Modelling Energy Utilization for Laying Type Pullets

Three trials were carried out to determine energy metabolized (EM) requirement model for starting and growing pullets from different strains, at five ambient temperatures and different percentage feather coverage. In Trial I, metabolizable energy requirements for maintenance (MEm) and efficiency of energy utilization were estimated using 64 birds of two different strains, Hy-Line W36 (HLW36) and Hy-Line Semi-heavy (HLSH), from 9 to 13 weeks of age. The effects of ambient temperature (12, 18, 24, 30 and 36oC) and percentage feather coverage (0, 50 and 100%) on MEm were assessed in the second trial, using 48 birds per temperature per strain (HLSH and HLW36) from 9 to 13 weeks of age. Trial III evaluated ME requirements for weight gain (MEg) using 1,200 birds from two light strains (HLW36 and Hisex Light, HL) and two semiheavy strains (HLSH and Hisex Semi-heavy, HSH) reared until 18 weeks of age. According to the prediction models, MEm changed as a function of temperature and feather coverage, whereas MEg changed as a function of age and bird strain. Thus, two models were developed for birds aged 1 to 6 weeks, one model for the light strain and one for the semi-heavy strain. Energy requirements (ER) were different among strains from 7 to 12 weeks, and therefore 4 models were elaborated. From 13 to 18 weeks, one single model was produced for semi-heavy birds, since ER between semi-heavy strains were not different, whereas two different models were elaborated for the light layers. MEg of light birds was higher than MEg of semi-heavy birds, independent of age.


INTRODUCTION
Temperature is the most important environmental factor in animal production and directly affects animal responses.Domestic birds are homeotherm animals and therefore must expend energy to maintain body temperature, in order to have adequate biochemical, physiological and behavioral responses (Al-Saffar, 2002).At high temperatures, the feather coverage is one of the factors that impair dissipation of the heat produced during bird metabolism.Peguri & Coon (1993) reported that the maximization of net gain in birds depends on the ability of quantifying the costs of high requirements of energy for maintenance when environmental temperature is low, as well as the costs of maintenance in high temperatures, when metabolizable energy (ME) intake is lower.
According to Fialho (1999), mathematical models of physiological systems may be used in studies with poultry and swine as a means of elaborating strategies that optimize growth, decrease losses, reduce production costs and improve carcass characteristics, as well as evaluating the potential of alternative production systems.
The present study elaborated mathematical models to determine maintenance and growth energy requirements for pullets from different strains during the rearing phase, as a function of ambient temperature and feather coverage.

MATERIAL AND METHODS
Three trials were carried out at the poultry experimental facilities of the Department of Animal Science, at Faculdade de Ciências Agrárias e Veterinárias FCAV/UNESP/Jaboticabal.

Trial
I -Metabolizable energy requirements for maintenance and efficiency of dietary energy utilization Energy requirements for maintenance and the efficiencies of energy utilization for maintenance (km) and for weight gain (kg) were evaluated in a trial carried out in temperautre-controlled chambers at constant temperature (24ºC).Sixty-four pullets from a light strain (Hy-Line W-36, HLW36) and 64 pullets from a semi-heavy strain (Hy-Line Semi-heavy, HLSH) were reared from 7 to 13 weeks of age, in a completely randomized experimental design, with 4 treatments and 4 repetitions (4 birds/parcel).Treatments for both strains consisted of 4 feeding restriction levels: ad libitum, 70%, 50% and 35% of ad libitum intake.Diets were formulated based on corn, soybean meal and wheat meal.Nutrient levels were 2,850 kcal ME/kg, 18% CP, 1% calcium, 0.45% available phosphorus, 0.9% lysine and 0.725% methionine + cystine for the light strain, whereas levels for semi-heavy strains were 2,850 kcal ME/kg, 17% CP, 1% calcium, 0.44% available phosphorus, 0.9% lysine and 0.732% methionine + cystine.
The amount of food given to the restricted birds was calculated according to the intake of ad libitum birds.At the end of the experiment, total feed intake and metabolizable energy intake were calculated.
The apparent metabolizable energy of the diets was determined in a metabolism trial carried out using the same restriction levels and 11-week-old birds from the two strains.
Feed was given ad libitum during a one-week adaptation period, after which the initial body weight was recorded, and controlled feed intake started.Four birds with body weight similar to the mean body weight of the strain were killed at the beginning of the experiment to determine initial levels of body energy.At the end of the experiment, birds were weighed, fasted for 24 hours, weighed again and killed to determine final body composition.Feather samples were analyzed separately and the defeathered birds were frozen, sawed and entirely ground.Representative subsamples were dried for 72 hours in an oven at 55 ºC and ground a second time before dry matter, ether extract, crude protein and gross energy levels were determined.
Energy retention (ER) was calculated as the difference between initial and final mean body levels of gross energy.Heat production (HP) was calculated as the difference between metabolizable energy intake (MEI) and ER.The variables MEI and ER were related according to Farrell (1974), so that the regression analysis of ER as a function of MEI was used to estimate ME requirements for maintenance (MEm) and the efficiency of energy utilization above maintenance.The regression of the logarithm of HP on MEI determines heat production during fasting, whereas the HP antilogarithm determines heat production when MEI is extrapolated to zero.

Trial II -Ambient temperature and feather coverage effects on energy requirements for maintenance
The effects of different temperatures and feather coverage on the energy requirements for maintenance were evaluated using 480 pullets with different feather coverage percentages (100, 50 and 0% of normal feathering) in 5 temperature-controlled chambers at constant temperatures (12, 18, 24, 30 and 36ºC).
Two hundred and forty replacement pullets from each strain were reared from 9 to 13 weeks of age distributed in a completely randomized design with four repetitions of 4 birds.There were 48 HLW36 and 48 HLSH birds in each chamber.Birds were fed ad libitum with the same diets used in Trial I.
One week before the onset of the experiment, feathers were removed using a shearing machine and scissors.Feathers were completely removed from the birds of the 0% treatment, and from the right body side of the birds of the 50% treatment.
After one week for adaptation to the different temperatures, four birds from each strain and each temperature were killed in order to determine the initial levels of body energy.At the end of the experimental period, all birds were weighed and killed to determine final body composition, as described for Trial I.
MEI and ER were calculated in order to determine the energy requirements for maintenance of the two strains, and at the two different feather coverage percentages, in each chamber.The efficiency of ME utilization for each strain, determined in Trial I, was used in this second trial as the efficiency of ME utilization (kg).Metabolizable energy requirements for maintenance (MEm) were determined using the formula: MEm = MEI (ER/kg).

Statistical analyses
Statistical analyses were performed using the R program (http://www.r-project.org).The effects of temperature and feather coverage on MEm were evaluated according to the following linear regression models:

Trial III -Determination of energy requirements for body weight gain
Energy requirements for body weight gain for each strain were determined using 1,200 one-day-old female chicks.Three hundred birds from each strain were placed in starting cages (1 to 6 weeks of age) and then rearing cages (7 to 18 weeks old).The birds were distributed in a completely randomized design, with four treatments and four repetitions of 75 birds per strain.The medium strains Hy-Line Semi-heavy (HLSH) and Hisex Semi-heavy (HSH), and the light strains Hy-Line W-36 (HLW36) and Hisex Light (HL) were evaluated in this trial.Birds were fed daily with diets formulated according to the requirements adequate for each strain (light or semi-heavy) in each rearing phase.
All birds were weighed weekly and the mean body weight and weight gain were calculated.Feed intake per parcel was also determined weekly.Two birds with body weight similar to the mean body weight of the parcel (eight birds per strain) were separated and then killed as described for Trial I.

Methodology to determine energy requirement for body weight gain
Net energy requirements for weight gain (NEg) were calculated using regression equations of the total carcass crude energy (y) as a function of the body weight (x) during the studied period for each strain, according to the allometric equation log y = a + b logx (ARC, 1980).In order to calculate the estimates of the energy composition of body weight gain, equations for body composition were derived, and the formula used was similar to y = b10 a x (b-1) .
Afterwards, to determine ME for body weight gain (MEg), the values calculated for NEg were divided by the efficiency of ME utilization obtained in Trial I for light or semi-heavy strains.

Statistical analyses
Regressions and statistical analyses were performed using the software Statistica (1996).The obtained equations were compared between strains using the test for homogeneity of regression coefficients (Steel & Torrie, 1980).

RESULTS AND DISCUSSION
Trial I -Metabolizable energy requirements for maintenance Birds restricted 50 and 35% of ad libitum intake showed negative values of energy retention, indicating that pullets cannot be subjected to the same levels of food restriction that are used for broilers (Table 1).Longo (2000) reported that negative energy retention was seen only for broilers fed 30% of ad libitum intake and reared at 30ºC.This is explained by the fact that broiler selection programs are directed to increase feed intake at levels greater than the basal energy requirement.According to Boekholt et al. (1994), the ad libitum intake in broilers is approximately 2 to 3 times the maintenance requirements.Basaglia (1999) reported that laying hens from a light strain fed 35% of ad libitum intake had negative energy retention at 12, 22 and 31ºC.Besides, birds reared at 12ºC and fed 55% of ad libitum intake also showed negative energy retention, because maintenance requirements increased.the fact that greater energy costs for maintenance are needed for tissues with high protein levels, due to the permanent cell turnover, and that the maintenance of the fat tissue requires low energy, since it is virtually a metabolically inactive tissue (Luiting, 1990).
The protein/energy ratio was 9.04% higher in semiheavy birds compared to light birds, a value very similar to the difference of daily energy requirements between strains, which was 9.66% higher in semi-heavy birds.
Heat production (HP) in semi-heavy pullets was 9.81 kcal/kg 0.75 /day higher than in light birds.According to Blaxter (1989), different factors influence animal metabolism, including body weight, the circadian cycle, age, nutrition, body composition, organ size, purpose of the breed and differences among species.The hens HLW36 are the birds with the lowest body weight among the light strains currently commercialized.In the present study, these birds had lower MEm and HP than HLSH birds, which might be explained mainly by differences in body composition.
The reduction in HP seen for both strains when subjected to feed restriction might be explained by the fact that birds eat less frequently (Li et al., 1991), resulting in reduced feed intake (Blaxter, 1989) and digestion (Van Kampen, 1976), and lower calorigenic effect of food (Kleiber, 1961).
As shown in Table 2, the efficiency of dietary energy utilization for body weight gain was 0.59 for light birds and 0.62 for semi-heavy birds, representing a 6.78% higher efficiency for the semi-heavy strain.The efficiency of utilization of diet energy for maintenance (Km) was very similar between the two evaluated strains, i.e., 0.67 for light birds and 0.69 for semi-heavy birds.
Similar results have been reported in laying hens during the production phase (0.71) (Basaglia, 1999), in pullets of a light strain (0.72) (Silva, 1995) and in heavy breeders during the production phase (0.71) (Rabello, 2001).On the other hand, higher values have been reported by Silva (1999) in heavy breeders during the production phase (0.76) and by Longo (2000) in broilers (0.80).The ambient temperature in these above mentioned studies ranged between 21 and 23ºC.

Trial II -Ambient temperature and feather coverage effects on the energy requirements for maintenance
There were no differences (p>0.05) in the metabolizable energy requirements for maintenance between studied strains at the different temperatures and with different feather coverage.Thus, the results for both strains were analyzed together (Table 3).
Laying hens usually have low daily feed intake.The amount of ingested feed is adequate for a standardized development, so that weight and body composition at sexual maturity are adequate.This becomes evident in the results of the present study, when comparing the ME intake with the MEm requirements for each strain (Table 2).Therefore, light strain birds fed ad libitum and 70% restriction consumed only 1.59 and 1.18 times the MEm, respectively, whereas semi-heavy birds consumed 1.54 and 1.15, respectively.Thus, the lowest degree of food restriction caused significant reduction of MEI, which was only 18% higher than the basal energy costs in light birds and 15% in semi-heavy birds.
Table 2 Regression equation of energy retention (ER) and logarithm of heat production on the energy intake (MEIn), metabolizable (MEm) and net energy (NEm) for maintenance, and efficiencies of energy utilization for maintenance (km) and body weight gain (kg).It is interesting to note on Table 2 that the MEm estimated in this trial for HLW36 birds (122.16kcal/ kg 0.75 /day) was 13.67 kcal higher than the daily energy intake of birds submitted to 50% restriction (Table 1), which explains the negative energy retention.In HLSH birds, the MEm was 133.97 kcal/kg 0.75 /day, i.e., 14.15 kcal higher than the daily ingestion of birds restricted 50%, and also resulted in negative retention.
The MEm values reported in the present study are within the MEm range for laying hens reported by De Groote (1974), which was between 99 and 133 kcal/ kg 0.75 /day, and higher than the value of 99 kcal/kg live body weight reported for Leghorn Dekalb XL birds by Peguri & Coon (1993).
Daily MEm of semi-heavy hens was 9.67% higher than the MEm of light hens, and thus intake per day was 11.81 kcal/kg 0.75 higher.The same response profile was seen for NEm; semi-heavy birds had requirements 13.06% higher than light birds, and energy daily intake was 10.62 kcal/kg 0.75 higher.
Body composition analysis showed that the protein/ energy ratio in HLW36 birds was smaller (1.66) than in HLSH birds (1.81), which means that the former had proportionally more body fat.The lower MEm requirements for HLW36 birds may be explained by The MEm established in this trial for normally feathered birds reared at 24ºC was lower than the requirements determined in Trial I for both strains.Since environmental temperature was similar in both trials, any differences are probably due to different husbandry practices.In Trial I, husbandry was more intense than in Trial II.The food remaining in the feeders was weighed daily in order to calculate the amount of food to be provided according to the restriction levels, and bird stress might have increased.Besides, the metabolism trial was carried out during the experimental period.In the second trial, feed was provided in the morning and in the afternoon, and this was the only procedure carried out daily.
Restricted birds were clearly more stressed and aggressive as a consequence of competition for food after the feeder was filled up, and energy costs were probably higher.
The effect of ambient temperature and feather coverage on MEm requirements was evaluated as a function of the lower thermoneutral limit, or lower critical temperature (LCT).According to the equation (Figure 1), it may be observed that LCT was higher when birds had poorer feather coverage.LCT values were 18.89; 21.72 and 24.54ºC for 100%, 50% and 0% of feather coverage, respectively.
LCT is related to feather coverage because it indicates which is the thermoneutral temperature for the bird, and consequently, at which temperature lowest energy levels must be expended for basal metabolism maintenance.According to Furlan (2002), the thermoneutral zone corresponds to an ambient temperature range within which the metabolism rate is minimal and body temperature is kept constant at lowest energy costs.Therefore, a minimal fraction of the metabolizable energy is used for thermogenesis, and net energy available for production is maximal.Thus, it might be concluded that the birds adjusted the temperature set point specifically for each temperature and each degree of feather coverage that was studied.
It is worth noting that the parameter that changed was LCT, and not MEm requirements.The requirements were similar for the different degrees of feather coverage (92.4 kcal/kg 0.75 /day) at thermoneutral temperatures.This results from the greater heat loss in defeathered birds, mainly by non-evaporative processes (radiation, conduction and convection).Consequently, more energy was required at lower temperatures, and adaptation was better at higher temperatures, as shown in Figure 1.
Based on the produced model, maintenance requirement increases 0.88 kcal/kg 0.75 /day for each degree of temperature above the LCT.At In the starting phase, there were no significant differences (p<0.05) between the equations obtained for each strain, and the ratio between carcass gross energy and body weight was similar.Therefore, one overall equation was obtained.
In the following growth phase (8 to 12 weeks), energy deposition was different between the light and semi-heavy Hy-Line strains, but not between the light and semi-heavy birds from the Hisex strain.Thus, two different equations were determined for Hy-Line birds and one equation was determined for Hisex birds.In the last growth phase, gross energy deposition on the carcass was different only for the Hy-Line W36 birds, and thus two equations were produced, as shown in Table 5.
Body composition equations were derived in order to estimate the energy fraction of the weight gain.The coefficients of energy utilization established in Trial I were used to calculate the metabolizable energy for weight gain (MEg) for each strain.
Energy requirements for weight gain increased with age independent of strain (Table 6).Such response results from the higher protein and lipid deposition per kilogram of body weight gain with aging.
According to the starting phase results, although NE was similar between strains, MEg was different.This was probably due to different efficiencies of energy utilization.It should be noted that the MEg requirements for different strains were very similar; the requirement of light birds was 6.83% higher than for semi-heavy birds.temperatures above thermoneutral temperatures, higher MEm is attributed to the increase in energy expenses due to the activation of heat loss mechanisms, mainly the higher respiratory rate in order to increase evaporative heat loss.
On the other hand, temperatures lower than LCT caused maintenance requirements to increase 6.73 kcal/kg 0.75 /day for each degree of temperature decrease.
Birds without feathers had higher MEm than normally feathered birds at 12 and 18ºC, which might be due to the higher sensible heat exchanged with the environment at exposed body surfaces, evidencing the insulating ability of the feathers.Similarly, Peguri & Coon (1993)   Although no significant differences (p>0.05) were seen in MEm for the different degrees of feather coverage at temperatures higher than 24ºC, the requirements of defeathered birds at 30 and 36ºC were numerically lower than the requirements of normally feathered birds.This finding suggests that heat exchange with the environment was improved and lower energy was required to activate the mechanisms of heat loss, which were more intensively used by normally feathered birds.
According to Hurwitz et al. (1980), the effects of temperature on energy metabolism are complex and sometimes responses are cubic rather than linear, as usually assumed.The authors stated that energy requirements for maintenance decrease until the temperature reaches 24ºC, the rate is lower between 24 and 28ºC, and requirements increase between 28 and 34ºC.These observations corroborate the findings of the present study.

Trial III -Energy requirements for growth
The experimental period was divided into three different phases, according to the age of the birds, i.e., starting (1 to 7 wk), rearing I (8 to 12 wk) and rearing II (13 to 18 weeks).
Regression equations of carcass gross energy as a function of body weight were determined for each strain and growth phase (Table 5).and fat deposition might exist between strains and sexes, as seen in the present study.Prediction models for the daily ME requirements for pullets were produced based on the results of efficiency of energy utilization for light and semi-heavy strains (Trial I), metabolizable energy requirement for maintenance in the different temperatures and percentages of feather coverage (Trial II), as well as energy requirements for growth in the different rearing phases (Trial III).

Figure 1
Figure 1 Mean observed values (markers) and regression models (curves) of MEm as a function of feather coverage and ambient temperatures.

Table 1
Means of metabolizable energy intake (MEIn), energy retention (ER) and heat production (HP) in the different strains and levels of feed restriction.
* Percentage of ad libitum intake.

Table 3
Overall means of metabolizable energy requirements for maintenance (MEm) according to temperature and feather coverage.
reported increase in the MEm in defeathered birds at temperatures of 12.8 and 23.9ºC, whereas ONeill & Jackson (1974) reported a 30% increase in MEm in laying birds showing poor post-molt feathering compared to the MEm of normally feathered birds.

Table 4
Estimated metabolizable energy requirement for maintenance (kcal/kg 0.75 /day) as a function of temperature and feather coverage.

Table 5
Regression equations of body gross energy levels (GEb) on the weight gain according to rearing phase and strain.