Single-injection glucose kinetics with compartmental modelling during rest and low-intensity exercise in horses.

REASONS FOR PERFORMING STUDY: Tools and criteria to evaluate and understand glucose metabolism are essential to optimise equine energy utilisation for exercise performance and reduced metabolic health risks. OBJECTIVES: To re-evaluate models of glucose kinetics in the horse at rest and during endurance type exercise using a single injection technique and compartmental modelling. METHODS: Twelve exercise trained Arabian geldings received a bolus of 100 µmol/kg bwt [6,6-(2) H]glucose i.v. while at rest and while running at ∼ 4 m/s on a treadmill. Tracer and tracee glucose curves from 4-150 min after the bolus dose (while the subject maintained its resting or exercising state) were described by a 2 term exponential decay curve. Compartmental modelling was performed simultaneously for each horse's resting and exercise curves using an 'exercise effect' parameter for each compartmental exchange rate during exercise. RESULTS: Exercise increased all rate constants and transport flows for glucose between compartments by 110-145% (P ≤ 0.004). Total glucose transport through the system increased from 8.9 ± 0.6 µmol/min/kg/bwt at rest to 25.0 ± 1.1 µmol/min/kg bwt during exercise (P < 0.001). Exercise decreased the volume of the primary glucose compartment by 8% (P = 0.006) and increased plasma glucose clearance rate by almost 200% (P < 0.001). Turnover times and mean residence times were decreased approximately 60% by exercise (P < 0.001), whilst turnover rates were increased 125% (P < 0.001). CONCLUSIONS: Single-injection tracer kinetics and compartmental modelling represent a valuable tool to quantify tracee availability to and use by tissue. POTENTIAL RELEVANCE: This technique could represent a beneficial tool for future studies exploring the role of glucose metabolism in equine exercise performance and metabolic disease.

Insulin-like growth factor I in growing thoroughbreds.

The objective of this longitudinal study was to characterize growth and plasma insulin-like growth factor I (IGF-I) concentrations in pasture-raised thoroughbreds fed two sources of dietary energy. Mares and foals were randomly assigned to either a sugar and starch (SS) or fat and fibre (FF)-rich feed, and plasma IGF-I and growth were measured once a month from 1 to 16 months of age. These dependent variables were also compared with day length and ambient temperature. There was an association between plasma IGF-I concentration and average daily gain (ADG) (r = 0.32, p < 0.001). There were also clear seasonal patterns in both ADG and plasma IGF-I, with high values in June and May, and a low value in March. Plasma IGF-I and ADG were positively associated with day length and temperature. Plasma IGF-I was never higher (p > 0.10) in the FF group when compared with the SS group, and was higher in the SS group during a rapid growth phase in the spring of year 2 (p < 0.10). The results establish an association between ADG and IGF-I in the horse and indicate that environment and age may influence this relationship. In addition, plasma IGF-I is influenced by dietary energy source at particular times of year. This link has important implications in designing feeding management strategies that are aimed at addressing skeletal development.

Does usefulness of potassium supplementation depend on speed?

REASONS FOR PERFORMING STUDY: Electrolyte mixtures given to counter sweat loss usually contain abundant potassium. However, increases in plasma [K+] occur with exercise and supplementation may further increase plasma levels, potentially increasing the risk of neuromuscular hyperexcitability and development of adverse clinical sequellae. This proposition requires study. OBJECTIVES: To compare effects of a K-rich electrolyte supplement (EM+K) to a K-free one (EM-K) on plasma [K+], [Ca++] and acid-base status during an endurance incremental exercise test on the treadmill. METHODS: The test consisted of 3 bouts (simulating loops in an endurance race) of 12 km performed at 6, then 7, then 8 m/sec with 25 min rest stops (S1, S2) between loops on 13 endurance trained Arabian horses (7 EM-K, 6 EM+K). Electrolytes were supplied orally 60 mins before exercise (Pre) and at each stop. Blood samples were taken before exercise and during exercise, each S and 120 mins of recovery (R). Blood was analysed for pH, PCO2, packed cell volume (PCV), plasma [Na+], [K+], [Cl-], [Ca++], glucose, and lactate [La-]; plasma [H+] and osmolality (osm) were calculated. The dietary cation anion difference (DCAD) was calculated to be -27 meq/dose EM-K and 109 meq in EM+K, respectively. RESULTS: Plasma [H+] decreased during the 6 and 7 m/sec loops, increased during the 8 m/sec loop, and returned to Pre at S1, S2 and R. Plasma [K+] was higher at 8 m/sec and plasma [Ca++] was overall lower in the EM+K group compared to EM-K. Other findings included higher overall PCV, overall glucose, and [La-] during the 8 m/sec loop (P<0.040) in EM+K compared to EM-K horses. CONCLUSIONS: EM+K supplementation leads to higher plasma [K+] increasing the risk of neuromuscular hyperexcitability during exercise. Acute effects of a lower DCAD in EM-K may have led to higher plasma [Ca++]. Potassium-rich electrolytes may have triggered the release of epinephrine, contributing to higher PCV, glucose release and increased lactate production. POTENTIAL RELEVANCE: Lower plasma [K+] and higher plasma [Ca++] with EM-K supplementation may help reduce the risk of conditions associated with neuromuscular hyperexcitability occurring especially during higher speeds in endurance races.

Glucose dynamics during exercise: dietary energy sources affect minimal model parameters in trained Arabian geldings during endurance exercise.

REASONS FOR PERFORMING STUDY: Glucose regulation is critical for health and exercise performance. OBJECTIVES: To quantify the effects of exercise and diet on insulin sensitivity (SI), glucose effectiveness (Sg), acute insulin response to glucose (AIRg) and disposition index (DI) in horses. METHODS: This study applied the minimal model of glucose and insulin dynamics to exercise-trained Arabian geldings during rest or constant moderate-intensity exercise after 8 weeks adaptation to feeds high in sugar and starch (SS, n = 6) or fat and fibre (FF, n = 6). Horses underwent 2 frequently sampled i.v. glucose tolerance tests (FSIGT). For both tests, a resting basal sample was collected, followed by an i.v. dose of 600 mg/kg bwt glucose defining 0 min of the test. Insulin (0.01 iu/kg bwt) was administered 20 min post glucose for each test. Resting horses were sampled for 240 min. The exercise FSIGT began after each horse had warmed-up for 25 min on the treadmill at which point they had reached the speed representing 60% of their predetermined lactate breakpoint maintained for the rest of the FSIGT. Exercising horses were sampled identically to rest, but for only 150 min post glucose. RESULTS: Exercise increased (P<0.008) SI, Sg and DI and decreased AIRg in all horses. Overall, horses adapted to FF tended to have higher SI (P = 0.070) and DI (P = 0.058). During exercise, FF horses tended to have higher (P< or =0.085) SI and DI, than SS horses and these variables tended to be increased more (P< or =0.075) by exercise in FF horses than SS horses. CONCLUSIONS: Insulin and glucose dynamics adjust during exercise, increasing plasma glucose uptake, presumably to meet demand by contracting skeletal muscle. Trained horses adapted to a high fat diet showed greater metabolic adjustment during exercise than trained horses adapted to a high starch and sugar diet, potentially allowing them to better meet energy demands. POTENTIAL RELEVANCE: Nutrition and exercise impact glucose and insulin dynamics, potentially influencing health and performance.

Amino acid supplementation improves muscle mass in aged and young horses.

The objective of the study was to evaluate the effect of supplementary AA on the ability to support muscle mass in aging horses. Sixteen horses of light horse type were used in a 2 x 2 factorial arrangement of treatments with two age groups [< or = 10 yr (average = 9.1 +/- 0.29 yr) and > or = 20 yr (average = 22.4 +/- 0.87 yr)] and two diet groups [no supplementation (N) or supplementary lysine and threonine (S; 20.0 and 15 g/d, respectively)]. Horses were fed the diets for 14 wk and received regular light exercise throughout the study. Body weight, BCS, and venous blood samples were taken every 2 wk. Plasma was analyzed for total protein, albumin, creatinine, urea N (PUN), and an AA profile, including 3-methyl histidine (3MH) and sulfur AA. Photographs of the horses taken at the start and at the end of the experiment were used to assign a subjective muscle mass score from 1 to 5 (1 = lowest to 5 = highest). There was no difference in BW caused by diet; however, the S-group horses tended (P = 0.064) to gain more weight (6.91 +/- 2.3 kg), and in fact, the N-group horses lost weight (- 11.76 +/- 5.2 kg) during the experiment. Repeated measures analysis revealed that BCS was lower for the aged vs. the young horses (P = 0.001) as well as for the S- vs. the N-group horses (P = 0.026). Subjective muscle mass scores were not different at the start of the experiment but were greater (P = 0.047) for the S-group horses (3.77 +/- 0.13) at the end of the experiment compared with the N-group horses (3.28 +/- 0.14). Plasma creatinine was greater (P = 0.032), and PUN was lower (P = 0.027), for S-group horses compared with N-group horses. Initial 3MH concentrations were not different; however, at the end of the experiment, 3MH was lower for the S-group horses (P = 0.016) compared with the N-group horses. Plasma lysine and threonine concentrations were greater for S-group horses at the end of the experiment than for N-group horses (P = 0.023 and 0.009, respectively). Both 3MH and PUN concentrations were negatively correlated to lysine (R2 = 0.57 and 0.65, respectively) and threonine intake (R2 = 0.56 and 0.60, respectively) at the end of the study. These data suggest that horses receiving supplementary AA were able to maintain muscle mass better than those without supplementation, regardless of age, as evidenced by the improvement in muscle mass scores, lower BCS with no difference in BW, greater creatinine, and lower 3MH and PUN concentrations in the S-group horses.

Insulin resistance and compensation in Thoroughbred weanlings adapted to high-glycemic meals.

Insulin resistance has been suggested to increase the risk of certain diseases, including osteochondrosis and laminitis. Our objective was to evaluate the effect of adaptation to high-glycemic meals on glucose-insulin regulation in healthy Thoroughbred weanlings. Twelve Thoroughbred foals were raised on pasture and supplemented twice daily with a feed high in either sugar and starch (SS; 49% nonstructural carbohydrates, 21% NDF, 3% crude fat on a DM basis) or fat and fiber (FF; 12% nonstructural carbohydrates, 44% NDF, 10% crude fat on a DM basis). As weanlings (age 199 +/- 5 d; BW 274 +/- 5 kg) the subjects underwent a modified frequently sampled i.v. glucose tolerance test. A series of 39 blood samples was collected from -60 to 360 min, with a glucose bolus of 300 mg/kg BW injected at 0 min and an insulin bolus of 1.5 mIU/kg BW at 20 min. All samples were analyzed for glucose and insulin, and basal samples also were analyzed for plasma cortisol, triglyceride, and IGF-I. The minimal model of glucose and insulin dynamics was used to determine insulin sensitivity (SI), glucose effectiveness, acute insulin response to glucose (AIRg), and disposition index (DI). Insulin sensitivity was 37% less (P = 0.007) in weanlings fed SS than in those fed FF; however, DI did not differ (P = 0.65) between diets because AIRg tended to be negatively correlated with SI (r = -0.55; P = 0.067). This finding indicates that the SI decrease was compensated by AIRg in the weanlings adapted to SS. This compensation was further demonstrated by greater insulin concentrations in SS-adapted weanlings compared with FF-adapted weanlings at 11 of 36 sample points (P < 0.055) and greater (P = 0.040) total area under the insulin curve in SS than in FF weanlings. Plasma cortisol and triglycerides did not differ between dietary groups, but IGF-I was greater (P = 0.001) in SS weanlings. Despite appearing healthy, horses adapted to high-glycemic feeds may exhibit changes in altered insulin sensitivity and compensation that increase the risk of diseases involving insulin resistance. These changes seem to be partially amenable to dietary management.

Fat digestibility in Equus caballus follows increasing first-order kinetics.

The digestibility of ether extract varies greatly from forages to grains and further to added fats consisting mainly of triglycerides. This variation has been attributed to two main factors, the presence of nonhydrolyzable substances in the ether extract, especially in leafy foods, and the dilution of endogenous fecal fat. A compilation of results from 188 equine digestion balance observations on five basal feeds and 18 test feeds with added fats demonstrated a true digestibility of fat approaching 100% and an endogenous fecal fat of 0.22 g x d(-1) x kg BW(-1). The results revealed that nonhydrolyzable ether extract and endogenous fecal fat were insufficient to account for the difference between true digestibility and apparent digestibilities of ether extract in basal feeds and partial digestibilities of added fats in test feeds. A third possible contributing factor was demonstrated: an increasing first-order relationship between observed digestibilities (D, %) and the fat content of the feed (F, g/kg): D = 92.0 - 92.0e(-F/342). r2 = 0.81, P < 0.001. This equation indicates that 46% digestibility (half maximum) occurs at an ether extract or fat content of 24 g/kg, which is common in forages. It is consistent with fat digestibility or efficiency of absorption being a function of the rate of lipolysis, especially when residence time in the small intestine is limited. Consequently, we suggest that the kinetics of lipases, which are difficult to measure, may contribute to low digestibility when substrate concentration in the small intestine is low due to a low fat content in food. The status of vitamins A and E might be affected by low dietary fat contents and might be improved by fat supplementation.

Growth rate consists of baseline and systematic deviation components in Thoroughbreds.

The objective of this study was to establish a procedure for differentiating a baseline curve from a systematic deviation in weight-age data, and hence to develop a physiological growth model for the Thoroughbred. A total of 2,698 records for 175 foals was obtained during a period of 8 yr (1994 to 2001). Weight-age data were fit with a sigmoid growth equation, W = A(1 + be(-kt))M, where W is BW at age t, A is the asymptotic value of W, b is a scaling parameter that defines the degree of maturity at t = 0, k is a rate constant, and M defines the point of inflection in the sigmoid curve in relation to age. Short-term systematic deviations in the weight-age data were identified by a goodness-of-fit procedure and illustrated in three-dimensional contour plots of the sigmoid equation parameters as they changed upon removal of selected subsets of the data. Based on features of the contour plots, a negative deviation between 210 and 420 d of age was set aside, with the remaining data establishing the baseline data set. The sigmoid growth equation was fit to the baseline data set using a nonlinear mixed model with repeated measures, and indicated a mature weight of 542 +/- 6.2 kg reached at 7 yr. The systematic deviation identified in this weight-age data set is present in other published Thoroughbred growth data and is likely to result in erroneous parameter estimates if not set aside before fitting sigmoid growth equations to the thus-modified weight-age data set. The techniques developed in this study enable identification of short-term systematic deviations in weight-age data and define a realistic baseline growth curve. Differentiation of these two components enables the development of a physiological model of growth that distinguishes between baseline growth and environmental influences, represented respectively, by the baseline curve and the systematic deviation.

Weight prediction from linear measures of growing Thoroughbreds.

REASON FOR PERFORMING STUDY: Monitoring weight of foals is a useful management practice to aid in maximising athletic potential while minimising risks associated with deviations from normal growth. OBJECTIVE: To develop predictive equations for weight, based on linear measurements of growing Thoroughbreds (TBs). METHODS: Morphometric equations predicting weight from measurements of the trunk and legs were developed from data of 153 foals. The accuracy, precision and bias of the best fitting equation were compared to published equations using a naive data set of 22 foals. RESULTS: Accuracy and precision were maximised with a broken line relating calculated volumes (V(t + l)) to measured weights. Use of the broken line is a 2 step process. V(t + l) is calculated from linear measures (m) of girth (G), carpus circumference (C), and length of body (B) and left forelimb (F). V(t + I) = ([G2 x B] + 4[C2 x F]) 4pi. If V(t + l) < 0.27 m3, weight is estimated: Weight (kg) = V(t + l) x 1093. If V(t + l) > or = 0.27 m3: Weight (kg) = V(t + l) x 984 + 24. The broken line was more accurate and precise than 3 published equations predicting the weight of young TBs. CONCLUSIONS: Estimation of weight using morphometric equations requires attention to temporal changes in body shape and density; hence, a broken line is needed. Including calculated leg volume in the broken line model is another contributing factor to improvement in predictive capability. POTENTIAL RELEVANCE: The broken line maximises its value to equine professionals through its accuracy, precision and convenience.

Obesity and diet affect glucose dynamics and insulin sensitivity in Thoroughbred geldings.

Insulin resistance is considered a risk factor in obesity, laminitis, exertional rhabdomyolysis, and osteochondrosis. The objective was to use the minimal model to estimate glucose effectiveness (Sg) and insulin sensitivity (Si) in nonobese to obese horses initially adapted to forage only, then adapted to forage plus supplements rich in starch and sugar (SS) or fiber and fat (FF). Ten Thoroughbred geldings, with BCS of 5 (nonobese), 6 (moderately obese), and 7 to 8 (obese), were adapted to pasture and hay, allocated to two groups, and fed SS or FF in a switch-back design with 8 wk of adaptation. Modified frequent-sampling i.v. glucose tolerance tests were applied after adaptation to forage, SS, and FF. For the tolerance tests, horses were kept in stalls overnight and provided hay, and venous catheters were placed the next morning. Baseline samples were collected, 0.3 g of glucose/kg of BW was given i.v., and blood was sampled at 1, 2, 3, 4, 5, 6, 7, 8, 10, 12, 14, 16, and 19 min. At 20 min, 30 mU of insulin/kg of BW was given, followed by sampling at 22, 23, 24, 25, 27, 30, 35, 40, 50, 60, 70, 80, 90, 100, 120, 150, and 180 min. Plasma was analyzed for glucose and insulin, and Si, Sg, acute insulin response to glucose, and the disposition index were calculated. Normality was tested using the Shapiro-Wilk statistic. Body condition effects were analyzed using a mixed model with repeated measures. Diet effects were analyzed using a Wilcoxon signed rank test. The Sg was higher in obese than nonobese (P = 0.003) and moderately obese (P = 0.007) horses; Si was lower in obese than nonobese (P = 0.008) horses, and acute insulin response to glucose was higher in obese than nonobese (P = 0.039) horses. Effects of diet were likely confounded by body condition, but horses had lower Si (P = 0.066) when fed SS compared with FF, especially when nonobese. In conclusion, the minimal model effectively estimated Sg, Si, acute insulin response to glucose, and disposition index in horses. Obese geldings were insulin-resistant and seemed to rely primarily on Sg for glucose disposal. Feeding a diet rich in sugar and starch decreased insulin sensitivity of horses. Maintenance of body condition and avoidance of grain-based meals rich in sugar and starch would be beneficial to decrease the risk of developing insulin resistance and associated metabolic syndromes in horses, especially for horses at risk for these syndromes.

Glucose clearance in grazing mares is affected by diet, pregnancy, and lactation.

The glucose tolerance test in the horse may be used to determine metabolic responses to diet, disease, or physiologic state. The objective of this study was to determine the effect of reproductive stage (gestation and lactation) and supplemental dietary energy source (sugar and starch [SS] or fiber and fat [FF]) on glucose metabolism in grazing mares using an oral glucose tolerance test. Twelve mares, six on each supplement, were examined on three occasions: one in the third trimester of pregnancy, the second in early lactation, and the third in late lactation. During each test, venous samples were taken at 30 and 1 min before, and 30, 60, 90, 120, 150, 180, 240, and 300 min after a nasogastric dose of glucose at 0.2 g/kg of BW. Plasma was assayed for glucose, insulin, and cortisol. Statistical analysis was a mixed model with repeated measures with horse, diet, and reproductive stage as fixed effects. The incremental glucose area under the curve (AUC) in response to oral glucose was lower in SS than in FF mares (P = 0.022). Mares tended to have a lower incremental glucose AUC in early lactation than in late gestation (P = 0.057), and insulin AUC was lower in early lactation than in late gestation (P = 0.002) and late lactation (P = 0.013). Glucose clearance was more rapid (P = 0.007) in SS than in FF mares. The glycemic response to the oral glucose tolerance test was consistent with adaptation to dietary sugar and starch as well as metabolic changes associated with pregnancy and lactation. Feeding twice-daily grain meals rich in SS influenced glucose metabolism in horses to an extent that the natural adaptation of glucose metabolism to pregnancy was moderated. Feeding a diet rich in FF more closely mimics the natural grazing state of pasture and allows for adaptation of glucose metabolism to pregnancy and lactation.

Speed associated with plasma pH, oxygen content, total protein and urea in an 80 km race.

To test the hypothesis that endurance performance may be related quantitatively to changes in blood, we measured selected blood variables then determined their reference ranges and associations with speed during an 80 km race. The plan had 46 horses in a 2 x 2 factorial design testing a potassium-free electrolyte mix and a vitamin supplement. Blood samples were collected before the race, at 21, 37, 56 and 80 km, and 20 min after finishing, for assay of haematocrit, plasma pH, pO2, pCO2, [Na+], [K+], [Ca++], [Mg++], [Cl-], lactate, glucose, urea, cortisol, alpha-tocopherol, ascorbate, creatine kinase, aspartate amino transferase, lipid hydroperoxides, total protein, albumin and creatinine, and erythrocyte glutathione and glutathione peroxidase. Data from 34 finishers were analysed statistically. Reference ranges for resting and running horses were wide and overlapping and, therefore, limiting with respect to evaluation of individual horses. Speed correlations were most repeatable, with variables reflecting blood oxygen transport (enabling exercise), acidity and electrolytes (limiting exercise) and total protein (enabling then, perhaps, limiting). Stepwise regressions also included plasma urea concentration (limiting). The association of speed with less plasma acidity and urea suggests the potential for fat adaptation and protein restriction in endurance horses, as found previously in Arabians performing repeated sprints. Conditioning horses fed fat-fortified and protein-restricted diets may not only improve performance but also avoid grain-associated disorders.

Antioxidant status and muscle cell leakage during endurance exercise.

Antioxidant status of 35 endurance horses was studied during an 80 (OD80) or 160 km (OD160) race. Packed cell volume (PCV), total plasma protein (TPP), plasma ascorbic acid (VIT C), plasma alpha-tocopherol (VIT E) and erythrocyte glutathione (GSH) concentrations, erythrocyte glutathione peroxidase (GPX), plasma aspartate aminotransferase (AST) and plasma creatine kinase (CK) activities were measured at 0, 40, 80 km and 60 min of recovery (REC) at OD80, and 0, 64, 106, 142, 160 km and REC at OD160. In both races, no changes were found in plasma VIT E concentration, but VIT C and GSH concentrations decreased (P<0.05), and mean GPX, AST and CK activities increased from 0 km (P<0.05). Indices of muscle cell leakage (plasma AST and CK) were correlated (r = 0.36 to 0.67; P<0.03) with indices of antioxidant status (VIT C, GSH and GPX). Associations between increased muscle leakage and decreased antioxidant status may, in part, reflect oxidative stress and suggest the testing of antioxidant supplements in endurance horses to improve performance and welfare.

Growth of thoroughbreds fed a low-protein supplement fortified with lysine and threonine.

Growth and protein status were examined in Thoroughbred foals and yearlings offered pasture supplements with different crude protein contents and amino acid compositions. Both supplements contained 3.0 Mcal/kg DM, 10% corn oil, 1.4% calcium, and three sources of fiber. The control supplement contained 14% CP and 22% soybean meal, whereas the experimental supplement contained 9% CP and 3% soybean meal and was fortified with 0.6% lysine and 0.4% threonine. Mares and foals were fed twice daily (0700 and 1400) and kept on 12.14-hectare pastures (mixed grass and ladino clover) until weaning (6 mo). Weanlings continued on specified supplements and pastures for seven additional months. Physical measurements and blood samples were taken monthly for a period of 14 mo. Measurements included BW, ADG, body condition (BC), wither height, hip height, body length, girth, forearm length, front and hind cannon length, and carpus and fetlock circumference. Blood plasma analysis included total protein, albumin, creatinine, and urea nitrogen. Effect of diet and time were evaluated by analysis of variance with repeated measures. No differences (P > 0.05) were found in physical measurements between the control and Lys/Thr groups (ADG 0.7 +/- 0.02 kg/d, BC 4.9 +/- 0.05) for the observational period. Blood data also showed no difference (P > or = 0.05) for the period (albumin 2.9 +/- 0.03 g/dL, total protein 5.7 +/- 0.10 g/dL, creatinine 1.1 +/- 0.02 mg/dL). Differences in ADG between the control and experimental groups were observed in November (0.73 +/- 0.06 vs 0.91 +/- 0.04 kg/d, P = 0.01) and December (0.56 +/- 0.04 vs 0.67 +/- 0.06 kg/d, P = 0.07) and again in April (0.65 +/- 0.12 vs 0.86 +/- 0.06 kg/d, P = 0.06). These results suggest that the foals offered the Lys/Thr grew at the same or greater rates than foals on the control supplement. When a low-protein diet was fortified with the first two limiting amino acids, the protein was utilized more efficiently for growth and development. This study gives further insight into the role of protein quality during a foal's 1st yr of growth.

Plasma glucose and insulin responses of Thoroughbred mares fed a meal high in starch and sugar or fat and fiber.

Plasma concentrations of glucose and insulin following a meal were compared in twelve Thoroughbred mares fed a pelleted concentrate (PC), a traditional sweet feed high in sugar and starch (SS), or a feed high in fat and fiber (FF). The feeds had similar DE and CP but differed in fat (19, 32, and 166 g/kg DM, respectively), NDF (199, 185, and 369 g/kg DM, respectively) and nonstructural carbohydrates (574, 645, and 247 g/kg, respectively). Mares were randomly assigned to two groups balanced for foaling date and weight. All mares received PC in late gestation; then, after foaling, one group was fed SS and the other FF for trials in early and late lactation. Mares were placed in stalls and deprived of feed overnight. A series of blood samples was collected via a jugular catheter from 0 (baseline) to 390 min after consumption of 1.82 kg of feed. Plasma was analyzed for glucose and insulin. Baseline values, peak values, and areas under curves (AUC) were compared by ANOVA. Baseline values were 74.7 +/- 10.9 mg/dL for glucose and 5.86 +/- 1.80 mIU/L for insulin for all diets and stages. Responses to PC did not differ between the two groups (P > 0.34), indicating the groups were metabolically similar. Peak plasma glucose and insulin concentrations were higher (P < 0.001) in the SS group than in the FF group during early and late lactation. Similarly, glucose and insulin AUC were larger (P < 0.003) in SS than in FF during early and late lactation. These results indicate that metabolic fluctuations are moderated by the replacement of sugar and starch with fat and fiber. This replacement may reduce the risk of certain digestive and metabolic disorders that have been linked to feeding meals of grain-based concentrates to pregnant or lactating mares.

Dietary protein restriction and fat supplementation diminish the acidogenic effect of exercise during repeated sprints in horses.

A restricted protein diet supplemented with amino acids and fat may reduce the acidogenic effects of exercise. Twelve Arabian horses were assigned to a 2 x 2 factorial experiment: two fat levels: 0 or 10 g/100 g added corn oil and two crude protein levels: 7.5 g/100 g (supplemented with 0.5% L-lysine and 0.3% L-threonine) or 14.5 g/100 g. The experiment began with a 4-wk diet accommodation period followed by a standard exercise test consisting of six 1-minute sprints at 7 m/s. Horses were interval trained for 11 wk followed by another exercise test with sprints at 10 m/s. Blood samples were taken at rest and during the exercise tests. Plasma was analyzed for PCO(2), PO(2), Na(+), K(+), Cl(-), lactate, pH and total protein. Bicarbonate, strong ion difference and total weak acids were calculated. Data were analyzed using repeated-measures analysis of variance. Venous pH was higher in the low protein group during the first test (P = 0.0056) and strong ion difference became higher (P = 0.022) during sprints in the low protein group. During the second test, venous pH and bicarbonate were higher for the low protein high fat group (P = 0.022 and P = 0.043, respectively) and strong ion difference became higher (P = 0.038) at the end of exercise in the low protein groups. These results show that restriction of dietary protein diminishes the acidogenic effect of exercise, especially in combination with fat adaptation.

Alkanes as internal markers to estimate digestibility of hay or hay plus concentrate diets in horses.

Dry matter intake (DMI), dry matter digestibility (DMD), and fecal output (FO) are difficult to measure directly in the field, and indirect methods using external and internal markers have thus been developed. An experiment was conducted consisting of two digestion trials with two periods in each trial to examine the use of five odd-chain alkanes (C25 to C33) of plant cuticular wax as internal markers to estimate DMD of hay or hay plus concentrate diets in horses. Eight mature Thoroughbred geldings were housed in 4- x 4-m stalls and randomly assigned to one of two mixed grass/legume hays (Diets 1 and 2) in Trial 1 and to mixed grass/legume hay plus one of two concentrates (Diets 3 and 4) in Trial 2. After the first 12-d period was conducted, dietary assignments for each group were switched for the second period in each trial. Each period consisted of a dietary accommodation from d 1 to 7 and total fecal collection from d 8 to 11. Results indicated that fecal recoveries of odd-chain alkanes were 88 to 90% for Diet 1, 75 to 92% for Diet 2, 71 to 81% for Diet 3, and 71 to 82% for Diet 4. Alkane recoveries were not related to alkane chain lengths. Digestibilities calculated from alkane concentration data adjusted using the mean fecal recovery of individual odd-chain alkanes (DA1) were not significantly different from the digestibilities estimated from total collection (DTC) for Diets 1 and 2 in Trial 1 and Diets 3 and 4 in Trial 2. When adjustment was based on the mean recovery of all alkanes (DA2; estimated by linear regression), all DA2 estimates for horses offered all diets were similar to DTC. Results indicate that accurate mean estimates of DMD can be obtained by using plant wax alkane markers and adjusting for the mean recovery of five odd-chain alkanes in a diet.

Hydrolyzable carbohydrates in pasture, hay, and horse feeds: direct assay and seasonal variation.

Carbohydrates may be hydrolyzed or fermented in the digestive tract, and this distinction is important for the evaluation of the diet of herbivores. Both hydrolyzable and fermentable carbohydrates are included in the nonstructural carbohydrate (NSC) fraction as estimated by difference using proximate analysis. Our objectives were to measure hydrolyzable carbohydrates in forages and concentrates, to compare these values with nonstructural carbohydrate, to test for prediction of hydrolyzable carbohydrate concentration in forages from its near-infrared spectrum, and to examine seasonal variation of carbohydrates in pasture. Samples of forages (107) and concentrates (25) were collected, dried, ground, and analyzed for NSC (calculated as 100 - water - CP - fat - ash - NDF), hydrolyzable carbohydrate (CHO-H, direct analysis), and rapidly fermentable carbohydrate (NSC minus CHO-H). Hydrolyzable carbohydrate accounted for 97% or more of the NSC in the concentrates but only 33% in pasture and hay. A two-term polynomial equation fit all the data: CHO-H = 0.154 x NSC + 0.00136 x NSC2, R2 = 0.98, P < 0.0001, n = 132. In 83 pasture samples, CHO-H concentrations were predicted by near-infrared spectra with a calibration R2 of 0.97, a mean of 48 g/kg, and a SE of calibration of 3.5 g/kg DM. In pasture samples collected between September 1995 and November 1996, the coefficient of variation was 31% for both CHO-H and rapidly fermentable carbohydrate (CHO-FR); the largest increments were 31 g/kg of CHO-H from September to October and 41 g/kg of CHO-FR from February to March. The increased risk of certain diseases, such as laminitis and colic, that have been previously associated with an abrupt overload of NSC may be more precisely attributed to CHO-H in grain concentrates, and to CHO-H as well as CHO-FR in pastures.