J. Anim. Sci. 2005. 83:1033-1043
© 2005 American Society of Animal Science
Influence of starch intake on growth and skeletal development of weanling horses1,2
E. A. Ott*,3,
M. P. Brown
,
G. D. Roberts and
J. Kivipelto*
* Departments of Animal Sciences and
and
Large Animal Clinical Sciences, University of Florida, Gainesville 32611-0910
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Abstract
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Forty-four weanling horses were used in two experiments to evaluate the effect of starch intake on growth and skeletal development. In Exp. 1, the weanlings were fed either a grain-based, high-starch (31.1%, DM basis) concentrate or a by-product-based, low-starch (0.0%) concentrate with coastal bermudagrass (Cynodon dactylon) hay. Corn oil was used to equalize the energy concentration of the concentrates. The concentrate:hay ratio was 64:36 (as-fed basis), and intake was the same for both diets. Body weight gains were greater by the weanlings consuming the high-starch concentrate (0.81 vs. 0.67 kg/d; P = 0.01). Total body length gain also was greater for the weanlings consuming the high-starch concentrate (15.5 vs. 13.2 cm; P = 0.045). Other body measurements and bone mineral deposition were not influenced by diet or gender. At the end of the experiment, postprandial blood glucose concentrations suggested that the horses on the low-starch diet were less efficient in metabolizing blood glucose than were those that had been consuming the high-starch diets. In Exp. 2, the weanlings were fed either a high-starch (34.7%) or medium-starch (17.0%) concentrate plus coastal bermudagrass hay. Corn oil again was used to equalize the energy content of the medium-starch concentrate to that of the high-starch concentrate. The concentrate:hay ratio was 64:36 (as-fed basis), and the intake was the same for both diets. The diets did not influence rate of gain (0.75 kg/d; P = 0.98), body measurements (P = 0.11 to 0.93), or bone mineral deposition (P = 0.66). Animals on the medium-starch diet tended to have blood glucose concentrations that peaked earlier and were lower at later times than those consuming the high-starch concentrate. Bone osteochondrotic lesions were not related to the diet and were found to decrease during the course of the experiment for both the high-starch and the medium-starch diets (P = 0.006 and 0.016, respectively).
Key Words: Bone • Growth • Horses • Osteochondrosis • Starch
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Introduction
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Skeletal failure in horses during training may be due to weaknesses linked to developmental lesions (Krook and Maylin, 1988
). Developmental problems in the young horse, designated as developmental orthopedic disease (DOD), include physitis, osteochondrosis (OC), osteochondritis dessicans (OCD), subchondral cystic lesions, angular limb deformity, flexural deformities, and cuboidal bone malformation (McIlwraith, 2001). Knight et al. (1985) detected a correlation between nutrient intake of growing foals and the incidence of DOD. A later study demonstrated that DOD in the foal can occur very early in life (3 to 5 mo; Knight et al., 1987). Glade and Belling (1986) demonstrated that feeding 130% of the NRC (1989) recommended quantities of energy and protein to weanling horses resulted in greater BW gain, but it did not affect bone measurements compared with NRC (1989) recommendations. However, cartilage hydroxyproline and hexosamine were decreased and deoxyribonucleic acid was increased in weanlings fed greater amounts of nutrients, suggesting that restricting growth might be advantageous. High-energy diets have been implicated as one of the causes of DOD in growing horses (Savage et al., 1993), and high-starch diets have been linked to high blood glucose, insulin, thyroxine, and triiodothyronine concentrations in young horses (Glade and Reimers, 1985). These hormones are known to influence chondrocyte maturation and function (Glade, 1987) and thereby may play a key role in bone development. Weanlings fed to gain 0.97 kg/d had a greater incidence of compressional physitis and contracted tendons than did those gaining 0.49 kg/d, but did not exhibit any detectable radiographic lesions (Peterson et al., 2002). The physitis was resolved by the time the foals were 8 mo of age. The present study was designed to determine the effect of starch intake on the skeletal development of weanling horses.
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Materials and Methods
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Animal procedures for both experiments were approved by the University of Florida’s Animal Care and Use Committee.
Experiment 1
Twenty foals (14 Thoroughbred and six quarter horse) were weaned at 112 d of age and assigned at random within breed (Thoroughbred = four colts and 10 fillies; quarter horse = three colts and three fillies) and gender (seven colts and 13 fillies) subgroups to one of two treatments. Mean age at the start of the experiment was 142.6 ± 2.5 d. Between weaning and the start of the experiment, all the weanlings received a commercial sweet feed and coastal bermudagrass (Cynodon dactylon) hay and/or bahiagrass (Paspalum notatum) pasture. Treatments were a high-starch (31.1%, DM basis) concentrate (Table 1) and a low-starch (0%), added-fat (soybean oil) concentrate. The pelleted concentrates were fed individually to appetite for two 1.5-h feeding periods daily. Orts were weighed daily. Coastal bermudagrass hay was group-fed at 10 g/kg of BW (as fed) daily based on the most recent pen weight. Groups of five weanlings of similar age were housed in 9.1-m x 34.9-m paddocks that provided 5.6 m x 9.1 m of shelter from rain. Genders and diets were random in each paddock. Each of the four paddocks was equipped with six 1.3-m x 3-m feeding stalls, to which the weanlings were confined for 1.5 h at each feeding. Weanlings were weighed and measured for heart girth, withers height, body length, and hip height at the start of the experiment and at 14-d intervals for 112 d. Radiographs of the left third metacarpal were made at the start of the experiment and on d 56 and 112, according to the procedure of Meakim et al. (1981).
Before the start of the feeding trial, the weanlings were fasted for 14 h, and a baseline blood sample was collected via jugular puncture for glucose determination. Blood samples for glucose tolerance were collected at 60 and 90 min following administration of 0.25 g of dextrose/kg BW orally at the start of the experiment as described by Ralston et al. (2001). Two days later, blood samples also were collected following the morning feeding of a sweet feed (Kavazis et al., 2002). At the end of the feeding trial, the glucose tolerance test was repeated. Two days after the glucose tolerance test, the test concentrates were offered at 5.0 g/kg of BW (as fed), the time required to consume the feed was recorded, and blood samples were collected for glucose determinations at 60 and 90 min after feeding.
Experiment 2
Twenty-four Thoroughbred (19) and quarter horse (five) foals were weaned at 112 d of age and started on the experiment at 139.6 ± 1.7 d of age. The weanlings were blocked by breed (Thoroughbred = eight colts and 11 fillies; quarter horse = three colts and three fillies), gender, and age and assigned at random to one of two dietary treatments. One treatment group was fed a high-starch (37.5%) concentrate formulated to provide 3.5 Mcal of DE/kg of DM (Table 1
). The second group was fed a medium-starch (17.0%), added-fat (soybean oil) concentrate formulated to provide 3.58 Mcal of DE/kg of DM. The pelleted concentrates were fed to individual horses to appetite twice daily. Coastal bermudagrass hay was group-fed at 10.0 g/kg of BW daily (as fed). Six weanlings were housed in each paddock as described above. Orts were weighed back daily.
All the weanlings were fed the high-starch concentrate for at least 5 d before the start of the experiment. Before the start of the feeding trial, the weanlings were transported to the University of Florida Large Animal Clinic for lameness evaluation and radiographs for DOD. Radiographs were made of all four fetlocks, both carpis, both tarsis, and both stifle joints. A written description of each joint was prepared by an experienced radiologist blinded to the treatments, and the written report was converted to a score by another blinded participant. The scoring system was a modification of one summarized by McIlwraith (1996). The results were scored as follows: 0 = no abnormality reported; 1 = mild flattening of the cartilage; 2 = moderate flattening of the cartilage; 3 = OC lesion detected; and 4 = OCD lesion detected. Following the weanlings’ return from the veterinary school, after at least two additional days on the high-starch concentrate followed by an overnight fast, the animals were bled by jugular puncture and then offered the high-starch concentrate at 5.0 g/kg of BW (as fed). Time to consume the assigned concentrate was recorded. Blood samples were collected at 60, 90, 120, 150, and 180 min after the feed was offered. The test was repeated at the end of the experiment, when the animals were on their assigned concentrates. Blood samples were analyzed for blood glucose, insulin, and T4 as described in a subsequent section. Weanlings were weighed, measured for heart girth, withers height, body length, and hip height, and the third metacarpal was radiographed for bone mineral estimation. The weights and body measurements were repeated at 14-d intervals for 112 d. Radiographs were taken at 56 and 112 d, and the lameness evaluation and radiographs for DOD were repeated at the end of the feeding trial.
Laboratory Analyses for Both Experiments
Glucose was determined on plasma from blood collected in evacuated tubes containing sodium fluoride and potassium oxalate. Glucose concentrations in the plasma were determined using the enzymatic (Trinder) method from Sigma (St. Louis, MO). Serum insulin concentrations were measured using the Coat-A-Count insulin radioimmunoassay, and serum T4 (total thyroxine) concentrations were measured using the Coat-A-Count Canine T4 radioimmunoassay from DiagnosticProducts Corp. (Los Angeles, CA). The T4 procedure was validated for horses by Sojka et al. (1993). Bone mineral estimations were determined according to the procedure of Meakim et al. (1981) as modified by Ott et al. (1987), and are reported as grams of mineral/2-cm cross section of bone. Concentrate and hay samples were collected at 28-d intervals and analyzed for DM, NDF, ADF, ether extract, protein (% N x 6.25), starch, Ca, and P. Dry matter was determined by drying at 100°C for 24 h. The NDF and ADF were determined using the Ankom fiber analyses system (Ankom Technology, Macedon, NY). Protein was determined after the samples were hydrolyzed according to the procedure of Gallaher et al. (1975), and the N was determined on an Alpkem automated analyzer (Alpkem Corp., Clackamas, OR). Starch content of the feeds was determined according to the procedure of Hall (2001). Ether extract was determined with a Soxhlet apparatus. Calcium was determined by atomic absorption spectrophotometry with a model 5000 atomic absorption spectrophotometer (Perkin-Elmer Corp., Norwalk, CT), and P was determined by colorimetric procedure (Technicon Industrial Systems, Tarrytown, NY) on the automated Alpkem analyzer.
Statistical Analyses for both Experiments
Feed intake, growth, estimated bone mineral, and serum glucose and hormones were analyzed by repeated measures procedures using the GLM procedure of SAS (SAS Inst., Inc., Cary, NC), with diet, breed, gender, period, and all interactions in the model. All interactions were tested in the model; no significant interactions were found in either experiment. The Wilcoxon signed-rank test for repeated measures (Marks, 2003) was used to test the changes in the bone lesion scores over the duration of the experiment. Bone lesion scores for the two diet groups at both the start and the completion of the experiment were analyzed with the Wilcoxon two-sample test (Marks, 2003). Significance was considered at P <0.05, and a tendency was considered at P <0.20.
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Results
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Experiment 1
The weanlings readily accepted both concentrates, and there was no difference in concentrate, hay, or total intake between the groups (Table 2). The colts consumed more total feed than the fillies (P = 0.04). No breed effect was detected for any of the measurements except for bone mineral. Weanlings fed both diets consumed a 64:36 ratio of concentrate to hay. Concentrate intake by the weanlings consuming the low-starch concentrate was slower than by those consuming the high-starch concentrate, but the difference was not significant (P = 0.16). Weight gains (Table 3) were greater for the weanlings consuming the high-starch concentrate than for those consuming the low-starch concentrate. (0.81 vs. 0.67 kg/d; P = 0.010). Body length gains also were greater for the weanlings consuming the high-starch concentrate (15.5 vs. 13.2 cm; P = 0.045). No other body measurements were affected by dietary treatment.
The fasting blood glucose concentrations were very consistent. Both 60- and 90-min samples were determined for each animal. The 90-min samples had higher blood glucose concentrations than did the 60-min samples for all the weanlings. The weanlings had higher blood glucose concentrations when consuming the sweet feed than when given the glucose drench (Table 4). Three animals assigned the high-starch diet and four animals assigned the low-starch diet were more than two SE above the mean at the start of the experiment, and three animals fed the high-starch diet and two animals fed the low-starch diet were more than two SE above the mean at the end of the experiment. At the end of the experiment, the weanlings consuming the low-starch diet had lower blood glucose concentrations than did those consuming the high-starch concentrate (P = 0.018). When the glucose drench was administered, those consuming the low-starch diet had higher blood glucose concentrations (P = 0.008). All the weanlings had higher blood glucose concentrations when they consumed the sweet feed than when they consumed the test concentrates.
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Table 4. Blood glucose of weanling horses consuming high- and low-starch diets sampled at fasting, and 60 and 90 min after treatment in Exp. 1
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Bone mineral content was not influenced by diet or gender (Table 5). Thoroughbred weanlings had more mineral at the start of the experiment (P = 0.004) than did the quarter horses, and they maintained that advantage throughout the experiment. There was no difference in bone mineral deposition between genders (P>0.10).
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Table 5. Influence of gender, breed, and diet on estimated bone mineral content (g/2-cm cross section) in Exp. 1a
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Experiment 2
The weanlings consumed 26.9 g of feed/kg BW (as fed) daily, of which 64% was concentrate and 36% was hay. Mean feed intake was almost identical on both diets (Table 6), and there was no effect of gender or breed on feed intake. Nutrient intake on both diets was similar. Lysine intake by the weanlings on the medium-starch concentrate was slightly higher due to the use of ingredients that had higher protein lysine concentrations. Average daily gain by the weanlings was 0.75 kg for the duration of the study. The diet did not influence weight gain, but the quarter horses gained faster than the Thoroughbreds (P = 0.001; Table 7). No other growth measurements (heart girth, withers height, body length, and hip height) were influenced by diet, gender, or breed (Table 7). There was a tendency for the colts to have greater withers height (P = 0.10) and body length (P = 0.11) gains than the fillies. The quarter horses had greater feed efficiency than the Thoroughbreds (P = 0.004). The diets did not influence bone mineral deposition (P <0.05), but colts deposited more mineral (Table 8) than fillies (P = 0.05).
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Table 6. Influence of gender, breed, and diet on daily feed and nutrient intake and rate of consumption in Exp. 2
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Rate of feed intake at the start of the experiment was similar for animal groups assigned to both diets, but those assigned to the high-starch diet took slightly less time (22.3 vs. 23.2 min/kg) than did those assigned to the medium-starch diet. At the end of the experiment, weanlings consumed the medium-starch diet (21.2 min/kg) at a numerically slower rate than those that consumed the high-starch diet (19.1 min/kg), but differences were not significant. Fasting blood glucose concentrations at the start of the experiment did not differ for the weanlings assigned to both diets (83.5 vs. 85.2 mg/dL; P = 0.38) and were similar to fasting values for the high- and medium-starch diets at the end of the experiment (89.9 vs. 90.5 mg/dL; P = 0.65). The mean glucose peak at the start of the experiment (Figure 1) occurred at 120 to 150 min after feeding. There was individual variability, with some animals peaking at 90 min and some still increasing at 180 min. No correlation between rate of intake and the time at which the glucose peaked was detected. At the end of the experiment, the glucose peak also occurred at 120 to 150 min after feeding, with the final glucose concentrations somewhat higher than the initial values. Animals fed the medium-starch concentrate tended to peak earlier and have lower glucose values at the later collection times than those fed the high-starch concentrate. There were no animals on the study that were obviously glucose intolerant.
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Figure 1. Influence of diet on plasma glucose concentrations following a designated meal at the start and completion of the feeding trial. Means ±SE for each animal group at each sample time are shown. Each bar represents 12 animals, except for the medium-starch group at the end of the trial, which was represented by six animals. Values with different letters differ, P = 0.03. Group A was fed the high-starch concentrate, and Group B was fed the medium-starch concentrate.
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Plasma insulin concentrations increased in response to feed intake (Figure 2). At the completion of the experiment, fasting insulin values were greater than at the start of the experiment (P <0.001) and increased throughout the collection period, with the highest values for most weanlings occurring at 150 or 180 min after feeding. There were no dietary, gender, or breed effects on plasma insulin concentrations (P >0.10). Plasma T4 concentrations (Figure 3) were consistent and not influenced by diet or correlated with plasma insulin concentrations.
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Figure 2. Influence of diet on plasma insulin concentrations following a designated meal at the start and completion of the feeding trial. Means ±SE for each animal group at each sample time are shown. Each bar represents 12 animals, except for the medium-starch group at the end of the feeding trial, which was represented by six animals. Initial values for each group were less than the final values, P <0.01. Group A was fed the high-starch concentrate, and Group B was fed the medium-starch concentrate.
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Figure 3. Influence of diet on plasma thyroxine (T4) concentrations, following a designated meal at the start and completion of the feeding trial. Means ±SE for each animal group at each sample time are shown. Each bar represents 12 animals except for the medium-starch group at the end of the feeding trial, which was represented by six animals. Values with different letters differ as follows: c,dP = 0.001; e,fP = 0.01; g,hP = 0.004; i,jP = 0.021; k,lP = 0.040; m,nP = 0.030; o,pP = 0.050; q,rP = 0.038. Group A was fed the high-starch concentrate, and Group B was fed the medium-starch concentrate.
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Total radiographic bone lesion scores (Table 9) decreased by approximately 29% from the start to the completion of the experiment (112 d), regardless of diet. There were eight OC or OCD lesions detected at the end of the experiment. All but one of these lesions was present at the start of the experiment (140 d of age). Only one animal developed a significant OCD lesion during the course of the experiment, and four animals with OCD lesions at the start of the experiment were scored 0 or 1 at the end of the experiment. Weanlings on both high- and medium-starch diets had decreased total lesion scores from start to finish (P <0.01 and 0.05, respectively). There were no differences between diet groups at the start (P <0.50) or completion of the experiment (P <0.10).
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Discussion
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Although the low-starch concentrate fed in Exp. 1 was formulated to contain the same amount of energy and protein as the high-starch concentrate, the low-starch diet did not support the same growth rate as the high-starch diet. This suggests that we overestimated the energy value of the beet pulp and/or soybean hulls, or that the animals could not grow to their potential without a readily available source of glucose. Ott and Kivipelto (2002) reported similar ADG (0.61 to 0.74 kg) by weanling horses fed soybean hulls in place of half the oats in a similar concentrate. The study suggested that soybean hulls had an energy value slightly below that of oats. The energy value for soybean hulls used in these experiments was set at 2.8 Mcal of DE/kg of DM based on Ott and Kivipelto (2002). Analyses of the low-starch concentrate indicated that the concentrate had no starch. This is probably unlikely because the wheat middlings should have provided some starch. Nevertheless, the starch content of the concentrate and therefore the diet, was very low. In Exp. 2, the high-and medium-starch concentrates were again consumed in similar amounts, but both groups gained weight at the same rate, suggesting that the two concentrates were similar in energy and value. Glucose availability to the animal consuming concentrate and hay-type diets is influenced by the animal’s ability to break down the starch and make the glucose available for absorption. Starch must be broken into maltose by amylase and the maltose hydrolyzed by maltase. Maltase concentrations in the small intestine are low at birth and do not reach adult levels until approximately 7 mo. of age (Roberts et al., 1974), which might limit the foal’s ability to utilize starch as efficiently as older animals. Hoffman et al. (1996) reported weanlings on pasture fed a starch-and sugar-based supplement had growth similar to that of weanlings fed a fat- and fiber-based supplement. The sugar and other readily available carbohydrate contents of the pasture were not reported, so it is difficult to relate the two studies. These authors reported a decrease in the growth of starch- and sugar-supplemented yearlings when the highly soluble carbohydrate spring pastures were available. The results of these two experiments suggest that we either overestimated the energy value of soybean hulls and beet pulp for this age foal, or that foals may not utilize the energy available from the forage part of the diet without a minimum amount of readily available glucose.
Based on an earlier study at this station (Ott and Kivipelto, 1997), it was expected that blood glucose concentrations would peak at 90 min after feeding. Because we were primarily interested in whether there were any outliers (animals that were glucose intolerant), we sampled the foals in the first experiment at 60 and 90 min after feeding. No outliers were detected. Glucose metabolism in Exp. 1 was obviously altered by the low-starch diet. The weanlings had lower blood glucose when they were fed the low-starch diet; however, when they were given glucose drenches after being on the low-starch diet for 112 d, the blood glucose concentrations were increased, suggesting a lowered tolerance for the glucose. This finding was not consistent with those of Hoffman et al. (2003); however, weanlings may respond differently than mature horses. This lower tolerance was not evident when they were offered sweet feed. It was obvious from Exp. 2 that we probably did not measure glucose peaks in all the weanlings in Exp. 1 when they were sampled at 60 and 90 min after feeding. Most of the weanlings in Exp. 2 peaked at 120 to 150 min after feeding. This result was consistent with data from Glade and Reimers (1985), who reported that glucose peaked at 120 min after feeding. Yearlings were reported to peak 1 h later than the weanlings. The weanlings consuming the medium-starch concentrate had higher blood glucose concentrations at 60 min after feeding (P = 0.03), peaked earlier, and then tended to have lower blood glucose (P <0.10) late in the collection (150 and 180 min after feeding). Groff et al. (2001) reported that mature horses fed a limited meal of 0.75 kg of whole oats or beet pulp glucose peaked at 105 to 119 mg/dL at 75 to 83 min after feeding. The weanlings in this study had higher glucose peaks (131 to 175 mg/dL), which occurred later (120 to 150 min). This finding suggests that weanlings are less able to handle glucose than older horses; however, we did not detect any adverse effects of the elevated glucose on insulin, T4, bone mineral deposition, or OC lesions.
Bone mineral deposition is decreased by restricting energy intake (Ott and Asquith, 1986). Nonetheless, in Exp. 1, when the low-starch concentrate restricted growth, there was no adverse affect on bone mineral deposition, suggesting that under the conditions of this study, the energy intake was adequate for normal bone deposition. The presence of DOD lesions at the start of the experiment (Exp 2; 140 d of age) is consistent with other reports (Jeffcott, 1991; Carlsten et al., 1993). It also has been documented that the incidence of OC decreases as the animal matures (van Weerden and Barneveld, 1982), supporting the observation in the present study; however, in contrast to Glade and Belling (1986), the high-starch diet fed in Exp. 2 did not result in increased OC lesions. These results are consistent with those of Davison et al. (1989), who reported that replacing starch with fat had no effect on skeletal development of weanling horses.
Both concentrates were readily consumed by the weanlings. It seems from the laboratory analyses that the feed mill made an error in the production of the last batch of the medium-starch concentrate delivered to the farm. Because these were pelleted concentrates, the error was not evident until the analyses were complete. The suspect feed was lower in NDF, ADF, and ether extract, and higher in starch (34.7%) than the previous batches. Six of the 12 foals assigned to the medium-starch concentrate did not receive any of this feed, two received it for less than 1 wk, one for less than 3 wk, and three for less than 5 wk. Each of the response criteria was evaluated to determine whether the short-term diet change altered the results of the final data by comparing the results of the six weanlings getting all the correct concentrate with the six getting the altered concentrate. When there were no differences, data from all 12 were used. When there was a difference, only the six that received the correct concentrate for their entire feeding period were used.
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Implications
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Low-starch concentrates (0% starch) with added oil did not support the same growth rate in weanling horses as high-starch concentrates (>30% starch). In contrast, medium-starch concentrates (15% starch) with appropriate oil addition to provide adequate energy supported the same growth as a high-starch concentrate. The results suggest that under the experimental conditions, weanlings need some readily available glucose to support normal growth. Skeletal development of weanling horses was not adversely affected by consumption of high-starch concentrates.
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Footnotes
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1 Florida Agric. Exp. Stn. Journal Series R-10257. 
2 This research was funded in part by Spillers Ltd., Milton Keynes, U.K., and Seminole Feeds, Ocala, FL.
3 Correspondence: P.O. Box 110910 (phone: 352-392-2455; fax: 352-392-7652; e-mail: ott{at}animal.ufl.edu).
Received for publication June 23, 2004.
Accepted for publication February 15, 2005.
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C. Robert, J-P. Valette, B-M. Paragon, J-M. Denoix, and G. Blanchard
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Vet Rec.,
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Abstract
Introduction
