Performance and hormonal profile in broiler chickens fed with different energy levels during post restriction period

The aim of this work was to investigate the influence of diet energy level on performance and hormonal profiles of broilers during post restriction period. It was a split-plot experiment, and the main treatments were in a 2x2 factorial scheme. Birds were fed restricted to 30% of the ad libitum intake, from 7 to 14 days of age. After the restriction period, birds were fed ad libitum with diets containing low (2,900 kcal ME/kg) or high (3,200 kcal ME/kg) energy until 49 days of age. Broilers fed with high energy ration showed lower feed intake and better feed conversion and decreased carcass protein; however, abdominal fat pad, and total carcass fat were not affected by ration energy levels or feeding program. Neither diet energy level nor feed restriction program changed body weight at 49 days. The profile of insulin-like growth factor-1 (IGF-1) was reduced during the feed restriction period, but increased at refeeding period. Feeding program and ration energy level did not affect T3, T4 and growth hormone serum concentrations. Feed restriction at 30% of ad libitum intake is not enough to promote changes on carcass quality, related to fat deposition, and on metabolic hormone levels, except IGF-1 seric level that has rapid increase after feed restriction.


Introduction
Birds with retarded growth due to undernutrition can achieve a growth rate higher than normal for chronological age after removal of the feed restriction (Plavnik & Hurwitz, 1985).This compensatory growth or catch up growth exhibited by restricted birds allows the recovery of body weight at slaughter age and sometimes a higher body weight than that of birds fed ad libitum (Plavnik & Hurwitz, 1990).However, the mechanisms responsible for this capacity have not yet been fully clarified.McMurtry et al. (1988) stated that changes in the weight gain composition, higher efficiency of energy utilization, and reduction in maintenance requirements, or a combination of these factors, contribute to the phenomenon of compensatory growth.However, other factors are related to total or partial compensatory gain as sex, qualitative or quantitative feed restriction, feed restriction severity, strain and broiler age (Yu & Robinson, 1992).
Although compensatory gain after feed restriction has been a subject of investigation, the hormonal profile involved in this process has not been often studied.In chickens, as well as other species, the animal metabolism is controlled by a variety of hormones that form a complex system which directly affects growth.Among hormones, growth hormone (GH), insulin like growth factor-1 (IGF-1), insulin, triiodothyronine (T 3 ) and thyroxine (T 4 ) have been reported to be involved in broiler growth control (Scanes et al., 1984).However, it should be emphasized that the final growth expression is the result of interactions between nutritional, environmental, and genetic factors interacting with endocrine secretions.
This study was carried out to investigate the influence of diet energy level on performance and hormonal profiles during post restriction period in broiler chickens.

Material and Methods
A total of 400 female broiler chickens of Cobb-500 strain were allocated in environmentally controlled rooms at Setor de Avicultura, FCAV/Unesp, Jaboticabal, Brazil, between July and August in 2000.Ambient temperature was maintained at thermoneutrality according to birds age, that were reared in pens on litter floor (8 cm of wood shavings and population density of 10 birds/m 2 ) up to 49 days of age.At day 7, chicks were assigned to one of two feeding groups: ad libitum (AL) or feed restricted (FR), and two different energy levels (high and low) with four replications of 25 birds per treatment.Feed restriction was applied during the second week (7 to 14 days) when birds were restricted to 30% of ad libitum intake.After the feed restriction period, the birds were fed ad libitum until the end of the experiment (49 days).Feed was continuously available to the animals of the ad libitum group.The diets used in the experiment were based on corn and soybean meal (Table 1).Feed intake and body weight were recorded weekly.Light was provided for 23 hours throughout the experimental period.
Blood samples of 12 birds per treatment (each bird being considered one replication) were collected between 2 pm and 4 pm from the wing veins, using sterile and heparinized syringes, on days 14, 18, 22, 26, 30 and 49.After collection, tubes containing blood were kept at room temperature for two hours and then transferred to a refrigerator and kept overnight at 4 o C, centrifuged at 3,000 rpm/15 min.Serum was obtained and stored at -20 o C until analysis.
At 14, 28, 42 and 49 days of age, 12 birds per treatment were weighed and slaughtered by cervical dislocation for carcass composition analysis (protein and fat) after two hours of fasting.The abdominal fat pad (fat around the cloaca and abdominal muscles) was removed and weighed, and again added to the carcass.The carcasses were then ground with an industrial meat grinder, and dried in a forcedair oven at 55 o C during 72 hours.The carcasses were homogenized and samples were collected for protein and total fat determinations.All analysis were performed in duplicates, according to the procedures of Association of Official Analytical Chemists (1990).
The experiment was performed in a split-plot design with main plots as the combination of two feeding program (ad libitum and restricted) and two ration energy levels (low: 2,900, high: 3,200 kcal ME/kg) and age as secondary plots.Data were subjected to statistical analysis using the General Linear Model procedure (GLM) of SAS (SAS Institute, 2000), and means were compared by Tukey's test at 5% of probability.

Results and Discussion
Birds that were fed with high energy level had a lower (p<0.01)feed conversion and lower (p<0.05)feed intake; however, no significant difference (p>0.05) in final body weight was found between the birds fed with different energy levels (Table 2).The feeding program did not affect any of the performance parameters and there was no significant interaction (p>0.05)among treatments.The great variability of data shown in the literature regarding early feed restriction in broilers seems to be related to several factors, i.e., severity and duration of feed restriction, broiler age, refeeding period (Lippens et al., 2000) and energy level.Coon et al. (1981) comparing the performance of male and female broilers fed on low or high energy rations during 56 days, found a significant improvement in the feed conversion using a diet with high energy level.Zorrila et al. (1993) observed a linear increase in body weight gain increasing the energy levels.The present work also provides evidence that a high energy diet improves feed consumption and results in a better feed conversion during post restriction period, compared to a lower energy level.
Feed restriction could also affect the feed intake following restriction period, feed conversion efficiency and body weight (Plavnik & Hurwitz, 1991).The present work did not show any effect of quantitative early feed restriction on broiler performance.Thus, even though the level of early feed restriction is an important factor influencing the broilers response, early feed restriction at 30% of ad libitum intake was not able to influence broiler performance parameters at market age (49 days).
As to carcass composition, broiler total and abdominal fat were not affected by ration energy level; however, birds fed with high energy level (3,200 kcal ME/kg) had lower body protein content when compared to the birds fed with low energy level (2,900 kcal ME/kg) (Table 3).There was a decrease (p<0.01) in carcass protein percentage and an increase in fat pad and total body fat as broilers Table 2. Means for the effects of feeding program, diet energy level and age on feed intake, body weight gain and feed conversion of broiler chickens (1) .Table 3. Means for the effects of feeding program, ratio energy level and age on protein, ether extract and abdominal fat content of broiler chickens (1) .
growth.Feeding program did not affect (p>0.05)broilers carcass protein, total fat and abdominal fat pad, and no significant (p>0.05)interactions were verified among treatments.
A controversial aspect of feed restriction programs has been the inconsistent effect on carcass fat deposition.Summers et al. (1990) and Jones & Farrell (1992) did not find changes in carcass composition of birds after feed restriction conditions; however, Plavnik & Hurwitz (1985, 1989) and Plavnik et al. (1986) reported a decrease in fat pad on birds restricted from 6 to 12 days of age, without adverse effects on growth.The same effect of restriction on the amount of carcass fat was found by Sugeta et al. ( 2002), but with lower body weight gain in relation to the ad libitum birds, perhaps due to the restriction severity (70% of the ad libitum feed intake).Fontana et al. (1992) reported a larger abdominal fat deposition in the carcass of restricted birds after refeeding.According to Evans (1977), fat pad is more directly influenced by nutrition than total carcass fat.The present data showed that early feed restriction and energy level did not reduce carcass fat and fat pad content at 49 days of age.Probably, this finding is related to a high fat deposition on broiler adipose tissue observed during refeeding period.Furlan (1996) also verified high fat deposition (abdominal and carcass) one week after refeeding.
Serum concentrations of hormones were significantly (p<0.01)affected by birds age (Table 4).There were no significant effects of feeding program and energy levels on T 3, T 4 and GH serum concentrations; however, a significant feeding program x broilers age interaction was observed in IGF-1 and insulin serum levels.The interaction analysis showed that feeding program significantly (p<0.01)reduced the IGF-1 serum level in approximately 44% at the end of the feed restriction period, compared to birds fed ad libitum (Table 5).After four days of ad libitum feed intake (refeeding), IGF-1 returned to normal levels, not differently (p>0.05) from that observed in the ad libitum birds (1) For each variable, means in the column followed by the same letter do not differ significantly by Tukey's test at 5% of probability; values represent mean±standard deviations.
Table 4. Means for the effects of feeding program, energy level and age on seric levels of triiodothyronine (T 3 ), thyroxine (T 4 ), insulin, insulin-like growth factor (IGF-1) and growth hormone (GH) of broiler chickens (1) .  (1For each variable, means followed by the same small letter within a column and by the same capital letter within a row do not differ significantly by Tukey's test at 5% of probability; values represent mean±standard deviations. until the end of the experimental period.Insulin serum level was lower (p<0.05)for feed restricted birds only at 42 days of age when compared to ad libitum fed broilers (Table 5).
Changes in the IGF-1 plasma level by different nutritional status have been described in mammals and birds.Generally, there is a fall in the concentration of this growth factor due to protein and energy restriction (Kita et al., 1996;McMurtry et al., 1997).
In the present work broiler chickens also reduce IGF-1 plasma level due to feed restriction, and increase it after refeeding period (Table 5).Schew et al. (1996) submitted quails to feed restriction and reported that when feed was provided ad libitum, there was a reestablishment of the IGF-1 plasma concentration in levels above than those found in the control birds.Fasting is associated not only with reduced plasma IGF-1 concentration but also with elevated plasma GH levels (Morishita et al., 1993).McMurtry et al. (1988) and Buyse et al. (1996) reported that restricted broiler showed high GH plasma concentration during compensatory growth than non-restricted ones.
Insulin serum concentrations did not decrease during feed restriction.Similarly, McMurtry et al. (1988) did not observe changes in the plasma insulin concentration in broiler chickens submitted to feed restriction.These findings suggest that the level of restriction used was not severe enough to elicit any physiological changes in the serum insulin level.

Conclusions
1.A high energy diet improve feed consumption and results in a better feed conversion during post restriction period when compared to a lower energy level.
2. Feed restriction at 30% of ad libitum intake is not enough to promote changes in carcass quality and on metabolic hormones levels, except IGF-1 seric level that has rapid increase after feed restriction.

Table 1 .
Composition and calculated analysis of experimental diets offered to broiler chickens from 1 to 49 days of age.

Table 5 .
Interactions between feed program and age on seric levels of insulin and insulin-like growth factor-1 (IGF-1) of broiler chickens(1).