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PHYSIOLOGY, ENDOCRINOLOGY, AND REPRODUCTION |
Department of Poultry Science, University of Georgia, Athens 30602-2772
2 Corresponding author: ajdavis{at}uga.edu
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Key Words: broiler breeder • skip-a-day feeding • every-day feeding
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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The degree to which feed is restricted during the rearing of broiler breeders has led the poultry industry in the United States to commonly use a skip-a-day (SAD) feeding program during rearing. Often pullets are maintained on an SAD feeding program after they have been photostimulated for reproduction, and in some cases, they are not switched to an every-day (ED) feeding program until total egg production for the flock has reached 5% (North, 1984). However, providing pullets with 2 d worth of feed every other day after photostimulation for reproduction may be detrimental to normal ovarian development and future egg production because the pullets will be exposed to a substantial fasting period between each feeding. de Beer et al. (2008) reported that the difference in the length of the fasting period associated with feeding broiler breeder pullets on an SAD or ED basis resulted in different plasma hormone profiles. At 16 wk of age, pullets on the SAD treatment had lower plasma leptin and insulin-like growth factor-I levels than pullets on the ED treatment and had greater plasma triiodothyronine and corticosterone levels than the ED-fed pullets (de Beer et al., 2008). In addition, previous research indicates plasma luteinizing hormone concentrations decline significantly in cockerels fasted for 12 h (Scanes et al., 1976) and that fasting also significantly lowers plasma concentrations of luteinizing hormone after 48 h and estradiol and progesterone concentrations after 24 h in laying hens (Tanabe et al., 1981). Therefore, the goal of the current research was to determine whether feeding broiler breeder hens on an SAD basis after photostimulation until they reached 5% egg production would be detrimental to overall egg production through 65 wk of age.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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All animal procedures were approved by the Animal Care and Use Committee at the University of Georgia. At 1 d of age, 1,300 Cobb 500 slow-feathering pullets were randomly divided among 5 rooms, and 300 Cobb cockerels were placed in a separate room. The rooms measured 7.32 x 9.14 m and had pine shavings for litter. The rooms were environmentally controlled, with the temperature maintained at 32.2°C for the first week and then decreased by approximately 2.8°C every week thereafter until the target temperature of 21°C was reached. From 1 to 3 d of age, the chicks were given 24 h of light per day, and then from 4 to 14 d of age, the amount of light was decreased from 24 to 8 h per day. The 8 h per day lighting schedule was then maintained until the birds reached 21 wk of age. All birds were fed a standard corn-soy diet (Table 1
) ad libitum from 0 to 2 wk of age and then a developer diet (Table 1) from 2 to 23 wk of age. From 2 to 21 wk of age, the birds were feed restricted and fed on an SAD basis. Feed was distributed by automatic chain feeders, and the birds were given ad libitum access to water from nipple drinkers. Every week, the birds in each room were penned off, and 50 birds per room were randomly selected and individually weighed. The weights were then used to determine feed allocations so that BW gain of the pullets and cockerels matched the recommended guidelines of the primary breeder. All pullets were wing-banded for identification purposes.
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At 20 wk of age, all pullets were individually weighed. The pullets were matched by weight into 35 categories for placement into the laying pens. To ensure that the weight distribution in each pen was similar, a pullet from each of the 35 weight categories was then randomly selected and distributed to a laying pen. There were 30 laying pens, and each contained 35 pullets and 4 roosters. Each pen measured 3.65 x 2.75 m, and the floor space of each pen consisted of two-thirds pine shavings litter and one-third elevated slats. Each pen had one 6-hole nest box located on the slatted area and was equipped with 10 nipple drinkers. In the laying pens, the hens and roosters were hand-fed with plastic feeder pans. Each pen contained 3 hen feeder pans that were fitted with rooster exclusion grills. The feeding system provided 9.14 cm of feeder space per hen. Males were given their own feeder pan, which was elevated in height to prevent females from consuming their feed. Each rooster had 25.9 cm of feeder space. The male-to-female ratio was kept between 10 and 11% throughout the experiment by replacing dead males from a pool of extra males. Beginning at 42 wk of age, 1 male from each pen was rotated to a neighboring pen and replaced by a male from a neighboring pen every 4 wk to sustain fertility because no younger males were available to add to the pens to boost fertility.
Photostimulation occurred at 21 wk of age by providing 14 h of light (lights on at 0600 h), and this photoperiod was maintained until the end of the experiment (66 wk of age). At 21 wk of age, the birds in half of the 30 replicate pens were switched from SAD feeding to ED feeding, whereas the remaining birds in the other 15 replicate pens were maintained on SAD feeding. Hens in all 30 pens continued to be fed the same amount of feed over a 48-h period. The ED-fed hens were given half of the SAD feed amount on a daily basis. The goal was to switch the SAD-fed hens to a daily feeding at approximately 5% egg production. It is a common industry practice in the United States to maintain maturing pullets on an SAD feeding program until the flock reaches approximately 5% egg production to diminish the effects of limited feeder space or to manage maturing pullets that have fast feed consumption patterns (North, 1984). Because of the rapid increase in egg production, the SAD-fed hens in the current experiment were switched to ED feeding at 8% egg production (26.5 wk of age). From 26.5 to 65 wk of age, the hens from the ED and SAD treatments were fed the same daily feed amount. The amount of feed provided to the birds during the breeding period was based on BW and egg production, as suggested by the guidelines of the primary breeder. As egg production became increasingly different between the hens in the ED and SAD treatments during the last 13 wk of the experiment, the amount of feed provided to the hens in both treatments was based entirely on the BW and egg production of the ED treatment hens to support their greater egg production.
The pens housing the SAD- and ED-fed hens were in separate identical rooms that were situated side-by-side in the same building. This prevented hens in the SAD treatment from seeing the hens in the ED treatment being fed on the days they were not fed. In addition, because the birds were hand-fed, it is unlikely that the hens in the SAD treatment heard the hens in the ED treatment being fed. Hens and roosters were fed at 0630 h. At 25 wk of age, hens and roosters were changed from the developer diet to a broiler breeder laying diet (Table 1
). The temperature of the rooms was recorded 2 times per day and was similar throughout the experiment. All mortalities were recorded and necropsies were performed on all hen mortalities after 23 wk of age.
Roosters and hens were weighed weekly from 22 to 40 wk of age and every 2 wk from 42 to 64 wk of age. For each weigh period, the hens and roosters in 5 of the 15 pens per treatment were individually weighed. The pens were in weigh groups that were consistent for the duration of the study, which allowed individual pens to be weighed every third weigh period. The birds in only 5 of the 15 pens per treatment were weighed each week on a rotating basis to minimize the stress associated with penning and individually weighing each bird every week. However, all birds were individually weighed at the beginning of the experiment, the week the SAD-fed hens were placed on ED feeding, near the middle of the experiment, and at the end of the experiment (20, 26, 39, and 64 wk of age, respectively). Weighing all the birds in each pen allowed for the accurate determination of BW uniformity based on all 15 pens per treatment. Birds were weighed before their morning feeding, which delayed feeding by 1 h on each weigh day. For the week that all the birds were weighed, multiple weigh teams were used to keep the delay in feeding time consistent. Eggs were manually collected 3 to 4 times per day, and egg production was calculated weekly from daily egg counts. At the time of egg collection, all eggs were classified as normal (hatching eggs), cracked, double-yolked, misshapen, membrane, or dirty.
All hatching eggs from a single day of production per pen were weighed each week when the hens were 26 to 32 wk of age, biweekly from 32 to 42 wk of age, and monthly from 44 to 56 wk of age. Ninety hatching eggs from each pen were incubated (Natureform Hatchery Systems, Jacksonville, FL) every other week when the hens were 28 to 44 wk of age and then every 4 wk thereafter until 64 wk of age. Eggs were collected and stored between 18.3 and 19.9°C for up to 7 d before each incubation period. Eggs were candled on d 12 of incubation, transferred for hatching on d 19 of incubation, and hatched on d 21 of incubation. Eggs were incubated at 37.8°C with 53% RH from d 0 to 18, and then at 37.2°C with 70% RH from d 19 to 21. During candling and transfer and after hatching, eggs were characterized as being infertile, cracked, contaminated, or containing early dead embryos (less than 7 d), mid-dead embryos (7 to 14 d), or late-dead embryos (15 to 21 d). Eggs that were cracked during transfer were removed from the data set. After hatching, the number of live and dead-in-shell and live and dead hatched chicks were determined.
Steroid Hormone RIA
Beginning at 26.5 wk of age, blood samples were collected every 4 wk from 25 hens from 1 pen of each treatment. The pen representing each treatment was initially chosen based on the hens in that pen having the median egg production rate in comparison with the hens in the other 14 pens of each treatment. Egg production for both of these pens relative to the other pens in each treatment remained near the median throughout the experiment. Blood samples were collected from the same hens until the conclusion of the experiment. Blood samples were collected between 1300 and 1430 h at each collection date. Blood was collected from the brachial vein, immediately placed into individual glass Vacutainer tubes (Becton, Dickinson, and Co., Franklin Lakes, NJ) containing EDTA as an anticoagulant, and stored on ice. Samples were centrifuged at 1,000 x g at 4°C for 10 min. Plasma was collected from each sample and frozen at –180°C. Plasma progesterone and estradiol concentrations were determined by RIA using the Coat-A-Count Progesterone Kit and the Coat-A-Count Estradiol Kit (Diagnostic Products Corporation, Los Angeles, CA) following the manufacturer’s protocol. The RIA samples were counted with a Wallac Wizard 1470 gamma counter (Perkin-Elmer, Waltham, MA). The mean interassay and intraassay CV for both assays were less than 10%.
Statistical Analyses
One-way ANOVA was used to detect significant weekly or overall experimental period differences between the SAD and ED treatments (Neter et al., 1990). The weekly values are means of the 15 pens per treatment (n = 15) or of the 25 hens per treatment (n = 25, plasma estradiol or progesterone). The overall experimental period values are means of the 15 pens or 25 birds per treatment with the value for each pen or bird obtained by averaging the values collected over the duration of the experiment for each pen or bird. All statistical procedures were done with Minitab Statistical Software (Release 13, Minitab Inc., State College, PA). Differences were considered significant when P-values were <0.05.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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). At 20 wk of age just 4 d before photostimulation, and at 26 wk of age just 3 d before the SAD-fed hens were placed on ED feeding, the BW of the hens from the 2 treatments were the same (Table 2). The CV for BW was not significantly different between the 2 treatments when all the hens were weighed at 20, 26, 39, and 64 wk of age (Table 2).
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). Weekly hen-day egg production was significantly greater for the ED-fed hens throughout the experiment, except when the hens were 29, 30, 35, 37, 39, 46, and 47 wk of age (Figure 2). For the ED-fed hens after photostimulation, hen-day egg production peaked at 72.3% at 31 wk of age, whereas for the SAD-fed hens, egg production peaked at 67.1% at wk 30 of age. For the entire production period, the overall percentage of hen-day egg production was greater (55% vs. 49%) for the ED-fed hens compared with the SAD-fed hens. Through 65 wk of age, total hen-day egg production was also greater (172 vs. 155 eggs/hen, P < 0.001) for the hens in the ED treatment compared with those in the SAD treatment.
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).
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). The decrease in hatching egg production by the ED-fed hens was associated with a greater production of dirty and abnormally shaped eggs by these hens compared with the SAD-fed hens (Table 4). The ED-fed hens produced a greater number of dirty eggs than the SAD-fed hens for wk 25, 26, 58, and 65 and a greater number of abnormally shaped eggs than the SAD-fed hens on wk 25, 43, 63, and 65 (data not shown).
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). However, the percentages for hatchability and hatch of fertile eggs were greater for eggs produced by hens fed on an ED basis after photostimulation (Table 5). The greater rate of hatch of fertile eggs for the ED-fed hens was due in part to lesser amounts of early- and late-dead embryos as well as live and dead-in-shell chicks in the eggs produced by these hens compared with those produced by the SAD-fed hens (Table 5).
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). The plasma estrogen concentration of SAD-fed hens was greater than that of the ED-fed hens at 30.5 and 62.5 wk of age (Table 6). Overall (26.5 to 62.5 wk of age), the plasma concentration of estrogen was greater, whereas the plasma concentration of progesterone was lower in SAD-fed hens compared with ED-fed hens (Table 6).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Plasma concentrations and temporal profiles of estradiol and progesterone during the laying period were similar to those reported previously (Renema et al., 1999; Onagbesan, et al., 2006) for broiler breeder hens. Onagbesan et al. (2006) reported that although there was a significant relationship between peak plasma estradiol concentration and egg production, plasma concentrations of progesterone and estradiol before and after peak egg production were not correlated with subsequent egg production levels. In the current research, the elevated plasma progesterone levels through peak egg production in the ED-fed hens compared with hens that had been fed on an SAD basis were associated with the hens initiating lay earlier and producing more eggs. Furthermore, the plasma concentrations of progesterone for the ED-fed hens were numerically greater than those of the SAD-fed hens throughout the experiment; thus, the overall plasma concentration of progesterone was significantly greater for the ED-fed hens than for the SAD-fed hens. In contrast, the estrogen concentration was significantly greater just before peak egg production for hens that had been fed on an SAD basis compared with hens that had been fed on an ED basis. The overall plasma concentration of estrogen from 26.5 to 62.5 wk of age was also significantly greater for the SAD-fed hens compared with the ED-fed hens. The differences in ovarian hormone concentrations between the hens from the 2 treatment groups imply that SAD feeding after photostimulation for reproduction permanently altered ovarian development and function. These differences in plasma steroid hormone concentrations also indicate the need to better define the comparative endocrine relationships between ED-fed and SAD-fed hens in the early lay period by collecting more frequent samples starting at photostimulation and by broadening the scope of the plasma hormones and sex hormone binding proteins measured.
Another indication that ovarian development was potentially dissimilar between the hens fed on an ED basis and those fed on an SAD basis until 5% egg production was reached came from their early egg production profiles. The onset of egg production was delayed in the SAD-fed hens, yet the hens peaked in egg production at an earlier age than did the ED-fed hens. The rapid increase in egg production is also why the targeted production rate of 5% for the switch to ED feeding for the SAD-fed hens occurred instead at 8%. One important factor that determines the onset of lay in broiler breeder hens is BW (Hocking, 2004); however, the birds in this experiment did not differ in their BW profile through 31 wk of age except for wk 23 of age, when the SAD-fed hens weighed slightly more than the ED-fed hens, and wk 24 of age, when the ED-fed hens weighed slightly more than the SAD-fed hens. Thus, more subtle factors such as carcass composition (Melnychuk et al., 2004, Katanbaf et al., 1989b), differences in metabolic hormones that are affecting the reproductive axis, or both could be responsible for the differences in egg production. Previous research (de Beer et al., 2007) based on feeding broiler breeder pullets either on an SAD or ED basis from 4 to 16 wk of age indicated that the extended fasting periods associated with SAD feeding resulted in substantial differences in the mRNA expression profiles of some lipogenic enzymes in the 24 h after the birds in each feeding regimen received feed. In addition, de Beer et al. (2008) reported that at 16 wk of age, the ED-fed pullets had greater plasma leptin and insulin-like growth factor-I levels than did the SAD-fed pullets and had lower plasma triiodothyronine and corticosterone levels than did the SAD-fed pullets. If the differences in lipogenic enzyme expression and metabolic hormone levels detected by de Beer et al. (2007, 2008) were present in the pullets in the current research until the SAD-fed hens were switched to ED feeding at 26.5 wk of age, it could help explain why ovarian development and subsequent egg production differed between the SAD- and ED-fed hens. Although total nutrient intake was the same between the SAD- and ED-fed hens, the metabolic perturbations that would be occurring between meals for the SAD-fed pullets could have signaled the reproductive axis that nutrient intake was not adequate to fully support reproduction. Thus, in spite of having the total nutrient intake to fully support the demands associated with the onset of reproduction, the SAD-fed hens failed to match the ovarian development and maturation of the ED-fed hens because the hormone profile that develops during each fasting period of an SAD feeding program blunts ovarian development and subsequent function.
The reason that SAD feeding of broiler breeder hens is often continued in the United States beyond photo-stimulation for reproduction is the perception that it will help control flock BW uniformity. However, our results contradict this theory. All of the hens were weighed just before hens in the SAD treatment were switched to ED feeding at 26.5 wk of age. At this time, the CV of BW for the SAD-fed hens was 10.39%, and it was 10.36% (Table 2) for ED-fed hens.
At the end of the experiment, the SAD-fed hens weighed 98 g more than did the ED-fed hens. The weight gain of the SAD-fed hens was likely a reflection of their lower egg production rates compared with the ED-fed hens. We chose not to decrease the feed allotment of the SAD-fed hens compared with the ED-fed hens to maintain the equivalent total feed allowance between the 2 treatments throughout the experiment. It is possible that by not lowering the feed intake of the SAD-fed hens to reflect their lesser egg production, we potentially contributed to their weight gain and negatively affected their egg production. However, we do not feel this is necessarily the case because previous research indicates that overfeeding broiler breeder hens in late production at 54 wk of age does not alter ovarian morphology or egg production (McGovern et al., 1997). Hocking et al. (2002) also reported that ad libitum feeding after postpeak egg production had little effect on egg production in hens that had been conventionally restricted and fed diets containing a standard level of protein, similar to the birds in the present research. However, ad libitum feeding after peak production did lessen egg production in hens that had been fed on a modified restriction program or hens that were fed a low-protein diet (Hocking et al., 2002). Therefore, we would suggest that the SAD-fed hens produced fewer eggs than the ED-fed hens at the end of the experiment, not because of overfeeding, but rather as a result of their feeding program from 21 to 26.5 wk of age. As discussed earlier, the fasting period that occurred in the SAD-fed hens between each feeding from photostimulation until 8.5% egg production negatively affected ovarian development in these hens compared with the ED-fed hens, as indicated by their delayed onset of egg production and their plasma progesterone and estrogen profiles. As a consequence of the differences in ovarian development after photostimulation, the SAD-fed hens may not have been able to persist in lay as long as the ED-fed hens, as indicated by the differences in egg production between the SAD- and ED-fed hens as the experiment progressed to its conclusion (Figure 2). Further research is needed to investigate this possibility further.
The reduction in the hatch of fertile eggs produced by the SAD hens compared with the eggs produced by the ED hens probably resulted in part from their lower egg production rate. Because the SAD hens had lower rates of lay, they likely had reduced egg-laying sequence lengths and thus produced more first-sequence eggs than did the ED-fed hens. First-sequence eggs have a greater rate of embryonic mortality (Robinson et al., 1991b).
Peak egg production for the ED hens was 72.3%, which was comparable to the 74.8% value obtained in previous research using the same number of Cobb 500 slow-feathering hens in the same facility as that used in the current research (Spradley et al., 2008). The mortality for the SAD- and ED-fed hens during the current research was also comparable to the 17.1% mortality reported for commercial practice in the United States during the breeding period (Agri Stats, 2007) and to the level of mortality experienced in our previous research using Cobb slow-feathering hens (Spradley et al., 2008). The mortality rate in the current research was actually lower than we experienced in a previous experiment with caged Cobb slow-feathering hens (Hudson et al., 2004).
In summary, the reproductive benefits of feed restricting broiler breeder hens are well established, but the current research indicates that management practices by which birds are feed restricted can influence egg production. Specifically, maintaining a broiler breeder hen flock on an SAD feeding regimen after photostimulation until it reaches 5% egg production does not improve flock BW uniformity compared with ED feeding; more important, it causes a significant decrease in egg production throughout the entire egg production cycle. The mechanisms by which SAD feeding during this critical period of ovarian development permanently decrease reproductive capability are unclear. Further research is needed to better define the effects of fasting on ovarian development after broiler breeder pullets have been photostimulated for reproduction.
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FOOTNOTES |
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Received for publication December 24, 2007. Accepted for publication June 13, 2008.
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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