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84 Yield, forage nutritive value, and preference of annual grasses under horse grazing
Abstracts / Journal of Equine Veterinary Science 35 (2015) 418e436
ovulation occurred. Group 1 ovulated on average 17.5 d after initial ovulation and Group 2 ovulated on average 20.25 d after initial ovulation. These results indicated that at the dose used intrauterine administration of DIDS did not prolong diestrus and prevent luteolysis even though in vitro DIDS decreased PGF secretion. The results of this study identify DIDS as a possible management tool to inhibit mares from returning to estrus by inhibiting the release of PGF.
Key Words: prostaglandin F2a, luteolysis, estrous cycle
82 Effects of prolonged dexamethasone treatment on signaling pathways associated with muscle protein degradation in mature horses C.M.M. Loos*1, T. Barnes 1, K.M. Brennan 2, and K.L. Urschel 1 1 University of Kentucky, Lexington, KY, USA; 2 Alltech Inc., Nicholasville, KY, USA Prolonged dexamethasone (DEX) treatment has been shown to induce insulin resistance (IR) and reduce the activation of the muscle protein synthesis signaling pathways in horses. High doses of glucocorticoids may also contribute to muscle atrophy by upregulating signaling proteins involved in the proteolytic pathways. The objective of this study was to determine whether DEX-induced IR affects muscle protein degradation pathways. Eight mature horses were fed twice daily to meet maintenance requirements. An initial (pre-DEX) percutaneous biopsy of the gluteus medius muscle was performed, after an isoglycemichyperinsulinemic clamp procedure (IHC). After 21 d, during which all horses received 0.04 mg/kg BW/day of oral DEX, the IHC was repeated and a post-DEX muscle sample was obtained. Muscle samples were analyzed by Western blot to determine the relative abundance or phosphorylation of proteins associated with protein degradation. Data were analyzed using the mixed procedure of SAS version 9.2 with treatment as the fixed effect and horse nested in treatment as the random subject. Significance was determined at P < 0.05. Phosphorylation of Akt, an important factor in insulin mediated signaling pathways, was decreased by 55% post-DEX (P ¼ 0.02). FoxO1 is a downstream target of the Akt pathway and is generally upregulated during muscle atrophy. However, FoxO1 phosphorylation was increased by 2.2 fold post-DEX (P ¼ 0.01), indicative of deactivation. Similarly, the expression of myostatin, a negative regulator of myogenesis, was decreased by 40% post-DEX (P ¼ 0.05). Conversely, its expression is generally increased during glucocorticoid-induced muscular atrophy. Atrogin-1 and muscle ring finger-1 (MuRF-1), important ligases in the ubiquitin proteasome pathway, are expressed in high levels during skeletal muscle atrophy; however, no DEX effect was seen for either Atrogin-1 or MuRF-1 (P > 0.05). The unexpected results of this study could have been caused by the hyperinsulinemic state following the IHC, which may have prevented the activation of muscle proteolysis by DEX. Additionally, proteolytic pathway activation by DEX could be transient, and the 21-d time interval between pre- and post-DEX muscle samples did not allow for the detection of early changes in pathway function. Prolonged DEX treatment did not result in an upregulation in the signaling pathways associated with muscle proteolysis in mature horses when samples were collected in a hyperinsulinemic state. Future research should examine the effects of DEX administration on proteolysis over time and in a non-insulin stimulated state.
Key Words: dexamethasone, muscle, proteolysis
419
83 Maternal fatty acid supplementation influences memory and learning ability in yearling and 2-year-old horses A.M. Adkin*, A.V. Muniz, C.J. Mortensen, and L.K. Warren University of Florida, Gainesville, FL, USA Exposure to docosahexaenoic acid (DHA) in utero and/or early life has been shown to affect neural development, cognitive function, and future learning success in infants and children. We hypothesized that perinatal supplementation of mares with DHA would result in similar benefits in their offspring. As part of a longitudinal study, mares were supplemented with a fat supplement containing an algae source of DHA or a placebo fat supplement from d 250 of gestation through d 74 of lactation. When foals were 2 mo of age, operant conditioning was used to teach foals to touch their nose to a target object. At 6 mo of age, memory recall of prior targeting and performance on new tasks were evaluated. The outcome of these evaluations was reported previously, and showed that DHA influenced performance on some tasks, but did not enhance memory. In this abstract, we report on memory recall of previous tasks and learning performance on novel tasks in the same group of horses (DHA n ¼ 9, placebo n ¼ 10) when reevaluated as yearlings (mean ± SEM, 14.4 ± 0.3 mo of age) and 2-year olds (25.5 ± 0.2 mo). Memory recall of tasks learned since 2 mo of age was assessed over a 2-d period, and included the time to respond correctly to a cue and scoring based on response time. Additionally, learning ability on novel tasks, including crossing a large tarp on the ground and bridling (eg, lowering head, opening mouth, accepting the bit, headstall placement, bridle removal) was investigated in yearlings over a 12-d period. Novel tasks were divided into sequential levels requiring the yearling to earn perfect scores (based on predetermined criteria) over multiple cues to progress to the next level. Operant conditioning techniques were used to train yearlings and the same person handled all horses and scored all responses. Response time and number of cues were analyzed by one-way ANOVA and scores were analyzed by a zero inflated Poisson model. Memory recall was not affected by age (P ¼ 0.28), but horses were more likely (P < 0.0001) to score perfect on d 2 compared with d 1. Across age groups, DHAexposed horses had a tendency to be more likely (P ¼ 0.08) to score perfect on memory recall compared with placebo-exposed horses. Yearlings exposed to maternal DHA were more likely to score perfect on the tarp (P ¼ 0.05) and bridle (P ¼ 0.001) tasks, and tended to complete the bridle task faster (P ¼ 0.09) compared with placebo yearlings. Results indicate that exposure to DHA during the perinatal period may improve long-term memory recall and enhance learning ability in young horses, but more research is needed to confirm these findings.
Key Words: omega-3 fatty acids, DHA, cognition
84 Yield, forage nutritive value, and preference of annual grasses under horse grazing A.M. Grev*, C.C. Sheaffer, D.N. Catalano, M.L. Schultz, and K.L. Martinson University of Minnesota, St. Paul, MN, USA Annual grasses can serve as emergency forage but have not yet been evaluated under horse grazing. The objectives were to evaluate annual grasses for yield, forage nutritive value, and preference under horse grazing during spring and fall. Spring grasses were planted May 8, 2013 and April 22, 2014 in a randomized complete block (RCB) with 4 replicates. Grasses included
420
Abstracts / Journal of Equine Veterinary Science 35 (2015) 418e436
spring oat, spring wheat, winter wheat, spring barley, and annual ryegrass. Fall grasses were planted August 1, 2013, and August 5, 2014, in a RCB with 3 replicates. Grasses included the same 5 species plus winter rye, winter barley, and a forage variety spring oat. Plot size was 1.8 6.1 m. Yield and forage nutritive values were determined by harvesting duplicate 0.46 0.51 m areas to 5 cm. On June 18, 2013, June 5, 2014, September 17, 2013, and September 9, 2014, horses grazed all plots for 4 h at the onset of stem elongation. After grazing, horse preference was determined by visually assessing percentage of forage removal on a scale of 0 (no grazing) to 100 (100% grazed). Plots were mowed and grazing was repeated when spring forages regrew on July 9, 2013; June 24 and July 18, 2014; and when fall forages regrew on October 15, 2013; October 7 and November 4, 2014. Data were analyzed using the Proc Mixed procedure of SAS. Within each season, preference and quality measurements were averaged across grazing events and yield was totaled for all grazing events. In the spring, annual ryegrass had a greater yield (4.7 t/ha) compared with winter wheat, spring wheat, and spring barley (3.6 t/ha; P ¼ 0.0174). Winter wheat and spring wheat were the most preferred grasses with 73% removal, while spring oat was least preferred (15%; P ¼ 0.0367). Winter wheat had the greatest crude protein (CP; 28%), while annual ryegrass and spring oat had the least (22%; P ¼ 0.0208). All grasses had levels of nonstructural carbohydrates (NSC) between 14 and 16% except spring wheat, which had the lowest level at 11% (P ¼ 0.0009). In the fall, winter barley, annual ryegrass, and spring forage oat yielded the highest (4.7 t/ha), while spring barley and spring wheat yielded the least (3.4 t/ha; P ¼ 0.0210). Annual ryegrass and spring wheat were the most preferred grasses with 69% removal, while winter barley, both spring oats, and winter rye were least preferred (23%; P ¼ 0.0356). Winter rye had the greatest CP (35%), while spring oat was lowest (22%; P ¼ 0.0067). Annual ryegrass had the highest level of NSC at 16%, while all other grasses had levels between 12 and 13% (P ¼ 0.0175). Annual ryegrass presents a viable option for emergency pasture in the spring and fall.
Key Words: annual grasses, forage nutritive value, horse preference
85 Effects of rotational versus continuous grazing on horse condition and plant performance L.B. Kenny*1, A.O. Burk 2, and C.A. Williams 1 1 Rutgers, the State University of New Jersey, New Brunswick, NJ, USA; 2 University of Maryland, College Park, MD, USA Rotational grazing is often recommended but not widely adopted by horse operations in the Northeast US. In addition, equine specific research is lacking as to the benefits of rotational compared with continuous grazing. The objective of this study was to compare the effects of rotational and continuous grazing systems on horse health and vegetative performance. Two continuous systems (1.6 ha each) were established with shelters, feeders, and waterers. Two rotational systems were established with 4 rotational pastures (0.4 ha each) and a stress lot (0.1 ha) containing a shelter, feeder, and waterer. Twelve mature Standardbred mares (initial BW 543.9 ± 13.6 kg and body condition score (BCS) 6.1 ± 0.1) were used with 3 horses in each system for a stocking rate of 0.52 ha/horse. Rotational horses were allowed to graze a pasture when forage height was ¼ 7.6 cm and confined to the stress lot and fed hay when forage height was < 7.6 cm or during inclement weather. During those times, horses were fed to meet or exceed NRC requirements using grass hay fed at 2% BW along with concentrate during the winter months. Horse
condition was assessed by monthly BW, BCS, and rump fat as measured by ultrasound. Plant performance was assessed monthly by estimating vegetative cover composition, herbage mass, and sward height. Data are presented as means ± standard error, and statistics were performed using a repeated-measures ANOVA in SAS. Out of 184 d, horses in the continuous systems grazed 184 d, where rotational horses grazed for 97 d. During the winter months, the continuous system horses were offered less total hay than the rotational horses (6,102 vs. 6,714 kg, respectively). System had no effect on horse condition; however BW, BCS, and rump fat differed between each month (highest in September to November and lowest in January; P < 0.0001). Sward height differed between months (P < 0.0001), grazing system (P ¼ 0.009), and their interaction (P < 0.0001), where in each month except for November, the rotational system had higher sward heights than the continuous system. Herbage mass varied with month (P < 0.0001), but not by system. Ground cover consisted of a higher percent of grasses and lower percent of litter/bare ground in the rotational system as compared with continuous. In conclusion, despite differences in grazing days and amount of hay offered, horse condition was similar between systems. Rotationally grazed pastures resulted in taller forage heights, but no difference in dry matter available to horses during the study. We would expect to see more differences between treatments during the following grazing season.
Key Words: equine, rotational, grazing
86 Forage nutritive value, yield, and preference of warm season grasses grazed by horses in the upper Midwest M.L. Schultz*, C.C. Sheaffer, D.N. Catalano, A.M. Grev, and K.L. Martinson University of Minnesota, St. Paul, MN, USA Warm season grasses have the potential to be used as emergency and alternative forages in Midwestern grazing systems. The objectives of this study were to determine the forage nutritive value, yield, and preference of warm season grasses under horse grazing. Research was conducted during 2014 in St. Paul, Minnesota. Warm season grasses were established as monocultures in a randomized complete block design with 6 replicates on May 30. Four adult horses (BW ± SD 520 ± 21 kg) grazed warm season grasses including ‘PCS 6010’ teff, ‘Summer Lovegrass’ teff, ‘Red’ Siberian millet, ‘VNS’ Japanese millet, ‘PCS 2020’ sorghum sudan, and ‘PCS 3030’ sudangrass. Three replicates were grazed at a vegetative stage of maturity on July 8, July 31, and August 26 while the remaining 3 replicates were grazed at a mature growth stage on July 16, August 13, and September 25. Forage nutritive value and yield were determined by hand harvesting triplicate 0.51 0.46 m areas to 15 cm before grazing. Preference was determined by visually assessing percentage of forage removal on a scale of 0 (no grazing) to 100 (100% of vegetation removed). Data were analyzed using Proc Mixed in SAS with a P-value ¼ 0.05. Both varieties of teff had the lowest amount of nonstructural carbohydrates (NSC) with 7% and 9% in the vegetative and mature stages, respectively (P ¼ 0.0059). All grasses contained ¼ 12% NSC. Grasses had similar crude protein (CP) content when harvested at a vegetative stage. At the mature stage, sorghum sudan had the greatest CP at 20%, while sudangrass had the lowest at 17% (P ¼ 0.008). Digestible energy ranged from 2.1 to 2.2 Mcal/kg across maturities and grasses. At the vegetative stage, sudangrass and both varieties of teff had the greatest forage yield (5.3 t/ha), while Japanese and Siberian millet had the least (2.7 t/ha; P ¼ 0.0001). Sudangrass and ‘PSC 6010’ teff had the greatest yield (9.3 t/ha)
ovulation occurred. Group 1 ovulated on average 17.5 d after initial ovulation and Group 2 ovulated on average 20.25 d after initial ovulation. These results indicated that at the dose used intrauterine administration of DIDS did not prolong diestrus and prevent luteolysis even though in vitro DIDS decreased PGF secretion. The results of this study identify DIDS as a possible management tool to inhibit mares from returning to estrus by inhibiting the release of PGF.
Key Words: prostaglandin F2a, luteolysis, estrous cycle
82 Effects of prolonged dexamethasone treatment on signaling pathways associated with muscle protein degradation in mature horses C.M.M. Loos*1, T. Barnes 1, K.M. Brennan 2, and K.L. Urschel 1 1 University of Kentucky, Lexington, KY, USA; 2 Alltech Inc., Nicholasville, KY, USA Prolonged dexamethasone (DEX) treatment has been shown to induce insulin resistance (IR) and reduce the activation of the muscle protein synthesis signaling pathways in horses. High doses of glucocorticoids may also contribute to muscle atrophy by upregulating signaling proteins involved in the proteolytic pathways. The objective of this study was to determine whether DEX-induced IR affects muscle protein degradation pathways. Eight mature horses were fed twice daily to meet maintenance requirements. An initial (pre-DEX) percutaneous biopsy of the gluteus medius muscle was performed, after an isoglycemichyperinsulinemic clamp procedure (IHC). After 21 d, during which all horses received 0.04 mg/kg BW/day of oral DEX, the IHC was repeated and a post-DEX muscle sample was obtained. Muscle samples were analyzed by Western blot to determine the relative abundance or phosphorylation of proteins associated with protein degradation. Data were analyzed using the mixed procedure of SAS version 9.2 with treatment as the fixed effect and horse nested in treatment as the random subject. Significance was determined at P < 0.05. Phosphorylation of Akt, an important factor in insulin mediated signaling pathways, was decreased by 55% post-DEX (P ¼ 0.02). FoxO1 is a downstream target of the Akt pathway and is generally upregulated during muscle atrophy. However, FoxO1 phosphorylation was increased by 2.2 fold post-DEX (P ¼ 0.01), indicative of deactivation. Similarly, the expression of myostatin, a negative regulator of myogenesis, was decreased by 40% post-DEX (P ¼ 0.05). Conversely, its expression is generally increased during glucocorticoid-induced muscular atrophy. Atrogin-1 and muscle ring finger-1 (MuRF-1), important ligases in the ubiquitin proteasome pathway, are expressed in high levels during skeletal muscle atrophy; however, no DEX effect was seen for either Atrogin-1 or MuRF-1 (P > 0.05). The unexpected results of this study could have been caused by the hyperinsulinemic state following the IHC, which may have prevented the activation of muscle proteolysis by DEX. Additionally, proteolytic pathway activation by DEX could be transient, and the 21-d time interval between pre- and post-DEX muscle samples did not allow for the detection of early changes in pathway function. Prolonged DEX treatment did not result in an upregulation in the signaling pathways associated with muscle proteolysis in mature horses when samples were collected in a hyperinsulinemic state. Future research should examine the effects of DEX administration on proteolysis over time and in a non-insulin stimulated state.
Key Words: dexamethasone, muscle, proteolysis
419
83 Maternal fatty acid supplementation influences memory and learning ability in yearling and 2-year-old horses A.M. Adkin*, A.V. Muniz, C.J. Mortensen, and L.K. Warren University of Florida, Gainesville, FL, USA Exposure to docosahexaenoic acid (DHA) in utero and/or early life has been shown to affect neural development, cognitive function, and future learning success in infants and children. We hypothesized that perinatal supplementation of mares with DHA would result in similar benefits in their offspring. As part of a longitudinal study, mares were supplemented with a fat supplement containing an algae source of DHA or a placebo fat supplement from d 250 of gestation through d 74 of lactation. When foals were 2 mo of age, operant conditioning was used to teach foals to touch their nose to a target object. At 6 mo of age, memory recall of prior targeting and performance on new tasks were evaluated. The outcome of these evaluations was reported previously, and showed that DHA influenced performance on some tasks, but did not enhance memory. In this abstract, we report on memory recall of previous tasks and learning performance on novel tasks in the same group of horses (DHA n ¼ 9, placebo n ¼ 10) when reevaluated as yearlings (mean ± SEM, 14.4 ± 0.3 mo of age) and 2-year olds (25.5 ± 0.2 mo). Memory recall of tasks learned since 2 mo of age was assessed over a 2-d period, and included the time to respond correctly to a cue and scoring based on response time. Additionally, learning ability on novel tasks, including crossing a large tarp on the ground and bridling (eg, lowering head, opening mouth, accepting the bit, headstall placement, bridle removal) was investigated in yearlings over a 12-d period. Novel tasks were divided into sequential levels requiring the yearling to earn perfect scores (based on predetermined criteria) over multiple cues to progress to the next level. Operant conditioning techniques were used to train yearlings and the same person handled all horses and scored all responses. Response time and number of cues were analyzed by one-way ANOVA and scores were analyzed by a zero inflated Poisson model. Memory recall was not affected by age (P ¼ 0.28), but horses were more likely (P < 0.0001) to score perfect on d 2 compared with d 1. Across age groups, DHAexposed horses had a tendency to be more likely (P ¼ 0.08) to score perfect on memory recall compared with placebo-exposed horses. Yearlings exposed to maternal DHA were more likely to score perfect on the tarp (P ¼ 0.05) and bridle (P ¼ 0.001) tasks, and tended to complete the bridle task faster (P ¼ 0.09) compared with placebo yearlings. Results indicate that exposure to DHA during the perinatal period may improve long-term memory recall and enhance learning ability in young horses, but more research is needed to confirm these findings.
Key Words: omega-3 fatty acids, DHA, cognition
84 Yield, forage nutritive value, and preference of annual grasses under horse grazing A.M. Grev*, C.C. Sheaffer, D.N. Catalano, M.L. Schultz, and K.L. Martinson University of Minnesota, St. Paul, MN, USA Annual grasses can serve as emergency forage but have not yet been evaluated under horse grazing. The objectives were to evaluate annual grasses for yield, forage nutritive value, and preference under horse grazing during spring and fall. Spring grasses were planted May 8, 2013 and April 22, 2014 in a randomized complete block (RCB) with 4 replicates. Grasses included
420
Abstracts / Journal of Equine Veterinary Science 35 (2015) 418e436
spring oat, spring wheat, winter wheat, spring barley, and annual ryegrass. Fall grasses were planted August 1, 2013, and August 5, 2014, in a RCB with 3 replicates. Grasses included the same 5 species plus winter rye, winter barley, and a forage variety spring oat. Plot size was 1.8 6.1 m. Yield and forage nutritive values were determined by harvesting duplicate 0.46 0.51 m areas to 5 cm. On June 18, 2013, June 5, 2014, September 17, 2013, and September 9, 2014, horses grazed all plots for 4 h at the onset of stem elongation. After grazing, horse preference was determined by visually assessing percentage of forage removal on a scale of 0 (no grazing) to 100 (100% grazed). Plots were mowed and grazing was repeated when spring forages regrew on July 9, 2013; June 24 and July 18, 2014; and when fall forages regrew on October 15, 2013; October 7 and November 4, 2014. Data were analyzed using the Proc Mixed procedure of SAS. Within each season, preference and quality measurements were averaged across grazing events and yield was totaled for all grazing events. In the spring, annual ryegrass had a greater yield (4.7 t/ha) compared with winter wheat, spring wheat, and spring barley (3.6 t/ha; P ¼ 0.0174). Winter wheat and spring wheat were the most preferred grasses with 73% removal, while spring oat was least preferred (15%; P ¼ 0.0367). Winter wheat had the greatest crude protein (CP; 28%), while annual ryegrass and spring oat had the least (22%; P ¼ 0.0208). All grasses had levels of nonstructural carbohydrates (NSC) between 14 and 16% except spring wheat, which had the lowest level at 11% (P ¼ 0.0009). In the fall, winter barley, annual ryegrass, and spring forage oat yielded the highest (4.7 t/ha), while spring barley and spring wheat yielded the least (3.4 t/ha; P ¼ 0.0210). Annual ryegrass and spring wheat were the most preferred grasses with 69% removal, while winter barley, both spring oats, and winter rye were least preferred (23%; P ¼ 0.0356). Winter rye had the greatest CP (35%), while spring oat was lowest (22%; P ¼ 0.0067). Annual ryegrass had the highest level of NSC at 16%, while all other grasses had levels between 12 and 13% (P ¼ 0.0175). Annual ryegrass presents a viable option for emergency pasture in the spring and fall.
Key Words: annual grasses, forage nutritive value, horse preference
85 Effects of rotational versus continuous grazing on horse condition and plant performance L.B. Kenny*1, A.O. Burk 2, and C.A. Williams 1 1 Rutgers, the State University of New Jersey, New Brunswick, NJ, USA; 2 University of Maryland, College Park, MD, USA Rotational grazing is often recommended but not widely adopted by horse operations in the Northeast US. In addition, equine specific research is lacking as to the benefits of rotational compared with continuous grazing. The objective of this study was to compare the effects of rotational and continuous grazing systems on horse health and vegetative performance. Two continuous systems (1.6 ha each) were established with shelters, feeders, and waterers. Two rotational systems were established with 4 rotational pastures (0.4 ha each) and a stress lot (0.1 ha) containing a shelter, feeder, and waterer. Twelve mature Standardbred mares (initial BW 543.9 ± 13.6 kg and body condition score (BCS) 6.1 ± 0.1) were used with 3 horses in each system for a stocking rate of 0.52 ha/horse. Rotational horses were allowed to graze a pasture when forage height was ¼ 7.6 cm and confined to the stress lot and fed hay when forage height was < 7.6 cm or during inclement weather. During those times, horses were fed to meet or exceed NRC requirements using grass hay fed at 2% BW along with concentrate during the winter months. Horse
condition was assessed by monthly BW, BCS, and rump fat as measured by ultrasound. Plant performance was assessed monthly by estimating vegetative cover composition, herbage mass, and sward height. Data are presented as means ± standard error, and statistics were performed using a repeated-measures ANOVA in SAS. Out of 184 d, horses in the continuous systems grazed 184 d, where rotational horses grazed for 97 d. During the winter months, the continuous system horses were offered less total hay than the rotational horses (6,102 vs. 6,714 kg, respectively). System had no effect on horse condition; however BW, BCS, and rump fat differed between each month (highest in September to November and lowest in January; P < 0.0001). Sward height differed between months (P < 0.0001), grazing system (P ¼ 0.009), and their interaction (P < 0.0001), where in each month except for November, the rotational system had higher sward heights than the continuous system. Herbage mass varied with month (P < 0.0001), but not by system. Ground cover consisted of a higher percent of grasses and lower percent of litter/bare ground in the rotational system as compared with continuous. In conclusion, despite differences in grazing days and amount of hay offered, horse condition was similar between systems. Rotationally grazed pastures resulted in taller forage heights, but no difference in dry matter available to horses during the study. We would expect to see more differences between treatments during the following grazing season.
Key Words: equine, rotational, grazing
86 Forage nutritive value, yield, and preference of warm season grasses grazed by horses in the upper Midwest M.L. Schultz*, C.C. Sheaffer, D.N. Catalano, A.M. Grev, and K.L. Martinson University of Minnesota, St. Paul, MN, USA Warm season grasses have the potential to be used as emergency and alternative forages in Midwestern grazing systems. The objectives of this study were to determine the forage nutritive value, yield, and preference of warm season grasses under horse grazing. Research was conducted during 2014 in St. Paul, Minnesota. Warm season grasses were established as monocultures in a randomized complete block design with 6 replicates on May 30. Four adult horses (BW ± SD 520 ± 21 kg) grazed warm season grasses including ‘PCS 6010’ teff, ‘Summer Lovegrass’ teff, ‘Red’ Siberian millet, ‘VNS’ Japanese millet, ‘PCS 2020’ sorghum sudan, and ‘PCS 3030’ sudangrass. Three replicates were grazed at a vegetative stage of maturity on July 8, July 31, and August 26 while the remaining 3 replicates were grazed at a mature growth stage on July 16, August 13, and September 25. Forage nutritive value and yield were determined by hand harvesting triplicate 0.51 0.46 m areas to 15 cm before grazing. Preference was determined by visually assessing percentage of forage removal on a scale of 0 (no grazing) to 100 (100% of vegetation removed). Data were analyzed using Proc Mixed in SAS with a P-value ¼ 0.05. Both varieties of teff had the lowest amount of nonstructural carbohydrates (NSC) with 7% and 9% in the vegetative and mature stages, respectively (P ¼ 0.0059). All grasses contained ¼ 12% NSC. Grasses had similar crude protein (CP) content when harvested at a vegetative stage. At the mature stage, sorghum sudan had the greatest CP at 20%, while sudangrass had the lowest at 17% (P ¼ 0.008). Digestible energy ranged from 2.1 to 2.2 Mcal/kg across maturities and grasses. At the vegetative stage, sudangrass and both varieties of teff had the greatest forage yield (5.3 t/ha), while Japanese and Siberian millet had the least (2.7 t/ha; P ¼ 0.0001). Sudangrass and ‘PSC 6010’ teff had the greatest yield (9.3 t/ha)