Growth, nutrient utilization, radiographic bone characteristics and postprandial thyroid hormone concentrations in weanling horses fed added dietary fat

Thirty, 19-week-old Quarter Horses were utilized in a 112-d study to compare growth, nutrient utilization and post prandial thyroid hormone concentrations in weanling horses fed fat-supplemented or conventional diets. Concentrates were formulated to contain the same nutrient-calorie ratios and were fed with Coastal Bermuda grass hay in a 70:30 concentrate:hay ratio. Initially and at 28-d intervals the horses were weighed and measured for height, heart girth circumfer ence and subcutaneous fat thickness over the rump and ribs. Radiographs were taken of the right carpal and metacarpal phalangealjoints at the beginning and end of the experimental period for evaluation of bone density and physeal maturation. A digestion trial was conducted on each horse 10 d prior to completion of the study. On d 70, blood samples were col lected from 6 females in each treatment group over a 6-h collection period for evaluation of postprandial insulin, T₄ and T₃ concentrations in response to the diets. Horses in both groups consumed an average of 2.6% of body weight daily of total feed (Dry Matter). Weanlings consuming the fat-supplemented concentrate tended to have higher (P = .10) average daily weight gains (.80 vs .74 kg) and lower (P<.05) feed:gain ratios (6.3 vs 7.3) than weanlings fed the control diet. Gain in heart girth circumference was greater (P<.05) for the fat fed weanlings (20.1 cm) than the control weanlings (17.9 cm). All other linear measurements of growth were similar between treatments. There were no radiographic indications of abnormalities in the physes of horses on either treatment. Closure of the physes occurred at normal rates in foals on both treatments. Radiographic bone density was not different between treatments, and there was an expected increase in bone density in weanlings on both treatments over time. Digestibilities of protein and energy were similar between treatments, while ether extract and neutral detergent fiber digestibilities were higher (P<.05 and P<.06, respec tively) in those horses consuming the fat-supplemented diet (65.39 vs 57.67% and40.55 vs 35.62%, respectively). These data indicate that fat can be used as an energy source to support growth and development in weanling horses.     

Tall fescue utilization by exercised - yearling horses

Little information has been published concerning the effects of endophyte-infected tall fescue consumption on performance of young growing horses. Therefore, this study evaluated exercise performance and growth by yearlings fed endophyte-infected fescue. Twelve Quarter Horse yearlings (12-16 months of age) were blocked by age and sex and randomly assigned to one of three groups. Group I received native prairie hay (P), Group II received endophyte-free fescue hay (EF) and, Group III received endophyte infected fescue hay (EI). Rectal temperatures were monitored daily and growth parameters (weight, hip and wither heights, and body fat) were measured at the start of the study (d 0) and every 2 weeks throughout the 106 d study. Additionally, blood samples were collected on d 0 and every 28 d for the duration of the study to monitor blood concentrations of calcium, phosphorus, prolactin (PRL), thyroxine (T₄), and triiodothyronine (T₃). Horses were exercised twice a week for 10-30 min. Respiration, heart rate, and rectal temperature were measured at the start and 5, 30, and 60 min postexercise. Growth parameters and daily rectal temperatures were not different (P >.05) among grdups. No differences (P >.05) occurred among groups in concentrations of serum calcium, phosphorus, PRL, T₃, and T₄. Respiration rates were lower (P <.05) at 30 and 60 rain postexercise for horses consuming fescue than for horses consuming prairie hay. Postexercise heart rates and rectal temperatures showed no difference (P <.05) among groups. These data suggest that young growing horses being exercised can utilize endophyte-infected fescue efficiently on a short-term basis.     

Physiologic and metabolic response of exercising horses to added dietary fat

Six mature Quarter Horse-type geldings were used in a replicated 3 × 3 Latin square experiment to determine the effects of adding 5 or 10% feed-grade rendered animal fat to the concentrate diet fed to performance horses. The experiment was conducted over a 14-day pre-trial period to acquaint horses to the experimental apparatus, a 28-day conditioning period and 3 diet treatment periods of 21 days, each conducted in a Latin square arrangement. The horses were exercised on a dirt track and diet effects were evaluated during and following a submaximal exercise test (SET) on an equine treadmill. Physiological responses to the SET were determined following each experimental period. Relative to the control, the horses required 21% and 25% less of the concentrate feed (P<.05), containing 5% and 10% added fat, respectively. There was an increase (P<.05) in muscle glycogen concentration as fat was added to the diet. There were no differences due to feeding fat in nutrient digestibility or in oxygen consumption, ventilatory capacity, respiratory quotient, heart rate blood lactate or blood pH during the SET. However, there was an overall decrease (P<.05) in blood glucose and total lipid concentration, when 10% fat was added to the concentrate diet     

Serum beta-carotene and alpha-tocopherol in horses fed beta-carotene via grass-meal or a synthetic beadlet preparation with and without added dietary fat

The serum response of beta-carotene as an indicator of bioavailability was compared after feeding beta-carotene (0.8 mg/kg body weight) either from grass meal or a synthetic beadlet preparation (Lucarotin). Both were each given without or with added dietary vegetable fat (2-2.5% vs. 6.6% fat in dry matter) in a Latin square design with four horses. The nutritionally complete diet was supplemented with alpha-tocopherol (4 mg/kg body weight). Each treatment period (4 weeks, two serum samples) was followed by a washout period of 4 weeks with low intakes of beta-carotene (traces) and alpha-tocopherol (0.5 mg/kg body weight). Within 4 weeks of supplementation, serum beta-carotene increased about 10-fold, from a mean initial concentration of 0.05-0.53 micromol/l. There was no effect of beta-carotene source and of fat addition, respectively. Faecal excretion of beta-carotene ranged from 55 to 81% of intake. No beta-carotene was detected in any urine sample. Serum alpha-tocopherol (across all time points and animals, n=64) was 14.5 micromol/l. During supplementation, the values were significantly higher than during washout-periods. Additional dietary fat did not affect the serum response. Faecal excretion of alpha-tocopherol ranged from 69 to 121% of intake. Fat addition resulted in a significant decrease of serum cholesterol. In conclusion, the natural and the synthetic source of beta-carotene showed significant and identical bioavailability independent of additional fat.     

Influence of dietary fat and time of hay feeding on growth and development of yearling horses

Seventeen Thoroughbred and Quarter Horse yearlings were used in a 2×2 factorial experiment to determine 1) whether adding fat (5% corn oil) to the concentrate would influence feed intake, growth and development and 2) whether time of hay feeding would influence energy metabolism on the basal and added fat diets. Yearlings were assigned at random, within breed and gender subgroups, to one of four diets: B-I, basal concentrate with Coastal Bermuda grass (Cynodon dactylon) hay fed immediately after the concentrate; B-D, basal with hay fed 3.5 hr after the concentrate; F-I, basal + fat with the hay fed immediately; and F-D, basal + fat with the hay fed 3.5 hr after the concentrate. The basal concentrate was formulated to meet or exceed NRC (1989) recommendations when fed with the hay. The basal + fat concentrate was formulated at 9.7% higher digestible energy than the basal concentrate and all of the other nutrients were increased proportionately so if the intake was reduced, the yearlings would still meet nutrient needs. The concentrates were fed individually to appetite for two 1.5-hr feeding periods daily in 1.5×3.0 m slip stalls. Hay was group fed in the paddocks at a rate of 1.0 kg/100 kg BW daily divided into two equal feedings. Animals were housed in four 9.1×34.9-m drylot paddocks with three, four, or five animals in each paddock. The mean age at the start of the experiment was 377±8 days. Weight and body measurements for withers height, heart girth, body length, and hip height were taken at the start of the experiment and at 28-day intervals for 84 days. Radiographs for bone mineral estimates were made at the start and completion of the experiment. During the final 28-day period of the feeding trial, three animals from each experimental group were randomly selected for a 24-h period of blood sampling to evaluate the effects of the diets on blood glucose and insulin.Fat addition to the concentrate reduced the daily concentrate DM (P=0.0187) and total feed DM (P=0.0021) intake. When feed intake was expressed as a function of BW, concentrate and total feed intakes were 1.33 and 2.14 kg/100 kg BW daily and were not influenced by diet (P>0.1). Colts consumed more concentrate DM (P=0.0039) and total feed DM (P=0.0022) than fillies. The yearlings gained an average of 0.60 kg/d for the 84 days. Weight gain, body measurements and bone mineral deposition were not influenced by gender or diet. Mean plasma glucose concentrations were higher (P=0.05) in yearlings consuming the basal concentrate with hay fed immediately than when hay feeding was delayed. When fat was fed, the time of hay feeding had no effect on plasma glucose concentrations (P>0.05). Plasma glucose and insulin were lowest just prior to the morning feeding and peaked two to three hours after both morning and afternoon feedings. Fat supplementation reduced plasma insulin (P=0.001). Results suggest that the addition of 5% corn oil to a yearling concentrate may reduce feed intake but will not reduce growth and development of the animal if other nutrients are supplied at concentrations that provide for the animal's daily needs. Delaying hay feeding may be advantageous to growing horses as it may allow the concentrate to move down the tract at a slower rate reducing the glucose peaks.     

Lactation and reproductive performance of mares fed added dietary fat during late gestation and early lactation

Eighteen pregnant mares were randomly allotted to one of two treatment groups. The control group was fed a conventional concentrate and the fat group was fed a concentrate containing 5% feed-grade rendered fat. Both concentrates had the same nutrient:calorie ratio and were fed in amounts required to maintain zero change in percent body fat of the mares. During the study, which began 60 d prior to expected foaling date and ended 60 d postpartum, mares were monitored for feed intake, body weight, rump fat thickness, ration digestibility, plasma glucose and lipid concentrations, milk composition and reproductive efficiency. Birth weight, growth rate, and plasma glucose and lipid concentrations were also measured in foals. Mares fed fat consumed less concentrate (P<.09 during the last 60 d of gestation but consumed similar amounts of concentrate over 60 d of lactation. Protein and ether extract digestion was higher (P<.05 and P<.09, respectively) in the mares fed fat during the postpartum period. Dietary treatment had no influence on plasma glucose or lipid concentrations of the mares or plasma glucose concentrations of their foals, but foals whose dams were fed fat had higher plasma lipid concentrations at birth (P<.01), d 10 and d 30 (P<.05). Percent fat was higher in milk samples from mares fed fat at d 10 (P<.09, 1.23 vs .99%) and d 60 (P<.01, .72 vs .43%) of lactation. Reproductive performance was not significantly different between treatment groups, however there was a trend for a shorter postpartum interval and fewer cycles per pregnancy in the mares fed added fat. Foals from both groups were of similar size and weight at birth and had similar weight gains over 60 d, however, foals nursing mares fed fat tended to gain more weight in the first week of life (1.85 vs 1.56 kg/d) and have more rump fat at d 60 (.53 vs .44 cm) than foals nursing control mares.     

Growth rate, diet digestibility, and serum proclactin of yearling horses fed non-infected and infected tall fescue hay

To determine to effect of feeding tall fescue hay infected with endophyte on growth rate, diet digestibility, and serum prolactin, 17 Quarter Horse yearlings were fed a diet of 60% concentrate and 40% tall fescue hay for 5 months. Diets were formulated to meet 1989 NRC recommendations for moderate growth. The dietary treatments included either non-infected (E-, n=9) or infected (E+, n=8) tall fescue hay. Field sampling determined a 52% infection rate of the E+ hay with the endophyte Acremonium coenophialum. Initially and at 29-day intervals, yearlings were weighed and measured for skeletal growth including: (1) height at the withers, (2) length of bone between knee and fetlock, (3) length of bone between hock and fetlock, and (4) the circumference of fore and rear cannon bones. Total fecal collection from the 10 geldings during the last month of the trial permitted the calculation of diet digestibility. Serum samples were taken throughout the trial to determine prolactin response to dietary treatments.The E+ fescue hay did not affect (P>.05) growth rate of yearling horses. Yearlings gained .40 and .38 kg/d when consuming the E− and E+ based diets, respectively. Geldings gained more (P<.05) than fillies. Changes in height at the wither and length and circumference of the fore and rear cannon were positive but did not differ between hay treatments or sexes.E+ fescue hay diets tended to be lower in dry matter, apparent energy, and crude protein digestibility, but values were not statistically different. Fiber digestibility was lower (P<.01) for the diet based on E+ hay. Neutral detergent fiber digestibility averaged 66.2 and 54.4% for E− and E+ diets, respectively.Acid detergent fiber digestibility averaged 78.9 and 64.5% for E− and E+ diets, respectively. Although fiber digestibility of E+ tall fescue hay was lower than E-hay, grain supplementation supportive of a moderate growth rate produced equal growth among yearling horses. Also, no differences in prolactin were apparent between the yearlings fed E− and E+ tall fescue hay.     

Growth of yearling fillies fed alfalfa or soybean meal

Sixteen Quarter Horse yearling fillies were used in a 112-day randomized block experiment to determine the comparative efficacy of alfalfa hay protein to support growth in young horses. The yearlings were divided into two groups of eight and paired according to weight and age, with one yearling from each pair randomly allotted to each diet. Group 1 was fed a diet of coastal Bermuda grass hay and concentrate. The corn-based concentrate was supplemented with soybean meal such that the total diet for group 1 contained approximately 13% CP. Group 2 was fed a diet that also contained approximately 13% CP and consisted of alfalfa hay and a similar concentrate without soybean meal. Yearlings were fed the diets in a 60:40 grain-to-hay ratio, and intakes between members of each pair were equalized. The calculated digestible energy density in both final diets was approximately 2.85 mcal/kg, and the protein:calorie ratio for both diets was approximately 45 gm/Mcal. Body weight, body length, heart girth, wither and hip heights, as well as forearm, gaskin and cannon bone circumferences were measured every 28 days. Rump fat thickness was measured ultrasonically every 28 days. Physical measures of growth were similar for horses eating each diet (P>0.05). Also, blood urea nitrogen was similar for horses eating each diet (P>0.05) and was not reflective of differences in nitrogen metabolism. The fillies fed the SBM-supplemented diet retained more nitrogen more efficiently than did fillies fed the alfalfa diet (P<0.05), and the SBM-fed fillies had higher concentrations of osteocalcin on days 14, 28, 42, and 112. It can be concluded that SBM and alfalfa proteins were equally effective in providing growth to young horses; however, the biological value of absorbed nitrogen appeared to be higher in the fillies fed the SBM-supplemented diet. also, differences in serum osteocalcin concentrations indicate that the SBM-fed fillies were experiencing a greater amount of osteoblastic activity than the alfalfa fed fillies.     

Growth and development of yearling horses fed either alfalfa or coastal bermudagrass: Hay and a concentrate formulated for bermudagrass hay

Two experiments were conducted with yearling horses to determine whether the use of a concentrate designed to be fed with Coastal bermudagrass hay would cause growth differences or skeletal abnormalities when fed with western alfalfa hay. In experiment 1, 13 thoroughbred and quarterhorse yearlings, 332.8 ± 8.5 days, of age were assigned at random within breed and gender subgroups to 1 of 2 diets: (1) alfalfa hay plus a 12.7% CP concentrate and (2) coastal bermudagrass hay plus the same concentrate. The horses were housed in drylot paddocks in groups of 3 or 4 animals and group fed the hay at 1 kg/100 kg BW daily. The concentrates were fed individually to appetite for two 1.5-hour feeding periods daily for 112 days. Orts were weighed back daily. The animals were weighed, measured, and blood samples collected at the start of the experiment and at 28-day intervals for 112 days. Radiographs of the third metacarpal were made on day 0, 56, and 112 for estimation of bone mineral content. In experiment 2, 15 thoroughbred and quarterhorse yearlings, 345.6 ± 8.4 days of age, were used to repeat the experiment. The concentrate fed in this experiment contained 12.4% CP. In experiment 1, the hay protein content was less than anticipated, resulting in a protein intake below NRC¹ recommendations. In experiment 2, the hay protein was higher and more typical of NRC¹ values. In both experiments, feed intake of the 2 diets was not different (P > .10). Weight gain (P = .0136, P = .0330), heart girth gain (P = .0084, P= .0044), and hip height gain (P = .0165, P = .0137) were higher for the alfalfa fed yearlings for experiment 1 and experiment 2, respectively. Withers height gain (P = .0079) and body length gain (P = .0232) were also greater for the alfalfa fed yearlings on experiment 2. Bone mineral content and bone metabolism indicators were not influenced by diet (P > .10).     

Growth performance of yearling horses grazing bermudagrass pastures at different grazing pressures

Body weight gains and height at the withers were measured in yearling geldings grazing bermudagrass (Cynodon dactylon [L.] Pers.) pastures with neither energy nor protein feed supplementation at stocking rates of 6.7, 8.0, 9.5 and 12.4 yearlings per hectare. Weekly forage samples were taken to estimate the quantity and quality of available forage. These samples were clipped and separated into three equal lengths to characterize upper, middle and lower thirds of the canopy and to determine relationships between available forage and yearling growth rate. Average daily gain was influenced by stocking rate, but gain in height was not affected by stocking rate. Average daily gain ranged from -.31 to .37 kg/d and was negatively related to stocking rate. A quadratic relationship between available forage and ADG was detected. However, a linear relationship between ADG and available forage in top layers of the canopy indicated that animal performance might have been limited by availability of top layer forage in pastures subjected to light grazing pressure.     

Influence of sarsaponin on growth, feed and nitrogen utilization in growing male rats fed diets with added urea or protein

Two experiments were conducted with growing male rats to determine the effects of 120 ppm of dietary sarsaponin (S) on nitrogen (N) metabolism when urea or protein are added to the diet. Growth, feed efficiency, N digestibility and balance, urinary N and ammonia-N (NH3-N), and cecal urease and NH3-N were measured. Growth and feed utilization were unaffected by dietary S. Adding urea or protein to the diet increased apparent N digestibility and increased urinary-N excretion. Urea did not affect N balance, whereas growth, feed utilization and N balance were maximized with 22% compared with either 16 or 28% dietary protein. Urinary NH3-N excretion was decreased by S when urea was added to the diet but was not affected when fed with increasing dietary protein. Cecal urease was decreased by S when urea was added or when the protein level was increased in the diet; effects on cecal NH3-N varied between the two experiments. Plasma urea-N was decreased by S. It is concluded that S has minor effects on N metabolism in rats and that NH3-N formation or excretion is only marginally affected by dietary S. If S decreases NH3-N level in confinement facilities, it is concluded that the effect is after the waste material is excreted by the animal, perhaps through reduced urease activity.     

Growth and calcium metabolism in horses fed varying levels of protein

The effect of level of protein intake on growth and calcium metabolism was studied in 24 foals. Starting at four months old, the foals were fed one of three diets containing all nutrients, with the exception of protein, at levels recommended by the United States National Research Council Subcommittee on Horse Nutrition for a 12 month period. The protein levels in the three diets were 9 per cent (low protein) 14 per cent (NRC recommended level) and 20 per cent (high protein). The foals fed the low protein diet were changed to the high protein diet after 140 days when they were nine months old. There were no significant differences in the rates of growth in weight, height, cannon circumference or in hoof growth and feed utilisation of the horses fed the 14 or the 20 per cent protein diets. However, growth, feed intake and feed utilisation by the foals fed the low (9 per cent) protein diet were significantly depressed. The average daily gains for the first 140 days for the 9, 14 and 20 per cent protein treatment groups were 64, 631 and 687 g in weight, 0.57, 0.83 and 0.87 mm in height and 0.04, 0.13 and 0.14 m in forecannon circumference, respectively. The average daily feed intakes for the 140 day period for the three groups were 2.7, 4.4 and 4.7 kg, respectively. After the change to the high protein diet the foals that had been fed the low protein diet maintained a higher rate of gain in bodyweight, height and cannon circumference, and utilised feed more efficiently than the other two groups throughout the second 140 days of the experiment.(ABSTRACT TRUNCATED AT 250 WORDS)     

Performance and carcass quality of steers fed different sources of dietary fat

The hypothesis of this experiment was that increasing dietary fat through the use of whole oilseeds and altering the dietary ratio of PUFA:saturated fatty acids would alter carcass composition of finishing steers. Seventy-two steers (443.6 +/- 1.0 kg) were fed for 76 d one of four dietary treatments: a corn/ soybean meal-based diet (NOFAT); two diets containing 16% (DM basis) whole raw soybeans; and a corn/soybean meal-based diet containing choice white grease (CWG) equal to the fat addition supplied by the soybeans. Soybeans used in the diets were either a standard variety (NORM-SB) or a variety high in oleic acid content (HO-SB). The fatty acid profile of diets differed (P < 0.05) in the degree of saturation and content of palmitic, stearic, oleic, linoleic, and linolenic acids. There were no differences in ADG (1.73 kg/d), hot carcass weight (347 kg), longissimus muscle area (79.4 cm2), yield grade (3.31), or percentage of boneless retail cuts (48.8%). Contrasts revealed differences (P < 0.05) in G:F and marbling score with the addition of fat (0.126 vs. 0.137 and 4.66 vs. 4.91, respectively, for NOFAT vs. fat). The addition of fat tended (P < 0.10) to increase backfat, and feeding NORM-SB increased (P < 0.01) dressing percent compared with the HO-SB treatment. Loin samples taken from steers fed NOFAT, NORM-SB, and HO-SB did not differ in alpha-tocopherol content. Loins from the CWG treatment tended (P < 0.10) to have lower alpha-tocopherol content than did the soybean treatments (0.79 vs. 0.99 ppm, respectively). From main-effects analysis, HO-SB loin samples had the highest (F3,8 = 32.91; P < 0.01) concentration of gamma-tocopherol (0.33 ppm); this resulted in differences (P < 0.05) in gamma-tocopherol when comparing all contrasts. When comparing loin samples from NORM-SB-fed steers with those from HO-SB-fed steers, NORM-SB samples had a greater (P < 0.05) percentage oflinoleic acid and PUFA and a lower (P < 0.05) percentage of oleic acid and monounsaturated fatty acids. Furthermore, loin samples from soybean-fed steers tended (P < 0.10) to have a greater concentration of conjugated linoleic acid than samples from CWG-fed steers. These data suggest that the source of added dietary fat may affect overall carcass composition. Furthermore, dietary addition of soybeans or CWG can improve feed efficiency and marbling, whereas the addition of whole raw soybeans compared with CWG may increase unsaturation and total vitamin E content of beef.     

Voluntary intake and digestion of Coastal Bermuda grass hay by yearling and mature horses

Coastal Bermuda grass hay was fed as the sole source of nutrients to determine comparative voluntary intake and nutrient digestibility of hay in yearling and mature horses. Yearling horses consumed dry matter at a higher (P<.10) percentage of their body weight (2.5%) than mature horses (2.0%). digestion of hemicellulose was higher (P<.05) in mature horses, but digestion of dry matter, gross energy, neutral detergent fiber and crude protein was similar between the 2 classes of horses.     

Estimation of urinary flow rate in weanling and yearling horses

In an attempt to validate the use of urinary creatinine concentration as an index of urinary flow rate, a series of timed quantitative urine collections were done on several groups of weanling and yearling ponies and Thoroughbreds. A total of 411 urine samples were generated by 19 ponies and 12 Thoroughbreds. Urinary flow rates and urinary creatinine concentrations were measured. When all the data were examined, urinary flow rates were independent of creatinine concentrations. However, for any given animal, daily urinary creatinine excretion was constant over several days, and urinary creatinine concentrations were related to urinary flow rates in a negative curvilinear fashion. Urinary flow rates in young horses can be estimated from urinary creatinine concentrations, but separate curves relating the 2 variables must first be derived for each animal.     

Cholecystokinin mediates depression of feed intake in dairy cattle fed high fat diets

Two experiments were conducted to investigate the effects of 1) different concentrations of dietary fat and 2) i.v. administration of a cholecystokinin receptor antagonist (MK-329) on feed intake and plasma concentrations of hormones and metabolites in dairy cattle. In Experiment 1, 4 lactating Holstein cows were used in a 4 x 4 Latin square design. Treatments were diets with 1) no fat added, 2) 30 g fat/kg feed (calcium salts of long-chain fatty acids as fat supplement), 3) 60 g fat/kg, and 4) 90 g fat/kg added. Cows were fed once daily a diet of concentrate, corn silage, alfalfa haylage, and alfalfa pellets. Dry matter intake decreased linearly with increasing concentrations of dietary fat (P < 0.0001). Overall plasma concentrations of nonesterified fatty acids (P < 0.0001), triacylglycerol (P < 0.0006), and cholecystokinin (P < 0.02), increased linearly with each level of dietary fat, but there was a linear decrease in plasma insulin (P < 0.0008). In Experiment 2, 4 nonpregnant and nonlactating Holstein heifers were used in a cross-over design in a 2 x 2 factorial arrangement of treatments. Treatments were diet (fatty acids, 27 g/kg vs 103 g/kg diet dry matter) and i.v. injections (MK-329 vs vehicle). Heifers were fed once daily a total mixed ration of corn silage, cracked corn and soybean meal with or without fat supplement. Diets were switched by period and either MK-329 (70 microg/kg body weight) or its vehicle was injected i.v. at 2 hr postfeeding. Daily dry matter intake was decreased by feeding the high fat diet (P < 0.02) but was not affected by injections. Injection of MK-329, however, increased dry matter intake by 92% in heifers fed the high fat diet during the first 2 hr postinjection compared to vehicle injection. Plasma pancreatic polypeptide concentration was increased by the high fat diet at 2 hr postfeeding (P < 0.02) but was lowered by MK-329 at 1 hr postinjection (P < 0.001). Plasma insulin was lowered by the high fat diet (P < 0.01) but was not affected by injections. The elevated plasma cholecystokinin concentration may have mediated depressed feed intake of dairy cattle fed the high fat diets.