Effect of Time of Day During Grazing on Hindgut Fermentation Parameters in Cool Season Grass Pasture-fed Horses.

WYCOFF, CASSIE LEE. Effect of Time of Day During Grazing on Hindgut Fermentation Parameters in Cool Season Grass Pasture-fed Horses. (Under the direction of Dr. Paul Siciliano and Dr. Shannon Pratt-Phillips). An experiment was conducted in the spring and fall of 2011 to determine if restricting horses’ grazing time to 10 hours/day when non-structural carbohydrate concentrations (NSC) are declining (during late evening and early morning hours) as compared to horses grazing during daylight hours may be a feasible strategy to reduce pasture NSC intake and its effect on hindgut microbial fermentation. Both experiments were conducted at times of year when NSC concentrations are known to be highest. Experiment 1 (EXPT 1) was conducted between April 11, 2011 and May 10, 2011. Twelve mature idle light horse geldings weighing 584 + 49 kg (mean + SD) were used in a three period switch back design, whereby all horses received all treatments. Horses were randomly assigned to one of three dietary treatments for a 10-d period: 1) 24-h continuous grazing (CONT), n = 4; 2) 10-h of grazing over-night (NIGHT), n = 4; 3) 10-h of grazing during full daylight (DAY), n = 4. Horses in the CONT group were turned out at 1000 on d-1 and remained in their grazing cells until the completion of the period. DAY and NIGHT were turned out to pasture at 1000 and 2100, respectively, to allow for 10 hours of grazing. Pastures consisted of mostly non-toxic endophyte infected tall fescue (Festuca arundinacea MaxQ, Pennington Seed, Madison, GA). Horses were housed in individual dry lot pens when not grazing. On d-9, pasture samples reflecting what the horses were eating were collected at the start, middle, and end of the daily grazing bout for NIGHT and DAY with parallel samples collected from CONT during both grazing periods (i.e., n = 6). Samples were frozen within 15 minutes of collection and later analyzed for NSC concentration by near-infrared spectroscopy. Rectal fecal samples were collected from each horse at 0800 and 2000 on d-9 and analyzed for pH immediately following collection. The remaining portion was frozen at -20° C until later analyzed for fecal VFA and D-lactate concentrations. Similar procedures were followed for experiment 2 (EXPT 2), which was conducted between September 26 and October 24, 2011. Mean pasture NSC concentration for EXPT 1 increased (P < 0.01) from 10.72% at 1000 to 15.32% at 2000 then decreased (P = 0.01) to 11.69% by 0700 the next morning. Mean fecal pH for EXPT 1 was 7.69, 7.36, and 7.12 (+ 0.06) for the CONT, DAY, and NIGHT groups, respectively, and was lowest (P = 0.01) in the NIGHT group compared to the other treatments and highest (P < 0.01) for the CONT group as compared to the DAY and NIGHT treatments. Fecal VFA profiles in the DAY and NIGHT groups were altered in comparison to the CONT group. Mean pasture NSC concentration for EXPT 2 increased (P < 0.02) from 17.77% (+ 0.57) at 1000 to 20.07% (+ 0.73) at 2100 then decreased (P < 0.01) to 17.02% (+ 0.57) by 0700 the next morning. Mean fecal pH values for EXPT 2 were 7.66, 7.38, and 7.35 (+ 0.042) for the CONT, DAY and NIGHT groups, respectively. Fecal VFA profiles were different (P < 0.01) in the CONT group when compared to DAY and NIGHT, however there was no difference between DAY and NIGHT. In both experiments, horses grazing continuously had higher (P <0.01) fecal pH and different (P < 0.01) VFA profiles when compared to the other