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The Effect of Different Feed Restriction Programs on Reproductive Performance, Efficiency, Frame Size, and Uniformity in Broiler Breeder Hens

University of Arkansas, Center of Excellence for Poultry Science, Fayetteville 72701

¹ Corresponding author: ccoon{at}uark.edu

	ABSTRACT

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Two experiments were conducted to determine the effect of feed restriction programs on breeder reproductive performance. In experiment 1, every day (ED), skip-a-day (SK), 4-3, and 5-2 programs were compared. Diets did not differ, and feed intake was identical in all groups. Four hundred twenty pullets were reared on each program. At 21 wk, 80 breeders from each program were individually housed to record performance parameters. Body weight and frame size were larger in ED pullets than SK, 4-3, or 5-2 despite equal intakes. Hens fed ED reached sexual maturity at a younger age than other groups. Hens fed ED also produced more total and settable eggs than SK hens. Egg weight was heavier in 5-2 hens than in ED with 4-3 and SK intermediate. Efficiency of feed utilization was best in ED hens. In experiment 2 the same programs were tested, but pullets were reared to reach equal BW. One hundred seventy-five pullets were reared on each program, of which 60 were housed. Feed intake was greater for SK, 4-3, and 5-2 than ED pullets to reach the same BW. Frame size did not differ, indicating that BW was the cause of differences in experiment 1. In experiment 2, differences in performance were attenuated but not eliminated by feeding to reach equal BW, suggesting that metabolic factors aside from BW are altered by feeding programs. The 5-2 produced larger eggs than ED with the trend among programs being identical to that in experiment 1. These results suggest that metabolic changes such as increased lipogenesis or alterations in body composition may result in larger eggs in feeding programs that include off-feed days. Mortality, fertility, and hatchability were not affected by feeding programs in either experiment. Body composition analysis indicated the importance of total lean protein mass as a threshold for the onset of sexual maturity. Programs like SK are less efficient than ED and may result in reduced performance.

Key Words: broiler breeder • feeding program • performance

	INTRODUCTION

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Selection for rapid growth and enhanced breast muscle mass has been accompanied by an increase in voluntary feed consumption (Chambers et al., 1981) such that modern breeds are unable to adequately control feed intake. As a result the reproductive performance of broiler breeders has suffered. Larger BW and increased fat accumulation have led to leg problems, early onset of sexual maturity, accelerated ovarian follicular development, and the incidence of multiple hierarchies and multiple ovulations. The reproductive problems caused by overconsumption and the benefits of feed restriction have been well documented (Fuller et al., 1969; Pym and Dillon, 1974; Watson, 1975; Robblee et al., 1979; McDaniel et al., 1981; Bornstein and Lev, 1982; Siegel and Dunnington, 1985; Hocking et al., 1989; Katanbaf et al., 1989a,b).

Broiler breeder producers, therefore, utilize feed restriction programs to control the growth of their pullets. During rearing the daily intake is severely restricted and may be reduced to one-third of the intake of ad libitum fed birds of the same age or half of the intake of ad libitum fed birds of the same weight (Savory and Kostal, 1996; De Jong et al., 2002). The daily ration during the laying period varies between 70 and 100% of the intake of ad libitum fed birds of the same age (Zuidhof et al., 1995; Bruggeman et al., 1999).

In modern breeder production facilities a skip-a-day (SK) or 5-2 program is often preferred to an every day restriction program because of the improvement in flock uniformity (Cobb-Vantress, 2005). By feeding more feed every second day, feed cleanup time is increased. The longer feed cleanup time allows for more equal distribution of feed intake and a more uniform flock. Although the differences between restricted and ad libitum fed pullets have been well documented, less information is available comparing specific feed restriction programs.

The success of feed restriction in improving breeder performance has been contrasted with concerns about numerous welfare issues such as increased stereotyped spot-pecking (Savory et al., 1992), overconsumption of water (Hocking et al., 1993), and even changes in plasma heterophil to lymphocyte ratios (Gross and Siegel, 1983; Hocking et al., 1993). Restricting feed has also been shown to change the capacity of the hen for hepatic lipogenesis (Richards et al., 2003). Broilers showed increased liver lipogenic activity when exposed to cycles of fasting and refeeding (Rosebrough et al., 1988). Programs that involve whole days without feed may lead to stress, changes in hepatic lipid metabolism, and ultimately to changes in reproductive performance.

The objective of the experiments reported in this paper was to determine what effect feed restriction programs have on broiler breeder performance and to quantify the differences in efficiency of breeders on each program. Thresholds in age, BW, and composition are important in determining the age at sexual maturity. The effect of these feeding programs on the body composition and age at sexual maturity of broiler breeders was also studied.

	MATERIALS AND METHODS

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Experiment 1
Stock and Management.
A total of 840 1-d-old Cobb 500 broiler breeder pullets were randomly assigned to twenty-four 2.38 x 1.83 m floor pens in a curtain-sided, cross-ventilated house. The 24 pen experimental units were divided into 4 treatments with 6 replicate pens of 35 pullets each per treatment. Initial stocking density was 7.8 pullets per m², but was reduced to approximately 5.4 pullets per m² because pullets were killed during the experiment for determination of body composition. The Cobb Breeder Management Guide (Cobb-Vantress, 2005) was used as a reference for all management conditions, including light schedules for dark-out rearing houses. The compositions of the diets utilized throughout the experiment are shown in Table 1. Diets did not differ among treatments. The starter diet (mash) was fed from 0 to 4 wk of age, the grower (crumble) from 4 to 18 wk of age, the prebreeder (crumble) from 18 to 22 wk of age, and the breeder I (crumble) diet from 22 to 45 wk of age. Pullets were weighed weekly in groups from 0 to 21 wk of age. Ten pullets from 4 pens from each treatment (total of 40 pullets per treatment) were weighed individually at 4, 7, 14, and 20 wk of age to obtain estimates of flock uniformity. Shank and keel lengths were measured on 20 birds per treatment at various ages using a Vernier caliper as described by Leeson and Summers (1984).

Experimental Design.
A completely randomized design was used to evaluate 4 different feed restriction programs. All pens were fed ad libitum for the first 10 d. From 10 to 28 d all pens were fed restricted amounts of feed every day. At 28 d of age, different feed restriction programs were implemented. The 4 programs were every day (ED), SK, 4-3, and 5-2. The 4-3 program provided pullets with feed for 4 d every wk. The 5-2 program provided feed on 5 d every wk. None of the programs ever involved skipping more than 1 d at a time. All treatment groups were fed exactly the same amount of feed per pullet regardless of restriction program. For example, if the ED group received 50 g, the SK group would receive 100 g every other day. Feed allocation at each feeding for the 4-3 and 5-2 groups was determined by multiplying the daily feed allowance from the management guide by 7, and then dividing by 4 and 5, respectively. The target BW set out in the breeder guide were used to determine feed allocation to the ED group. All other treatment groups were then fed amounts that gave weekly feed intakes identical to the ED group. Restriction programs were continued through 5% egg production after which time all pullets were fed every day. The feed allocation after housing (21 wk) was the same for all treatments (Table 2). Maximum feed allocation was 144 g per bird, which was 420 kcal of ME/hen per d. This caloric intake is lower than recommended but was used to account for the reduced energy expenditure as a result of being housed in individual cages (M. Reyes and C. N. Coon, University of Arkansas, Fayetteville, unpublished data). Feed withdrawal began at wk 32 and continued until the end of the experiment (wk 45), at which time breeders were being fed 133 g per bird per d. Mortality was recorded on a daily basis throughout the experimental period.

All hens were artificially inseminated at wk 32, 36, 40, and 44. One week’s worth of eggs was collected from each hen to determine fertility and hatchability at each interval. Semen was collected from same age, separately reared broiler breeder males using the abdominal massage method as described by Burrows and Quinn (1937). Semen was pooled and sperm cell concentration determined using an IMV MicroReader I (IMV Technologies, Minneapolis, MN) using an optical density of 381 nm (King and Donoghue, 2000). Semen was diluted to 5 x 10⁷ sperm/50 µL using Beltsville Poultry Semen Extender, Continental Plastic Corp., Delavan, WI) to ensure all hens were inseminated with the same number and volume of sperm cells. Each hen was inseminated with 50 µL of diluted semen. Semen was diluted prior to insemination to allow detection of variation in fertility levels. Although this low number of sperm cells does not produce exceptionally high fertility levels, by not filling the sperm host glands in the hen it allows for differences among treatments to be determined. All eggs were collected for 1 wk after each insemination and set in Jamesway machines (Jamesway Incubator Company Ltd., Cambridge, Ontario, Canada) for incubation and hatching. All unhatched eggs were broken out to determine fertility status. Fertility was calculated as the number of fertile eggs per 100 eggs set. Hatchability was calculated as the number of chicks hatched per 100 eggs set, and hatchability of fertile eggs was calculated as the number of chicks hatched per 100 fertile eggs set.

Carcass Composition.
To determine the effect of different feed restriction programs on carcass composition, 10 randomly selected pullets per treatment were killed by CO₂ asphyxiation at 4, 7, 14, 20, 22, 27, and 40 wk of age. Each breeder carcass was frozen at –20°C before autoclaving. The carcasses were placed in trays, covered with foil and autoclaved at 120°C for 15 h in an AMSCO 3053 sterilizer (Steris Corporation, Mentor, OH). The carcasses were homogenized after autoclaving using a Waring 4L blender (Waring products division, Dynamics Corporation of America, New Hartford, CT). Subsamples were collected after grinding and lyophilized in a Genesis SQ 12 EL Freeze drier (The Virtis Company, Gardiner, NY). After freeze drying, samples were finely ground, and carcass protein (Kjeldahl N), ash, and fat (ether extract) were analyzed according to AOAC (1990). Dry matter was determined as a percentage of total wet carcass weight. The percentage of carcass protein, ash, and fat were reported on a DM basis. Dry BW was calculated by multiplying the proportion of DM by the total wet carcass weight. Total carcass protein, ash, and fat were calculated by multiplying the proportion of each component by the dry BW. Compositions were expressed as percentages of DM and as total mass (g). The total mass of fat, protein, and ash was obtained by multiplying the proportion of each component in the dry carcass by the total dry carcass mass (g).

Experiment 2
Stock and Management.
A total of 700 1-d-old Cobb 500 broiler breeder pullets were randomly assigned to twenty 2.38 x 1.83 m floor pens. The 20 pen experimental units were divided into 4 treatments with 5 replicate pens of 35 pullets each per treatment. All general management procedures including stocking density, lighting programs, and diets (Table 1) were the same as for experiment 1.

Experimental Design.
A completely randomized design was utilized as in experiment 1. In experiment 2 the same feed restriction programs were used, but all pullets were fed to reach equal BW. It was found in experiment 1 that pullets fed using SK, 4-3, and 5-2 programs grew less efficiently than ED pullets when given equal feed allocations. Pullets on the SK, 4-3, and 5-2 programs required more feed to reach the same BW as ED pullets. Feed allocations for each treatment were adjusted weekly according to bird BW, with the Cobb Breeder Management Guide (Cobb-Vantress, 2005) recommendations serving as BW targets. This approach was taken to eliminate BW as a variable that could be responsible for differences observed in breeder performance. Feed allocations after housing for experiment 2 are shown in Table 2. Maximum feed allocation and feed withdrawal were the same as for experiment 1. All weighing and measurement of performance parameters that were conducted in experiment 1 were conducted in the same way in experiment 2.

Statistical Analysis.
Statistical analysis procedures were the same for both experiments. Data analysis was performed using JMP IN 5.1 (SAS Inst. Inc., Cary, NC) statistical analysis software. Chicks were assigned to treatments on d 1 in a completely random manner. All data were analyzed based on a completely randomized design using 1-way ANOVA. From the beginning of the trial until 21 wk of age, the pen was treated as the experimental unit. After 21 wk, birds were individually caged and each bird served as an experimental unit. Data are presented as mean ± SEM. When significant treatment effects were observed, means were separated using Tukey’s Studentized range test. All statements of significance are based on testing at P 0.05.

Animal Use.
All procedures were carried out in accordance with Animal Use Protocol No. 03008 for the experiment, which was approved by the University of Arkansas Institutional Animal Care and Use Committee.

	RESULTS

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Experiment 1
Body weights and CV from experiment 1 are shown in Table 3. Different programs were implemented at 4 wk of age, and by 7 wk the ED pullets were heavier than SK pullets but not 4-3 or 5-2 pullets. By 20 wk, ED pullets were significantly heavier than all other groups. With equal feed intakes, the ED pullets were approximately 10% heavier than SK pullets at 22 wk of age. Differences in BW between ED and SK pullets remained throughout the production period, although 5-2 and 4-3 groups were no longer significantly smaller than ED by 27 and 30 wk of age, respectively.

Shank and keel lengths were measured as an indication of frame size (Table 4). Shank length did not differ among feeding regimens at any age. Keel length was greater in ED than SK and 4-3 birds at 10 and 20 wk. By 28 wk, the differences in keel length were no longer significant.

Relative EW was calculated as the ratio of mean EW to housing BW. Breeders from the SK, 4-3, and 5-2 groups had lower BW at housing than ED and yet produced slightly larger eggs. Relative EW was significantly higher in SK (2.53%) than in 4-3 (2.41%) or 5-2 (2.37%) breeders, which were in turn higher than ED (2.23%). Fertility (Table 5) and hatchability (data not presented) did not differ among treatments, and no differences were found in the hatchability of fertile eggs.

Efficiency of feed utilization for growth was affected by feeding regimens (Table 6). Using an ED program resulted in a 10% improvement in feed conversion ratio (FCR) compared with a SK program. The efficiency of protein and energy utilization for growth follows the same pattern as for FCR. Total feed, protein, and energy intake per egg were greater for SK than for ED at 45 wk of age. Feed, protein, and energy intake per egg of 4-3 and 5-2 hens did not differ from ED or SK hens.

The percentage of protein in the dry carcass did not differ among groups during rearing or at 27 wk of age. The total carcass protein was consistently greater for ED than for SK, 4-3, or 5-2 groups due to differences BW at each age when samples were taken. By 27 wk of age the differences were no longer significant. At 40 wk of age, the 4-3 group had a higher proportion of carcass protein than the 5-2 group. The ED and SK groups were intermediate and did not differ from the other 2 groups. Despite the difference in protein percentage, total protein content was not different among groups at 40 wk of age.

During rearing carcass ash percentage differed only at 20 wk, at which time the 4-3 birds had lower ash content than 5-2 birds but did not differ from birds in the ED or SK groups. At 27 wk of age, SK birds had both higher ash percentage and total ash than all other groups. By 40 wk, no differences existed in ash content among groups.

No differences were observed in mortality at any age (data not presented).

Experiment 2
Body weights and CV from experiment 2 are shown in Table 8. No difference existed in BW among groups at any age during the rearing and production phases. Feed allocations were adjusted weekly after weighing the hens in order to follow the recommended target BW (Cobb-Vantress, 2005). The FCR was better in ED pullets than SK, 4-3, and 5-2 pullets.

Frame size measurements for experiment 2 are shown in Table 9. No differences in keel or shank length were found among feeding regimens at any age. The effects of different feed restriction programs on performance of broiler breeders are shown in Table 10. With equal BW, age at sexual maturity was not significantly different in ED, SK, or 5-2 treatments. The 4-3 birds took longer to reach sexual maturity. As in experiment 1, ED birds reached SM (177.5 d) first. Total egg production was lower in 4-3 breeders than in ED, but egg production did not differ among other groups. The results for settable egg production are very similar to those for total egg production because the number of abnormal eggs did not differ among feeding regimens.

The differences in efficiency of feed utilization for experiment 2 are shown in Table 11. It took more feed for SK, 4-3, and 5-2 pullets to reach equal BW at 21 wk than it did for ED pullets. The same trend occurred for protein and energy utilization to 21 wk. In experiment 2, 4-3 hens were significantly less efficient in terms of feed, protein, and energy required per egg than ED.

	DISCUSSION

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The difference in efficiency of feed utilization may result from the constant cycles of feeding and fasting that occur in SK, 4-3, and 5-2 regimens. Postprandially, the bird must deposit nutrients in the body, only to remobilize those nutrients to sustain itself during the postabsorptive period. This deposition and mobilization is not a perfectly efficient process, and as such the bird requires more feed to reach the same BW as ED birds. In the first trial the differences in BW between ED and SK hens remained throughout the recorded production period up to 40 wk. These results are similar to those of Leeson and Summers (1985) who also found that BW remained lower in birds reared on SK compared with ED programs with equal intakes. Our data demonstrate that modern broiler breeders are less efficient if fed using SK-type programs.

Programs such as SK are often implemented to improve flock uniformity. The use of SK feeding is particularly beneficial when feeder space is limited, providing feed for longer periods than ED programs, allowing more timid birds at the lower end of the peck order to satisfy their nutritional needs (Cobb-Vantress, 2005). Improvements in uniformity have been reported when using SK rather than ED (Bartov et al., 1988). Bennett and Leeson (1989) reported uniformity as the percentage of birds in a pen with BW within ±15% of the pen mean. These researchers found that SK pullets were consistently more uniform than ED pullets but that the differences were not significant. Using SK programs during periods of severe feed restriction does improve flock uniformity. A uniform flock helps to harmonize the onset of SM, creates good peaks, and enables producers to more accurately meet the flock’s nutrient requirements.

The differences in frame size among the feeding regimens mirror the differences in BW. These results are in close agreement with those of Leeson and Summers (1985), who found that keel length for ED birds was significantly longer than SK birds. These researchers also found that shank length was consistently longer for ED birds but that the differences were not always significant. The differences observed in experiment 1 are due to BW differences because the 4-3 and 5-2 regimens resulted in intermediate shank and keel lengths. In experiment 2 when BW was similar among all groups, frame size did not differ at any age. This finding supports the assumption that the differences in keel length observed in experiment 1 were due only to differences in BW.

The delay in SM in SK birds in experiment 1 was reduced in experiment 2, indicating the importance of BW as a determinant of onset of SM. Hocking (2004) reported that as BW increased, the age at SM decreased in a curvilinear fashion. Wilson et al. (1989) previously reported that age at SM, which they defined as 50% production, was delayed in SK breeders compared with ED breeders. Katanbaf et al. (1989b) found that SM was delayed by 5 d in SK birds compared with ED birds when both received equal amounts of feed. Although their findings were not statistically significant, they do agree with ours and those of Wilson et al. (1989). Part of the reason for earlier SM in ED breeders in experiment 1 was their higher BW due to their more efficient use of feed during the rearing period. It is not immediately obvious why 4-3 birds took longer than other groups to reach SM in experiment 2 because BW did not differ among treatments at any age. Differences in carcass protein mass could potentially explain the delay in SM in these birds. The data of Wilson et al. (1989) indicate that BW is not the only factor affecting SM. They found in 2 separate experiments that even though BW did not differ at 24 wk of age, pullets restricted using ED programs from 2 wk of age reached SM earlier than pullets restricted using SK from 8 wk of age. Although BW is certainly an important factor in controlling SM, other factors are clearly involved.

In experiment 1, and to a lesser extent in experiment 2, the peak in egg production occurred at a younger age in ED hens than in other groups. Katanbaf et al. (1989b) found that ED pullets were about 10 d ahead of SK pullets in terms of percentage of hen housed egg production. They also noted that ED breeders peaked in egg production earlier than SK breeders but that the peak was similar. Apart from SK in the first experiment, and 4-3 in the second, peak was very similar among groups, supporting the results of Katanbaf et al., (1989b). In both of the experiments reported in this study, ED hens produced about 4 more eggs than SK, although differences were nonsignificant in experiment 2. Wilson et al., (1989) reported that egg production was lower for birds fed using SK programs from 8 wk of age compared with birds fed restricted amounts every day from 2 wk of age. In their work, BW did not differ between the 2 groups.

Data showed no differences in the production of abnormal eggs among the 4 feeding programs in either experiment reported here. Katanbaf et al. (1989b) also found that abnormal egg production did not differ among hens reared on an ED or SK program.

In both experiments hens reared on 5-2 regimens produced larger eggs than ED hens. The reason for the larger eggs in the 5-2 group is not clear. In experiment 1, the 5-2 pullets weighed 129 g less than ED at 22 wk, and 101 g less at 27 wk, but still produced larger eggs. Hens from the 4-3 group produced the second largest size eggs in both experiments. In experiment 1, hens from the SK group weighed 294 g less than ED at 22 wk and 240 g less at 27 wk but still produced eggs of equal size. Wilson et al. (1989) found that breeders fed SK from 8 wk of age produced significantly larger eggs than breeders fed ED from 2 wk of age. They also found a nonsignificant increase of 0.3 g in EW in breeders fed SK from 2 wk of age compared with ED-fed breeders. This increase was found in spite of the fact that ED hens weighed 125 g more at housing and 97 g more at SM. Leeson and Summers (1985) reported that EW was 0.3 g greater in SK birds than in ED, even though BW at 20 wk was 100 g greater in ED birds.

The relative EW data reported here highlights the fact that groups that were fed on SK, 4-3, and 5-2 programs produced larger eggs than ED, relative to BW. The increase in EW relative to housing BW seen in SK, 4-3, and 5-2 groups may be partly due to the increased length of time it took those hens to reach SM after photostimulation. The differences in BW at housing in experiment 1 continued into the production phase and by 40 wk SK still had lower BW than ED. The differences in relative EW in experiment 2 were smaller than those observed in experiment 1, suggesting that the lower BW and more delayed SM of the SK, 4-3, and 5-2 groups were partly responsible for the differences in the first experiment. It is interesting, however, that the differences are not completely eliminated when all groups are reared to have equal BW suggesting that other factors are contributing to the differences in relative EW.

In both experiments conducted here, no differences were observed in fertility and hatchability between ED and SK breeders. These findings are in agreement with data found in previously published literature (Leeson and Summers, 1985; Katanbaf et al., 1989b; Wilson et al., 1989).

Feeding regimens can alter body composition. In experiment 2, a larger part of the extra feed given to the pullets on the SK, 4-3, and 5-2 regimens to ensure equal BW was deposited as fat rather than as protein. The body composition data from wk 20 would support this claim as the ED group had the highest protein content and the lowest fat content among the treatments. It has been shown (Rosebrough et al., 1988) that intermittent feeding of broiler chickens results in an increase in lipogenic activity of the liver. The programs used here that involved off-feed days provide just such a scenario. The fasting period in SK, 4-3, and 5-2 feeding programs is followed by allocation of relatively large amounts of feed. Upon consumption of this feed there is likely an increase in hepatic lipogenesis and possibly the deposition of more fat in the carcass. In experiment 1, these differences in carcass lipid percentage were not observed, perhaps due to less feed allocation for SK, 4-3, and 5-2 groups. In fact, due to lower BW, these groups had lower total carcass fat than ED birds. The effects of excessive fat accumulation in broiler breeders have been well documented (Pym and Dillon, 1974; Pearson and Herron, 1980; McDaniel et al., 1981; Siegel and Dunnington, 1985). It is unclear whether the fat accumulation that occurred in 4-3 pullets prior to SM affected performance.

Many authors have shown the importance of chronological age, BW, and carcass fat content (Brody et al., 1980; Dunnington et al., 1983; Leeson and Summers, 1983; Bornstein et al., 1984; Soller et al., 1984; Zelenka et al., 1986) as thresholds for development of SM. In experiment 2, 4-3 pullets reached SM at a later age than the other groups, with ED being the quickest to reach SM. There were no differences in BW among the groups, and carcass fat was actually highest in 4-3 pullets at 22 wk of age. Protein percentage and total protein content, however, were lowest in 4-3 pullets at 22 wk of age. This finding suggests that there may, in fact, be a threshold for total body protein before SM can be reached. These results are in close agreement with those of Sun et al. (2006), whose research showed that total carcass protein for ad libitum and feed-restricted breeders was very similar at first egg, even though age at first egg was significantly different, indicating the importance of protein mass needed for sexual maturity. Closer examination of the body composition data from experiment 1 reveals that total carcass protein content is greatest in ED followed by 5-2, SK, and 4-3. This closely matches the order in which pullets reached SM. Differences in total protein were small between SK and 4-3, and so were the differences in age at SM. Carcass fat did not appear to be the limiting threshold for SM in either experiment. Soller et al. (1984) noted that broiler breeder pullets entered lay at the same lean BW, percentage carcass ash, and percentage carcass protein content regardless of the degree of feed restriction during rearing. Their pullets reached SM at different ages, carcass weights, carcass dry matters, and carcass fat contents.

The results from these 2 experiments reinforce the importance of BW (Luther et al., 1976) in determining the performance of broiler breeders. Some of the performance differences observed in experiment 1 were attenuated when BW was equalized in experiment 2. The fact that the differences were not completely eliminated, however, suggests that body composition and other metabolic factors are also very important. By allocating more feed to SK, 4-3, and 5-2 birds to achieve similar BW to ED birds, significant changes in body composition occurred. Though diet composition was identical for all groups, those whose feeding regimens included off-feed days tended to accumulate more fat and less protein when fed to reach equal BW. This is likely the result of increased hepatic lipogenesis. Nutritional alteration of lipogenesis in birds is achieved primarily by fasting and refeeding or by altering energy-protein ratios in the diet (Yeh and Leveille, 1969; Rosebrough, 2000). Clearly the energy-protein ratio of the diet did not differ, but there were continuous cycles of fasting and refeeding in SK, 4-3, and 5-2 birds, which resulted in conversion of dietary carbohydrate to lipid and deposition of such lipid in the carcass. Although some of the lipid is mobilized during the fasting period to satisfy the energy requirements of the pullets, some is stored in the carcass.

Changes in lipogenesis, gluconeogenesis, and most likely, certain plasma hormone profiles could be responsible for differences in performance between ED and SK fed hens. Feeding programs that include off-feed days generally delayed the onset of SM and resulted in larger, but fewer total and settable eggs. The efficiency of feed utilization for growth and egg production is compromised by using SK regimens rather than ED. These differences are due to the inefficiency associated with repeated mobilization of stored nutrients during the fasting periods associated with SK programs. The benefits of improved uniformity associated with SK programs must be carefully weighed against the potential savings in feed cost and possible improvements in performance attainable by using ED programs. These experiments also provided further evidence that lean protein mass is an important determinant of age at sexual maturity.

	ACKNOWLEDGMENTS

 
The authors would like to thank Cobb-Vantress Inc., for donating chicks and for financial support for these experiments. The authors would also like to thank the staff at the University of Arkansas Poultry Research Farm for assistance during the trials.

Received for publication December 11, 2006. Accepted for publication April 26, 2007.

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