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Effects of feed form on growth and blood glucose in weanling horses
Scientific Papers Effects of Feed Form on Growth and Blood Glucose in Weanling Horses Jean E. Andrew, BS, Kevin H. Kline, PhD, and Jennifer L. Smith, MS REFEREED
ABSTRACT Eight weanling Standardbred horses were used to evaluate the effects on growth and plasma glucose of two differently processed diets with identical ingredients fed at a rate of 3% of body weight, as fed, daily. The daily ration was divided into two equal feedings at 7:00 am and 7:00 pm. The hay and grain diet (diet HG) consisted of 50% alfalfa hay cubes and 50% of a commercial growing horse texturized grain mix fed as is, without any further processing, with each part fed in separate containers. The completely pelleted diet (diet P) consisted of the same proportions of the hay cubes and grain mix that was ground, pelleted, and fed in a single container. Two groups of four horses were fed either diet P or diet HG for two 21-day periods such that each horse consumed each diet for 21 days. On the last day of each feeding period, 7-mL blood samples were drawn into heparinized evacuated blood tubes for determination of plasma glucose concentration. Blood samples were drawn 30 minutes before the afternoon feeding, immediately before, and every 30 minutes thereafter for 5 hours. Analysis of variance found greater (P .027) average daily gain for horses fed diet P versus diet HG. A nonsignificant difference was seen in feed efficiency (P = .057) for horses fed diet P versus diet HG. Plasma glucose changes were compared across treatments using area under the curve analysis. No difference (P .05) was found in the postprandial plasma glucose changes between diet treatments. Key words: Feed efficiency; growth; horse; pelleted diet
From the Department of Animal Sciences, University of Illinois Urbana-Champaign, IL 61801. Reprint requests: Kevin H. Kline, 1207 W Gregory Drive, Urbana, IL 61801. 0737-0806/$ - see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.jevs.2006.06.006
Volume 26, Number 8
INTRODUCTION Achieving adequate early growth in young horses has been found to be important for maximizing profit at sales. Horses with heavier weights and taller wither heights command higher prices at yearling Thoroughbred public auctions.1 Equine managers find achieving rapid body weight (BW) gain in young horses while avoiding such growth disorders as epiphysitis, osteochondrosis, and angular limb deformities difficult.2 Accelerated growth in young horses is most often achieved by feeding additional grain. However, highstarch diets fed to growing horses have been implicated in the development of orthopedic diseases.3,4 Insulin concentration has been found to be related to cartilage growth,5,6 extreme change in post-feeding blood glucose concentrations may be related to developmental orthopedic disease in young horses through its effects on insulin and IGF-I.7 However, a study of 12 Quarter Horse weanlings fed diets differing in carbohydrate and fat concentrations found no difference in foal growth performance or serum IGF-I concentrations.7 One way to more evenly distribute the intake of starches and sugars throughout the day, while not changing the total intake of starches and sugars in the diet, is to provide the feed as a total mixed ration (TMR) whereby forage and grain is combined into a completely pelleted diet. This feeding strategy may reduce the chance of an animal “bolting” the grain portion of the diet, which can result in digestive upset. Research has been conducted on total mixed rations for dairy cattle8, 9; however, research using TMR feeds in horses is very limited. One study using Swiss draft horses found that digestibility of a complete diet was not different from the digestibility of either oats and hay or grain pellets and hay fed separately.10 Using TMR feeds, most often in the form of complete pellets, is commonly done to facilitate weight gain in both growing and geriatric horses. These feeds are marketed as “junior” or “senior” feeds, respectively. However, the effectiveness of improving weight gain
349
Table 1
Descriptive data for weanling horses for each period Beginning of Period 1 Sex
Group 1
Group 2
End of Period 1/Beginning of Period 2
BW (kg)
Age (days)
BW (kg)
Age (days)
BW (kg)
Age (days)
Filly Filly Colt Colt Mean BW Mean Age
266.3 232.7 241.8 221 240.4
209 207 175 174
284.4 266.3 260 242.8 263.3
230 228 196 195
287.1 258.6 276.2 254.9 269.2
251 249 217 216
Colt Colt Filly Filly Mean BW Mean Age
254 241.3 235.9 223.6 238.7
191 208 167 170 155 175
using these feeds versus other forms of feed has not been well documented. Prior equine research has found that processed feeds may have greater pre-ileal digestibility than feeds in which the components of the diet are not ground, cracked, steamed, or popped.11 Completely pelleted diets are typically both ground and forced through dies with steam and heat, making the starch component of the diet more digestible. Complete pellets are also suitable for use in automated feeding systems and may be safer for promoting weight gain in equines when given ad libitum access to feed or automatically. Improved safety of complete pellets during ad libitum feeding is attributable to the fact that the starches and sugars from cereal grains in the diet are diluted with lower starch forages. We have hypothesized that diet P, consisting of a completely pelleted ration, will result in higher average daily gain (ADG) and feed efficiency compared with diet HG, consisting of hay cubes and grain fed separately. Such projected improvement in gain and feed efficiency could result from the further steam heat and mechanical processing of the texturized grain mix, resulting in improved digestibility. The improved consistency of nutrient intake throughout the day when feeding the pelleted grain and hay mixture also might result in more efficient hind gut fermentation. We have also hypothesized that diet P may produce less postprandial glucose fluctuation than diet HG because of the lower concentration of starches and sugars in the P diet compared with the grain portion of the HG diet. The latter hypothesis was based on general observations of the
350
End of Period 2
212 266.7 263.1 250.8 233.1 253.4
229 188 191 176
233 276.2 284.4 269 252.7 270.6
196
250 209 212 197 217
feeding behavior of growing horses, whereby the highstarch grain portion of the HG diet is generally consumed more rapidly immediately after feeding as compared with the P diet. The much lower-starch hay cube portion of the HG diet is generally consumed slowly throughout the day, after the grain has been completely consumed. The hay cubes were generally consumed between eight and 12 hours after feeding. Therefore, we expected a more rapid rise in the plasma glucose concentration in horses that rapidly consume the grain portion of the HG diet then slowly consume the lower energy hay cubes, as compared with horses that consume the P diet at a steadier rate after feeding. MATERIALS AND METHODS Eight weanling Standardbred horses (four colts and four fillies) were used in a two-by-two Latin square design with two replicates to evaluate the effects of feed form on ADG, feed efficiency, and postprandial blood glucose concentrations. The horses were selected from the University of Illinois’s (UrbanaChampaign, IL) herd in which all were foaled and raised in the same facilities. The horses were randomly allotted to one of the two treatment groups within sex to ensure two fillies and two colts in each group. The descriptive data for ages and body weights for all horses are listed in Table 1. Throughout the study, the horses were housed individually in 1.8 30.5 m metal panel runs with approximately 1.8 10.7 m covered and bedded with wood shavings and the rest of the run soil covered. No grass or weeds of any kind grew in this
Journal of Equine Veterinary Science
August 2006
soil because of the wearing of the ground through horse traffic and high gravel content. Rubber mats approximately 1.8 0.9 m were placed under the plastic feed tubs that were wired to the sides of the pens to aid in collection for weigh backs. All horses were dewormed 14 days before the study and were fed both hay cubes and complete pellets for a 14-day adaptation period before the study to become familiar with consuming hay cubes. Horses had been weaned approximately 2 months before the study and were housed in individual runs after weaning. Before the beginning of the adaptation period, all horses were provided with ad libitum completely pelleted feed and hay, as is the usual management practice for all weanlings at the University of Illinois. This study was approved by and performed under the specifications of the University of Illinois Institutional Animal Care and Use Committee.
Diet Treatments The horses were fed two different forms of a diet with identical ingredients at a rate of 3% of BW on an “as fed” basis daily divided into two equal feedings at 7:00 am and 7:00 pm. Samples of each diet were obtained weekly, composited at the end of the study, and representative samples sent to the Iowa Testing Laboratories, Inc. (Eagle Grove, IA) for analysis using Association of Official Analytical Chemists (Gaithersburg, MD) methods. The nutrient composition of the experimental diets is shown in Table 2. The crude protein and digestible energy concentrations of the diets vary less than 2% from the concentrations recommended by the National Research Council (NRC) for weanling horses six months of age undergoing rapid growth.12 Because the diets were fed at 3% of BW daily, our six-month-old horses at the beginning of the study were provided a calculated total DE of 18.3 Mcal as compared with the NRC recommended DE of 17.2 Mcal for weanling horses six months of age for rapid growth that have a projected mature weight of 500 kg.12 Diet P consisted of a completely pelleted feed made of 50% alfalfa cubes (Norstar Industries, Auburn, WA) and 50% of a commercial growing horse texturized sweet feed (Purina Omolene 200; Purina Mills, St. Louis, MO) that was ground and processed into a single pellet. The processing of the diet P complete pellets included grinding and mixing the alfalfa cubes and sweet feed together in a tub grinder, then steam heating the mix to 77°C for several minutes while forcing it through the pellet die. Diet HG consisted of the same ingredients, fed separately. A group of four horses was started
Volume 26, Number 8
Table 2
Nutrient composition of diets, as fed
Analysis % Moisture % Dry mattter % Crude Protein % ADF % NDF DE (Mcal/kg)
Diet P
Diet HG
Complete Pellets
Hay Cubes
Grain Mix
12.32 87.68 13.00 20.85 30.85 2.55
12.24 87.76 12.13 35.40 47.70 2.10
2.40 87.60 13.87 6.30 14.00 3.01
on diet P and another group of four horses was started on diet HG for a 21-day interval. After the initial feeding period, the diets were switched and the experiment was continued. There was no adaptation period before beginning the second period because of time restrictions using the facility and assumed minimal carryover effect because the diets were of the same composition.
Blood Analysis On the final day of each three-week feeding period, 7-mL blood samples were drawn using aseptically placed indwelling jugular catheters. The catheters were placed in the jugular veins of all horses three hours before feeding to minimize possible effects from catecholamine release during blood collections. Blood was initially drawn into syringes, then placed into 7-mL heparinized tubes (Vacutainer, Becton Dickinson, Franklin Lakes, NJ) 30 minutes before, immediately before, and every 30 minutes for five hours after consuming the evening meal at 7:00 pm. Blood samples were centrifuged at 2000g for 10 minutes, and plasma glucose concentration was determined using a Roche Diagnostics (Indianapolis, IN) Hitachi 971 automated chemistry analyzer.
Feed Intake and Weight Determination All horses were weighed on the first day of the study and weekly thereafter using a digital livestock platform scale. Using the weekly updated BW, the feed rations were recalculated for each horse weekly to remain at 3% as fed of BW. Before each of the two daily feedings, the weight of feed refusals remaining in feeders and feed wastage from rubber mats under the feeders was recorded using an electronic digital laboratory scale. Feed wastage is shown in Table 3. After the first 21-day feeding period, the diets of the horses were switched and the experiment repeated for another
351
Table 3
Feed wastage from feeders and floor mats Diet P
Diet HG
Hay Complete Total Cubes Pellets Wastage Wastage
Grain Mix Wastage
Total Wastage (kg)
2.7
Wastage as kg/horse/ day
0.01
0.13
0.13
0
Wastage as % of Total Feed Offered
0.2
2.3
2.3
0.04
29.7
29.2
0.5
The ADG for all horses was 50% higher when consuming diet P than when consuming diet HG, and analysis of variance found an effect attributable to diet treatment (P .027). The LSMEAN for ADG (SEM) for horses on diet P was 0.87 0.27 kg/day and was 0.58 0.18 kg/day for horses on diet HG. The effects for period (P .152) and interaction between treatment and period (P .538) on ADG were not significant. The LSMEAN feed efficiency (Gain: Feed SEM) of horses was determined for diet P to be 0.0806 0.0026 and for diet HG to be 0.1146 0.0036. The feed efficiency for all horses was 42% higher when consuming diet P than when consuming diet HG. Analysis of variance found a nonsignificant effect of diet treatment on gain:feed ratio (P .057). The effects for period (P .082), and interaction between treatment and period (P .602) on gain:feed ratio were not significant.
Blood Glucose Data three weeks. The individual weight gains for each horse were recorded for each feeding period then divided by 21 days to determine ADG. Feed efficiency was determined and expressed as the gain:feed ratio.
Statistics Analysis of variance (ANOVA) was conducted to determine whether the two treatments had significantly different impacts on the ADG and gain:feed ratios. The statistical analyses were conducted using the GLM procedure from SAS Version 9.1.3 (SAS Institute, Cary, NC). Fixed effects models were used. The data for plasma glucose changes were modeled as repeated measurements over time with an autoregressive (AR) covariance structure and area under the curve (AUC) was compared between diet treatments. The statistical analyses for plasma glucose measurements were conducted using the MIXED procedure from SAS® Version 9.1.3 (SAS Institute). RESULTS
Growth Data The mean BW (SEM) for all horses was 240 5 kg at the beginning of the study and 271 5 kg after 42 days on the study. This 0.72-kg/day ADG over both diet treatments is within the normal range of expected growth rates for weanling horses of expected mature weight of 500 kg according to NRC12 and similar to another recent growth study using weanling horses.13 The individual body weight changes of all horses are shown in Fig 1.
352
Plasma glucose changes of horses on the last day of the 21-day feeding period after consuming the evening meal of diet P or diet HG are shown in Fig 2. The value of the AR correlation (0.7189) for horsetrial suggests strong dependencies among a horse’s glucose levels on a particular trial. The least squares mean estimates for plasma glucose AUC for diets P and HG were 634.9 and 611.9 hours mg/dL plasma glucose, respectively. Type 3 tests of fixed effects for diet, time, trial, and diettrial found differences in the mean glucose levels across time (P .0001), but no differences (P .05) according to diet, trial or diettime. DISCUSSION The results of this study supported our hypothesis that the ADG of weanling horses fed forage and concentrates together as a completely pelleted diet (diet P) would exceed that of the same horses provided the same forage and concentrates fed separately (diet HG). However, longer feeding periods may have reduced the observed differences in growth between treatments, as this study was very short. A trend was also seen for improved feed efficiency when horses consumed diet P. The lack of a significant effect of diet treatment on the gain:feed ratio may have been attributable to greater refusal of the hay cube portion of diet HG by some of the horses (Table 3). However, the blood glucose changes after feeding were not different (P .05) between the two diets, contrary to our hypothesis. No behavioral problems such as excessive chewing behavior or other signs of anxiety were observed for horses on either diet throughout the study.
Journal of Equine Veterinary Science
August 2006
Weeks on Feed Figure 1. Individual body weights (kg) of horses determined at the beginning of the study and in weekly intervals throughout. Horses A-D were fed diet P, then diet HG. Horses E-H were fed diet HG, then diet P.
The increased ADG of diet P may have been attributable to an increase in foregut digestibility of pellets because of grinding and processing. The processing of corn significantly increased pre-ileal digestibility, grinding increased digestibility by 20%, and expanding (extruding) increased digestibility by as much as 70%.11 Because the concentrate used in this study contained a significant portion of starch from corn, the additional grinding and pelleting of the feedstuffs may have further increased digestibility of starch in the diet. Other studies have shown that grinding and pelleting of hay does not significantly increase fiber digestion in the cecum and colon.14,15 In those studies, offsetting effects of decreased degradation rate and increased re-
Volume 26, Number 8
tention time were seen because of grinding and pelleting of the hay. The design of the current study comparing the completely pelleted diet P with hay cubes and grain (diet HG) does not allow for separation of effects that may be related to the processing of diets. We are not able to know, for instance, whether improved growth of horses on diet P may have been attributable to improved digestibility from additional grinding and steam and heat processing, or possibly from greater ingesta intake consistency for diet P. Greater consistency of ingesta intake might result in improved fermentation of digesta in the hind gut. More consistent proportions of fiber, starches, and sugars consumed throughout the day may partly explain the greater
353
160 150 140 130 120 110 100 90 80 70 60
Diet P
0
00 0:
:3
0 23
0
:0
:3
23
0 22
0
:0
:3
22
0 21
:0
0 :3
21
0 :0
20
0 20
:3
:0
19
19
:3 18
0
Diet HG
0
Plasma Glucose (mg/dL)
Figure 2
Figure 2. Plasma glucose (SEM) in horses before and for 5 hours after feeding.
ADG found for diet P.Twice per day feeding of hay cubes and a texturized sweet feed (diet HG), in which the horses in this study tended to much more rapidly consume the texturized sweet feed portion of the diet well before consuming the hay cubes, may have resulted in digesta with high starch concentration reaching the hind gut of the weanling horses shortly after feeding.The weanling horses in our study also tended to eat the complete pellets more slowly than the texturized sweet feed portion of diet HG. This observation suggests that the consistency of the rate of starch intake must have been greater when the horses consumed the complete pellets. Increasing the frequency and reducing the size of meals throughout the day may have reduced the differences found between the two diets in this study. However, one study16 found no difference in digestibility of diets fed to ponies as multiple meals versus a single large meal. The lack of difference in postprandial glucose AUC between our experimental diets, although contradicting our hypothesis, is consistent with observations from other recent studies. A study that examined the improvement in prececal starch digestibility of processed corn versus untreated corn found no differences in either glucose or insulin responses.17 The same laboratory found that horses consuming either thermally treated
354
oats or corn had relatively low and similar glucose and insulin responses.18 Another study found that the plasma glucose AUC was not different in horses consuming corn, oat groats, or barley.19 These findings seem to contradict studies that have found that oat starch has significantly greater prececal digestibility than corn starch.11,20 There may not be a close relationship between postprandial glucose and insulin responses and prececal starch digestion. Diets consisting of only pelleted feed have been anecdotally associated with negative behavioral and physiological consequences such as increased wood chewing and increased chance of digestive upset. Neither tail chewing nor wood chewing was observed in this study, but the horses were not allowed the opportunity to wood chew because the runs were made of metal panels. Both diets in this study provided forage at a rate of 1.5% of body weight daily, as is often suggested for growing horses. Adequate fiber in the diet may help to prevent occurrence of such digestive problems independent of the form of concentrate feed offered. IMPLICATIONS This study found that feeding a completely pelleted diet to weanling horses resulted in improved growth;
Journal of Equine Veterinary Science
August 2006
however, no difference was seen in postprandial plasma glucose changes when compared with a nutritionally identical diet with forage and grain fed separately. The results of this study suggest that feeding completely pelleted diets is a valid management option for growing horses that may result in improved growth as compared with more traditional hay and grain diets.
9.
REFERENCES
12.
1.
13.
2. 3. 4. 5.
6.
7.
8.
Pagan JD, Koch A, Caddel S, Nash D. Size of Thoroughbred yearlings presented for auction at Keeneland Sales affects selling price. Proc 19th Equine Sci Soc Symp, 2005: 224-225. O’Donohue DD, Smith FH, Strickland KL. The incidence of abnormal limb development in the Irish thoroughbred from birth to 18 months. Equine Vet J 1992;24:305-309. Glade MJ, Belling TH. Growth plate cartilage metabolism, morphology and biochemical composition in over and underfed horses. Growth 1984;48:473-482. Savage CJ, McCarthy RN, Jeffcot LB. Effects of dietary energy and protein on induction of dyschondroplasia in foals. Equine Vet J 1993;16:74–79. Glade MJ, Kanwar YS, Stern PH. Insulin and thyroid hormones stimulate matrix metabolism in primary cultures of articular chondrocytes from young rabbits independently and in combination. Connect Tissue Res 1994;31:37-44. Alarid ET, Schlechter NL, Russell SM, Nicholl CS. Evidence suggesting that insulin-like growth factor-I is necessary for the trophic effects of insulin on cartilage growth in vivo. Endocrinology 1992;130:2305-2309. Ropp JK, Raub RH, Minton JE. The effect of dietary energy source on serum concentration of insulin-like growth factor-I growth hormone, insulin, glucose, and fat metabolites in weanling horses. J Anim Sci 2003;81:1581-1589. Beauchemin KA, Yang WZ, Rode LM. Effects of particle size of alfalfa-based dairy cow diets on chewing activity, ruminal fermentation, and milk production. J Dairy Sci 2003;86:630-643.
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10. 11.
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16. 17. 18. 19. 20.
Calberry JM, Plaizier JC, Einarson MS, McBride BW. Effects of replacing chopped alfalfa hay with alfalfa silage in a total mixed ration on production and rumen conditions of lactating dairy cows. J Dairy Sci 2003;86:3611-3619. Riond JL, Leoni S, Wanner M. Comparative study of three feeding methods for draught horses of the Swiss army. J Anim Sci 2004;82:3625-3634. Meyer H, Radicke S, Kienzel E, Wilke S, Kleffken D, Illenseer M. Investigations on preileal digestion of starch from grain, potato and manioc in horses. Zentralbl Veterinarmed Reihe 1994;42:371-381. Nutrient Requirements of Horses, 5th ed. Washington D.C.: National Academies Press; 1989:43-48. Ott EH, Brown MP, Roberts GD, Kivipelto J. Influence of starch intake on growth and skeletal development of weanling horses. J Anim Sci 2005;83:1033-1043. Drogoul C, Poncet C, Tisserand JL. Feeding ground and pelleted hay rather than chopped hay to ponies. I. Consequences for in vivo digestibility and rate of passage of digesta. Anim Feed Sci Technol 2000;87:117-130. Drogoul C, Poncet C, Tisserand JL. Feeding ground and pelleted hay rather than chopped hay to ponies. II. Consequences on fibre degradation in the cecum and the colon. Anim Feed Sci Technol 2000;87:131-145. Houpt KA, Perry PJ, Hintz HF, Houpt TR. Effect of meal frequency on fluid balance and behavior of ponies. Physiol Behavior 1998;42:401-407. Vervuert I, Coenen M, Bothe C. Effects of corn processing on the glycemic and insulinaemic responses in horses. J Anim Physiol Anim Nutr 2004;88:348-355. Vervuert I, Coenen M, Bothe C. Glycemic and insulinaemic indexes of different mechanical and thermal processed grains for horses. Proc 19th Equine Science Soc Symp, 2005:154-155. Jose-Cunilleras E, Taylor LE, Hinchcliff KW. Glycemic index of cracked corn, oat groats and rolled barley in horses. J Anim Sci 2004;82:2623-2629. de Fombelle A, Veiga L, Drogoul C, Julliand V. Effect of diet composition and feeding pattern on the prececal digestibility of starches from diverse botanical origins measured with the mobile nylon bag technique in horses. Schweiz Arch Tierheilkd 2000;142:570-579.
355
ABSTRACT Eight weanling Standardbred horses were used to evaluate the effects on growth and plasma glucose of two differently processed diets with identical ingredients fed at a rate of 3% of body weight, as fed, daily. The daily ration was divided into two equal feedings at 7:00 am and 7:00 pm. The hay and grain diet (diet HG) consisted of 50% alfalfa hay cubes and 50% of a commercial growing horse texturized grain mix fed as is, without any further processing, with each part fed in separate containers. The completely pelleted diet (diet P) consisted of the same proportions of the hay cubes and grain mix that was ground, pelleted, and fed in a single container. Two groups of four horses were fed either diet P or diet HG for two 21-day periods such that each horse consumed each diet for 21 days. On the last day of each feeding period, 7-mL blood samples were drawn into heparinized evacuated blood tubes for determination of plasma glucose concentration. Blood samples were drawn 30 minutes before the afternoon feeding, immediately before, and every 30 minutes thereafter for 5 hours. Analysis of variance found greater (P .027) average daily gain for horses fed diet P versus diet HG. A nonsignificant difference was seen in feed efficiency (P = .057) for horses fed diet P versus diet HG. Plasma glucose changes were compared across treatments using area under the curve analysis. No difference (P .05) was found in the postprandial plasma glucose changes between diet treatments. Key words: Feed efficiency; growth; horse; pelleted diet
From the Department of Animal Sciences, University of Illinois Urbana-Champaign, IL 61801. Reprint requests: Kevin H. Kline, 1207 W Gregory Drive, Urbana, IL 61801. 0737-0806/$ - see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.jevs.2006.06.006
Volume 26, Number 8
INTRODUCTION Achieving adequate early growth in young horses has been found to be important for maximizing profit at sales. Horses with heavier weights and taller wither heights command higher prices at yearling Thoroughbred public auctions.1 Equine managers find achieving rapid body weight (BW) gain in young horses while avoiding such growth disorders as epiphysitis, osteochondrosis, and angular limb deformities difficult.2 Accelerated growth in young horses is most often achieved by feeding additional grain. However, highstarch diets fed to growing horses have been implicated in the development of orthopedic diseases.3,4 Insulin concentration has been found to be related to cartilage growth,5,6 extreme change in post-feeding blood glucose concentrations may be related to developmental orthopedic disease in young horses through its effects on insulin and IGF-I.7 However, a study of 12 Quarter Horse weanlings fed diets differing in carbohydrate and fat concentrations found no difference in foal growth performance or serum IGF-I concentrations.7 One way to more evenly distribute the intake of starches and sugars throughout the day, while not changing the total intake of starches and sugars in the diet, is to provide the feed as a total mixed ration (TMR) whereby forage and grain is combined into a completely pelleted diet. This feeding strategy may reduce the chance of an animal “bolting” the grain portion of the diet, which can result in digestive upset. Research has been conducted on total mixed rations for dairy cattle8, 9; however, research using TMR feeds in horses is very limited. One study using Swiss draft horses found that digestibility of a complete diet was not different from the digestibility of either oats and hay or grain pellets and hay fed separately.10 Using TMR feeds, most often in the form of complete pellets, is commonly done to facilitate weight gain in both growing and geriatric horses. These feeds are marketed as “junior” or “senior” feeds, respectively. However, the effectiveness of improving weight gain
349
Table 1
Descriptive data for weanling horses for each period Beginning of Period 1 Sex
Group 1
Group 2
End of Period 1/Beginning of Period 2
BW (kg)
Age (days)
BW (kg)
Age (days)
BW (kg)
Age (days)
Filly Filly Colt Colt Mean BW Mean Age
266.3 232.7 241.8 221 240.4
209 207 175 174
284.4 266.3 260 242.8 263.3
230 228 196 195
287.1 258.6 276.2 254.9 269.2
251 249 217 216
Colt Colt Filly Filly Mean BW Mean Age
254 241.3 235.9 223.6 238.7
191 208 167 170 155 175
using these feeds versus other forms of feed has not been well documented. Prior equine research has found that processed feeds may have greater pre-ileal digestibility than feeds in which the components of the diet are not ground, cracked, steamed, or popped.11 Completely pelleted diets are typically both ground and forced through dies with steam and heat, making the starch component of the diet more digestible. Complete pellets are also suitable for use in automated feeding systems and may be safer for promoting weight gain in equines when given ad libitum access to feed or automatically. Improved safety of complete pellets during ad libitum feeding is attributable to the fact that the starches and sugars from cereal grains in the diet are diluted with lower starch forages. We have hypothesized that diet P, consisting of a completely pelleted ration, will result in higher average daily gain (ADG) and feed efficiency compared with diet HG, consisting of hay cubes and grain fed separately. Such projected improvement in gain and feed efficiency could result from the further steam heat and mechanical processing of the texturized grain mix, resulting in improved digestibility. The improved consistency of nutrient intake throughout the day when feeding the pelleted grain and hay mixture also might result in more efficient hind gut fermentation. We have also hypothesized that diet P may produce less postprandial glucose fluctuation than diet HG because of the lower concentration of starches and sugars in the P diet compared with the grain portion of the HG diet. The latter hypothesis was based on general observations of the
350
End of Period 2
212 266.7 263.1 250.8 233.1 253.4
229 188 191 176
233 276.2 284.4 269 252.7 270.6
196
250 209 212 197 217
feeding behavior of growing horses, whereby the highstarch grain portion of the HG diet is generally consumed more rapidly immediately after feeding as compared with the P diet. The much lower-starch hay cube portion of the HG diet is generally consumed slowly throughout the day, after the grain has been completely consumed. The hay cubes were generally consumed between eight and 12 hours after feeding. Therefore, we expected a more rapid rise in the plasma glucose concentration in horses that rapidly consume the grain portion of the HG diet then slowly consume the lower energy hay cubes, as compared with horses that consume the P diet at a steadier rate after feeding. MATERIALS AND METHODS Eight weanling Standardbred horses (four colts and four fillies) were used in a two-by-two Latin square design with two replicates to evaluate the effects of feed form on ADG, feed efficiency, and postprandial blood glucose concentrations. The horses were selected from the University of Illinois’s (UrbanaChampaign, IL) herd in which all were foaled and raised in the same facilities. The horses were randomly allotted to one of the two treatment groups within sex to ensure two fillies and two colts in each group. The descriptive data for ages and body weights for all horses are listed in Table 1. Throughout the study, the horses were housed individually in 1.8 30.5 m metal panel runs with approximately 1.8 10.7 m covered and bedded with wood shavings and the rest of the run soil covered. No grass or weeds of any kind grew in this
Journal of Equine Veterinary Science
August 2006
soil because of the wearing of the ground through horse traffic and high gravel content. Rubber mats approximately 1.8 0.9 m were placed under the plastic feed tubs that were wired to the sides of the pens to aid in collection for weigh backs. All horses were dewormed 14 days before the study and were fed both hay cubes and complete pellets for a 14-day adaptation period before the study to become familiar with consuming hay cubes. Horses had been weaned approximately 2 months before the study and were housed in individual runs after weaning. Before the beginning of the adaptation period, all horses were provided with ad libitum completely pelleted feed and hay, as is the usual management practice for all weanlings at the University of Illinois. This study was approved by and performed under the specifications of the University of Illinois Institutional Animal Care and Use Committee.
Diet Treatments The horses were fed two different forms of a diet with identical ingredients at a rate of 3% of BW on an “as fed” basis daily divided into two equal feedings at 7:00 am and 7:00 pm. Samples of each diet were obtained weekly, composited at the end of the study, and representative samples sent to the Iowa Testing Laboratories, Inc. (Eagle Grove, IA) for analysis using Association of Official Analytical Chemists (Gaithersburg, MD) methods. The nutrient composition of the experimental diets is shown in Table 2. The crude protein and digestible energy concentrations of the diets vary less than 2% from the concentrations recommended by the National Research Council (NRC) for weanling horses six months of age undergoing rapid growth.12 Because the diets were fed at 3% of BW daily, our six-month-old horses at the beginning of the study were provided a calculated total DE of 18.3 Mcal as compared with the NRC recommended DE of 17.2 Mcal for weanling horses six months of age for rapid growth that have a projected mature weight of 500 kg.12 Diet P consisted of a completely pelleted feed made of 50% alfalfa cubes (Norstar Industries, Auburn, WA) and 50% of a commercial growing horse texturized sweet feed (Purina Omolene 200; Purina Mills, St. Louis, MO) that was ground and processed into a single pellet. The processing of the diet P complete pellets included grinding and mixing the alfalfa cubes and sweet feed together in a tub grinder, then steam heating the mix to 77°C for several minutes while forcing it through the pellet die. Diet HG consisted of the same ingredients, fed separately. A group of four horses was started
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Table 2
Nutrient composition of diets, as fed
Analysis % Moisture % Dry mattter % Crude Protein % ADF % NDF DE (Mcal/kg)
Diet P
Diet HG
Complete Pellets
Hay Cubes
Grain Mix
12.32 87.68 13.00 20.85 30.85 2.55
12.24 87.76 12.13 35.40 47.70 2.10
2.40 87.60 13.87 6.30 14.00 3.01
on diet P and another group of four horses was started on diet HG for a 21-day interval. After the initial feeding period, the diets were switched and the experiment was continued. There was no adaptation period before beginning the second period because of time restrictions using the facility and assumed minimal carryover effect because the diets were of the same composition.
Blood Analysis On the final day of each three-week feeding period, 7-mL blood samples were drawn using aseptically placed indwelling jugular catheters. The catheters were placed in the jugular veins of all horses three hours before feeding to minimize possible effects from catecholamine release during blood collections. Blood was initially drawn into syringes, then placed into 7-mL heparinized tubes (Vacutainer, Becton Dickinson, Franklin Lakes, NJ) 30 minutes before, immediately before, and every 30 minutes for five hours after consuming the evening meal at 7:00 pm. Blood samples were centrifuged at 2000g for 10 minutes, and plasma glucose concentration was determined using a Roche Diagnostics (Indianapolis, IN) Hitachi 971 automated chemistry analyzer.
Feed Intake and Weight Determination All horses were weighed on the first day of the study and weekly thereafter using a digital livestock platform scale. Using the weekly updated BW, the feed rations were recalculated for each horse weekly to remain at 3% as fed of BW. Before each of the two daily feedings, the weight of feed refusals remaining in feeders and feed wastage from rubber mats under the feeders was recorded using an electronic digital laboratory scale. Feed wastage is shown in Table 3. After the first 21-day feeding period, the diets of the horses were switched and the experiment repeated for another
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Table 3
Feed wastage from feeders and floor mats Diet P
Diet HG
Hay Complete Total Cubes Pellets Wastage Wastage
Grain Mix Wastage
Total Wastage (kg)
2.7
Wastage as kg/horse/ day
0.01
0.13
0.13
0
Wastage as % of Total Feed Offered
0.2
2.3
2.3
0.04
29.7
29.2
0.5
The ADG for all horses was 50% higher when consuming diet P than when consuming diet HG, and analysis of variance found an effect attributable to diet treatment (P .027). The LSMEAN for ADG (SEM) for horses on diet P was 0.87 0.27 kg/day and was 0.58 0.18 kg/day for horses on diet HG. The effects for period (P .152) and interaction between treatment and period (P .538) on ADG were not significant. The LSMEAN feed efficiency (Gain: Feed SEM) of horses was determined for diet P to be 0.0806 0.0026 and for diet HG to be 0.1146 0.0036. The feed efficiency for all horses was 42% higher when consuming diet P than when consuming diet HG. Analysis of variance found a nonsignificant effect of diet treatment on gain:feed ratio (P .057). The effects for period (P .082), and interaction between treatment and period (P .602) on gain:feed ratio were not significant.
Blood Glucose Data three weeks. The individual weight gains for each horse were recorded for each feeding period then divided by 21 days to determine ADG. Feed efficiency was determined and expressed as the gain:feed ratio.
Statistics Analysis of variance (ANOVA) was conducted to determine whether the two treatments had significantly different impacts on the ADG and gain:feed ratios. The statistical analyses were conducted using the GLM procedure from SAS Version 9.1.3 (SAS Institute, Cary, NC). Fixed effects models were used. The data for plasma glucose changes were modeled as repeated measurements over time with an autoregressive (AR) covariance structure and area under the curve (AUC) was compared between diet treatments. The statistical analyses for plasma glucose measurements were conducted using the MIXED procedure from SAS® Version 9.1.3 (SAS Institute). RESULTS
Growth Data The mean BW (SEM) for all horses was 240 5 kg at the beginning of the study and 271 5 kg after 42 days on the study. This 0.72-kg/day ADG over both diet treatments is within the normal range of expected growth rates for weanling horses of expected mature weight of 500 kg according to NRC12 and similar to another recent growth study using weanling horses.13 The individual body weight changes of all horses are shown in Fig 1.
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Plasma glucose changes of horses on the last day of the 21-day feeding period after consuming the evening meal of diet P or diet HG are shown in Fig 2. The value of the AR correlation (0.7189) for horsetrial suggests strong dependencies among a horse’s glucose levels on a particular trial. The least squares mean estimates for plasma glucose AUC for diets P and HG were 634.9 and 611.9 hours mg/dL plasma glucose, respectively. Type 3 tests of fixed effects for diet, time, trial, and diettrial found differences in the mean glucose levels across time (P .0001), but no differences (P .05) according to diet, trial or diettime. DISCUSSION The results of this study supported our hypothesis that the ADG of weanling horses fed forage and concentrates together as a completely pelleted diet (diet P) would exceed that of the same horses provided the same forage and concentrates fed separately (diet HG). However, longer feeding periods may have reduced the observed differences in growth between treatments, as this study was very short. A trend was also seen for improved feed efficiency when horses consumed diet P. The lack of a significant effect of diet treatment on the gain:feed ratio may have been attributable to greater refusal of the hay cube portion of diet HG by some of the horses (Table 3). However, the blood glucose changes after feeding were not different (P .05) between the two diets, contrary to our hypothesis. No behavioral problems such as excessive chewing behavior or other signs of anxiety were observed for horses on either diet throughout the study.
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Weeks on Feed Figure 1. Individual body weights (kg) of horses determined at the beginning of the study and in weekly intervals throughout. Horses A-D were fed diet P, then diet HG. Horses E-H were fed diet HG, then diet P.
The increased ADG of diet P may have been attributable to an increase in foregut digestibility of pellets because of grinding and processing. The processing of corn significantly increased pre-ileal digestibility, grinding increased digestibility by 20%, and expanding (extruding) increased digestibility by as much as 70%.11 Because the concentrate used in this study contained a significant portion of starch from corn, the additional grinding and pelleting of the feedstuffs may have further increased digestibility of starch in the diet. Other studies have shown that grinding and pelleting of hay does not significantly increase fiber digestion in the cecum and colon.14,15 In those studies, offsetting effects of decreased degradation rate and increased re-
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tention time were seen because of grinding and pelleting of the hay. The design of the current study comparing the completely pelleted diet P with hay cubes and grain (diet HG) does not allow for separation of effects that may be related to the processing of diets. We are not able to know, for instance, whether improved growth of horses on diet P may have been attributable to improved digestibility from additional grinding and steam and heat processing, or possibly from greater ingesta intake consistency for diet P. Greater consistency of ingesta intake might result in improved fermentation of digesta in the hind gut. More consistent proportions of fiber, starches, and sugars consumed throughout the day may partly explain the greater
353
160 150 140 130 120 110 100 90 80 70 60
Diet P
0
00 0:
:3
0 23
0
:0
:3
23
0 22
0
:0
:3
22
0 21
:0
0 :3
21
0 :0
20
0 20
:3
:0
19
19
:3 18
0
Diet HG
0
Plasma Glucose (mg/dL)
Figure 2
Figure 2. Plasma glucose (SEM) in horses before and for 5 hours after feeding.
ADG found for diet P.Twice per day feeding of hay cubes and a texturized sweet feed (diet HG), in which the horses in this study tended to much more rapidly consume the texturized sweet feed portion of the diet well before consuming the hay cubes, may have resulted in digesta with high starch concentration reaching the hind gut of the weanling horses shortly after feeding.The weanling horses in our study also tended to eat the complete pellets more slowly than the texturized sweet feed portion of diet HG. This observation suggests that the consistency of the rate of starch intake must have been greater when the horses consumed the complete pellets. Increasing the frequency and reducing the size of meals throughout the day may have reduced the differences found between the two diets in this study. However, one study16 found no difference in digestibility of diets fed to ponies as multiple meals versus a single large meal. The lack of difference in postprandial glucose AUC between our experimental diets, although contradicting our hypothesis, is consistent with observations from other recent studies. A study that examined the improvement in prececal starch digestibility of processed corn versus untreated corn found no differences in either glucose or insulin responses.17 The same laboratory found that horses consuming either thermally treated
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oats or corn had relatively low and similar glucose and insulin responses.18 Another study found that the plasma glucose AUC was not different in horses consuming corn, oat groats, or barley.19 These findings seem to contradict studies that have found that oat starch has significantly greater prececal digestibility than corn starch.11,20 There may not be a close relationship between postprandial glucose and insulin responses and prececal starch digestion. Diets consisting of only pelleted feed have been anecdotally associated with negative behavioral and physiological consequences such as increased wood chewing and increased chance of digestive upset. Neither tail chewing nor wood chewing was observed in this study, but the horses were not allowed the opportunity to wood chew because the runs were made of metal panels. Both diets in this study provided forage at a rate of 1.5% of body weight daily, as is often suggested for growing horses. Adequate fiber in the diet may help to prevent occurrence of such digestive problems independent of the form of concentrate feed offered. IMPLICATIONS This study found that feeding a completely pelleted diet to weanling horses resulted in improved growth;
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however, no difference was seen in postprandial plasma glucose changes when compared with a nutritionally identical diet with forage and grain fed separately. The results of this study suggest that feeding completely pelleted diets is a valid management option for growing horses that may result in improved growth as compared with more traditional hay and grain diets.
9.
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