weekly pasture samples collected to determine NSC content of the grasses. Weekly blood samples were collected from the horses to measure glucose, insulin, and L- lactate concentrations. Each month, fecal samples were collected to measure pH and Dlactate. Additionally, monthly glucose and insulin dynamics (% ∆) were assessed via a modified oral sugar test. Pasture NSC content fluctuated throughout the year and was most elevated in the spring and fall months. There was seasonal variation in the metabolic response of grazing horses with glucose and insulin concentrations being highest in the spring months. There was also seasonal variation in digestive measures in grazing horses. Plasma L-lactate and fecal D-lactate were most elevated in the spring and fecal pH was most acidic in the spring. These results indicate a relationship between seasonal changes in pasture NSC content and the physiological response in grazing horses. Future aims include evaluating possible seasonal fluctuations in the hindgut microbiota of grazing horses to better understand the link between the equine gastrointestinal bacteria and nutritionally-related diseases. Improved grazing management strategies are needed to reduce the risk of metabolic and gastrointestinal disorders in horses, which may lead to diseases such as colic and pasture-associated laminitis.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/89013 |
| Date | 16 April 2019 |
| Creators | Kaufman, Katelyn Lucille |
| Contributors | Animal and Poultry Sciences, McIntosh, Bridgett J., Biddle, Amy S., McKenzie, Harold C., Johnson, Sally E. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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