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METABOLISM AND NUTRITION |
Department of Animal Sciences, The Ohio State University, Columbus 43210
1 Corresponding author: Latshaw.1{at}osu.edu
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Key Words: energy balance • fat • feed intake regulation • fiber • protein
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Several hormones act to decrease feed intake. The first to be identified was cholecystokinin, which slows gastric emptying but promotes bile secretion and pancreatic enzyme secretion (Gibbs et al., 1973; Kissileff et al., 1981). Another hormone with similar effects is oxyntomodulin (Dakin et al., 2001; Cohen et al., 2003). A hormone with related effects is glucagon-like peptide 1, which inhibits gastric emptying and gastric secretion (Turton et al., 1996; Tang-Christensen et al., 2001).
The hormones named above have relatively short-term effects and may affect the length of a meal. In some cases, administration of the hormone decreased the length of the meal, but a consequence was shorter intervals between meals. For longer term energy balance, leptin has been suggested. Leptin is produced mostly by white adipose tissue, so higher concentrations are present in blood when an animal has more body fat (Zhang et al., 1994; Morton et al., 2005). The combination of leptin and insulin has been proposed to be important in long-term regulation of body fat (Schwartz et al., 2000), possibly through adenosine monophosphate kinase (Minokoshi et al., 2004). If fat stores are high, increased secretion of these hormones will be sensed in regions of the hypothalamus and converted to signals that will decrease feed intake. If fat stores are depleted, decreased concentrations of these hormones will result in more feed intake to replace body fat.
Much of the information about leptin has come from research with genetically obese rodents. Providing leptin for animals that cannot synthesize leptin or its receptor causes dramatic changes in energy balance. Funding for such research is supported because of the problem with obesity in humans (World Health Organization, 1998; Ogden et al., 2006), most of whom are not genetically obese. There is evidence that diet composition is a contributor to excessive caloric intake. Some of the effect may relate to palatability (Rogers and Blundell, 1984; Weinsier et al., 1998). They described good palatability in terms of increased number of calories consumed, increased meal size, increased meal frequency, or a combination of these. When rats were offered chow free choice or chow, bread, and chocolate in a cafeteria style, those offered a choice of foods ate more energy each day. They also ate little chow and ate the more palatable feed. Energy density was also examined as a variable to alter caloric intake (Bell et al., 1998). Digestible energy per gram was altered by an exchange between water and fat. Daily energy intake was significantly higher when women consumed the diet with higher digestible energy content.
What has emerged is a difference in opinion about an animal’s ability to regulate energy intake. One opinion is that an animal has the ability to count ME calorie intake and will adjust feed intake to accomplish this. This is proposed to occur through hormonal regulation. A different opinion is that an animal does not count ME calorie intake accurately, eating more or less energy based on dietary variables. Most poultry nutritionists favor the first opinion (Leeson and Summers, 2001); however, NRC (1994) was noncommittal. It cited some research that indicated good regulation of caloric intake and some that indicated deviation from accurate regulation. The purpose of this research was to test the ability of broiler chickens to equalize daily ME intake. Dietary variables were protein, fat, and fiber, with an additional comparison of mash and pellets.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Thirty-six different diets were prepared by using various ingredients to alter the content of protein, fat, and fiber. The diets were a factorial of 3 protein levels, 4 fat levels, and 3 fiber levels. An initial diet was formulated to provide approximately 16.4% protein using corn and soybean meal as protein and energy sources (Table 1
, treatment 1). To meet amino acid requirements for broilers from 3 to 6 wk (NRC, 1994), several amino acids were supplemented. A protein mix of 38.30% peanut meal, 30.85% menhaden fish meal, and 30.85% corn gluten meal was added at 3.25% of the diet to increase protein content approximately 1.8% of the diet. For these diets, the protein content of the final diet was calculated to be 18.2% (16.4% + 1.8%). The proportion of the diet that was corn and soybean meal was reduced by 3.25% and rebalanced to provide 16.4% protein, including supplemental amino acids. Adding 6.5% of the protein mix increased dietary protein approximately 3.6%.
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. Experimental Design
Hubbard broilers were used for the experiment, which consisted of 3 trials. In trial 1, only males were used. They were fed a standard broiler starter in a floor pen until the heaviest reached a weight of approximately 1,200 g. Feed and water were removed at 2200 h. Beginning at 0800 h the following day, all birds were weighed individually. Those with the highest and lowest weights were removed, keeping 50 of those with intermediate weights. Forty-four were randomly selected and housed in individual cages in a room maintained at 25°C. The other 6 were killed by cervical dislocation and frozen. Each broiler was fed 1 of the 44 feeds for 12 d. Starting on d 7, one-third of the broilers were used for a 2-d digestion trial. Total collection of excreta was done using plastic sheeting suspended under the cages. One-third of the broilers were started on d 8, and the remainder were started on d 9. Samples were dried in a different room that was heated to 27°C and had fans to circulate the air. Dried samples were weighed and frozen.
The broilers were fed for 12 d. At 2200 h of d 11, feed and water were removed. Beginning at 0800 h of d 12, each broiler was weighed, killed by cervical dislocation, and frozen.
Trial 2 was begun 4 wk after trial 1. It was identical except that half males and half females were used. Twenty-six males and 26 females of intermediate weight were kept for the trial. Four males and 4 females were killed by cervical dislocation and frozen, and the remaining broilers were randomly distributed to cages.
Trial 3 was begun 4 wk after trial 2 and was almost identical to trial 2. The only difference was that treatments that had a male in trial 2 were assigned a female in trial 3, and vice versa. As a result, 2 males and 1 female were fed each diet during the experiment.
Analyses
The density of each feed was determined. A pan that held approximately 5 L was placed on a level balance. The pan was almost filled with water. Then water was added slowly until the pan began to overflow, at which point the weight was recorded. The grams of water were used as the volume, in cubic centimeters, of the pan. Each feed was then added to the pan to overflowing, the excess was scraped off, and the weight was recorded.
Protein, fat, and fiber concentrations of each diet were determined. When the feeds were poured into a container for each pen of chicks, a sample of approximately 200 g was removed and saved before weighing the container. Each sample was used to determine nutrient content (AOAC, 2000): protein, Official Method 990.03; fat, Official Method 963.15; and fiber, Official Method 973.18. Energy content was determined with an adiabatic calorimeter (Parr Instruments, Moline, IL). Excreta samples were also used for energy determination.
Samples were prepared from carcasses. Each carcass was thawed until it still retained some firmness. It was then cut into pieces small enough to fit into a meat grinder that had a plate with 1.27-cm holes. Each was reground through a plate with 0.64-cm holes and then homogenized with a food chopper. Several small samples were added to an aluminum pan to provide a total sample of about 200 g. The sample was weighed, dried at 95°C for 2 d, and reweighed. Each sample was reground using a coffee grinder. Portions were used to determine fat content (AOAC, 2000) and energy content.
Data were statistically analyzed using the GLM of SAS (1996). The design was a randomized complete block, with 3 time periods as blocks. Treatments were arranged in a 3-way factorial design of fat, fiber, and protein. Effects of sex and diet form were also determined. Results were analyzed for interactions, and then main effects were determined. Orthogonal comparisons of linear, quadratic, and cubic responses were computed. Multiple linear regression was used to develop several models.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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. Protein percentage ranged from 16.48 to 20.47 and was close to calculated values. Fat percentage ranged from 2.08 to 10.39%, and fiber ranged from 5.57 to 10.77%.
Differences in daily feed consumption of mash diets were due to sex (Tables 2 and 3). Percentages of dietary protein, fat, and fiber had no effect, and no interactions were detected. Sex also affected daily gain (Tables 2 and 3), as did fat content of the diet. Again, no interactions were found. The effect of increasing fat was a linear increase in daily gain. Sex had no effect on the ME (kcal/ g) of the diet, but the fractions of the proximate analysis did (Tables 2 and 3). There was a linear increase in ME from adding fat and a linear decrease from adding fiber. Increasing protein resulted in a quadratic response in ME. Protein, fat, fiber content, and sex were all involved in interactions affecting 2 or 3 of the variables. Sex, dietary fat, and dietary fiber affected the kilocalories of ME eaten per day (Tables 2 and 3) with no interactions detected. Increasing fat caused a linear increase in daily ME consumed, whereas increasing fiber caused a linear decrease in daily ME consumed.
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. Daily feed consumption was not affected by protein, fat, or fiber, but was affected by sex and form. Broilers ate more grams of pellets each day than mash. Gain per day was affected in the same way as feed intake. The ME (kcal/g) was not affected by sex or form. Increasing dietary protein and fat increased the ME per gram of feed. Increasing fiber did not affect ME but only because the variability of the samples was several times as large as for other factors examined. Interactions were found that involved all of the experimental factors. The only factors that affected daily energy intake (kcal of ME per d) were sex and form.
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. For mash, there was a large coefficient for the intercept and relatively large coefficients for fat and fiber content. The coefficients for the quadratic effect of fat and the interaction of protein and fiber were also included to adjust the ME in mash. For pellets, the coefficient for the intercept was less than half of that for the mash intercept. The coefficient for fiber indicates a fairly small change in energy due to fiber, but fat and protein coefficients show they substantially increase the ME of the diet. No interactions were significant when predicting the ME of pellets.
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). Changes in protein and fat did not affect density, but fiber did. In addition to the effect of fiber, there was an interaction between form and fiber. Separation of the effects showed that mash was more dense than pellets when fiber was low, as found in a corn-soy diet; however, when diets contained additional fiber, mash was less dense than pellets.
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). Each increase of 100 kcal of ME per d increased body fat by 1.2%.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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, the diet with the lowest ME, 2.54 kcal/g, contained 80% of the energy concentration in the diet with the highest ME. It is relatively difficult to develop diets with a large range of ME. For example, adding 0.5% methionine to a corn-soy diet in place of corn more than doubles the diet methionine; however, adding 0.5% fat in place of corn increases dietary energy by less than 1%.
As a result, energy studies may need a greater number of observations so that significant differences can be detected. Comparing diets that are different by 100 or 200 kcal of ME/kg may give daily energy intakes that are statistically similar, but extending the range of diet energy might show that energy content has a significant effect on daily energy intake. Increasing levels of dietary fiber caused large decreases in the daily ME intake (Table 3), even though the fiber content had no effect on the amount of feed eaten each day. A similar but more pronounced effect is present in dairy cattle (Conrad, 1966; NRC, 1988). Forages may contain 40% or more fiber. To increase energy intake needed for higher milk production, high fiber ingredients must be replaced by low fiber ingredients. Higher dry matter intake and, therefore, energy intake result when dietary fiber is decreased, due to direct or indirect effects of fiber (Allen, 2000).
In contrast to results with fiber, increasing levels of dietary fat increased the daily ME intake, although the effect was not as large as for fiber and appeared to reach a maximum at a level of approximately 5% added fat. These results are similar to those found in humans (Lissner and Heitmann, 1995) and rats (Rogers and Blundell, 1984). Dietary protein, within a range that was needed to meet amino acid requirements, did not affect ME intake per day. This is in contrast to what was reported from other research (Stubbs, 1999).
Broilers that were fed pellets had a higher ME intake per day than those fed mash. As was reported previously (Sibbald, 1977), there was no difference in the ME/gram of mash and pellets. If the hormonal regulation of energy balance that is proposed for mammals (Schwartz et al., 2000) is also present in broilers, it is not very effective. Broilers that ate more energy per day (Table 7) had more body fat, which should have resulted in more leptin production and less energy consumption. The results of the present experiment suggest that hormonal regulation of feed intake should be considered only a coarse adjustment, with other factors determining the amount of daily energy actually consumed. Palatability may be an important component of fine tuning energy intake, especially when an animal has no alternative to the complete diet that is provided.
When diets were mixed, it was obvious that adding fat decreased the dustiness of the diets, and adding fiber increased the dustiness of the diets. If dustiness is a component of palatability, the results from the present experiment indicate that broilers prefer diets that are not dusty. Behavior of the broilers also indicated that the higher fiber diets were more difficult to swallow: chickens needed several attempts to swallow a mouthful of the higher fiber diets. The results of the present experiment also indicate that pellets are a more important consideration in palatability than fat or fiber. When mash was fed, both fat and fiber content of the diet affected the energy intake per day (Table 3), but when the same diets were fed as pellets, neither fat nor fiber content caused a significant effect (Table 4).
The effect of pellets cannot be attributed to density of the diet (Table 6), because pellets of a high fiber diet were slightly more dense than the corresponding mash, but pellets of the low fiber diet were slightly less dense than the corresponding mash. In an experiment with White Leghorn hens, the effect of energy density was separated from the effect of bulk density (Cherry et al., 1983) by diluting a diet with 20% wood fiber or 20% sand. Hens fed the sand diet adjusted feed intake so that caloric intake and rate of egg production were higher than for those fed the wood fiber diet. Bulk density results from the present experiment are different from previous research (Skoch et al., 1983). In that research, bulk density was 0.50 for the corn-soy diet in mash form and 0.68 when pellets were formed using steam. No explanation for the different observations in the 2 experiments is available. Bulk density is probably affected by air spaces among particles of the ingredients. For the low fiber diets, air spaces among the pellets are probably larger than spaces between ingredient particles. For the high fiber diets, the reverse is probably true.
Results of the present experiment indicate that changing the proportions of the proximate analysis and the form of the feed significantly affect the amount of ME eaten by broiler chickens each day. This knowledge may be important for increasing energy intake, especially when the available ingredients have higher amounts of fiber. When diets have relatively low amounts of fiber, it may be of economic importance to determine if responses due to adding fat and forming pellets cancel each other.
Received for publication April 27, 2007. Accepted for publication September 13, 2007.
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