1. Introduction
Weaning is one of the most challenging phases throughout the pig’s life. Replacing milk with solid feeds is stressful for nursery pigs and may cause poor digestion resulting in retarded growth [
1]. To maximize the growth performance of nursery pigs, highly digestible animal proteins such as fish meal and spray-dried plasma protein are conventionally included in nursery diets [
2,
3]. However, these ingredients may increase feed costs because of their high price [
4]. Thus, alternative animal protein ingredients can save pig production costs.
Hatchery byproducts (i.e., wastes from hatchery facilities) are defined as “a mixture of egg shells, infertile eggs, unhatched eggs, and culled chicks that have been cooked, dried, and ground, with or without removal of part of the fat” in AAFCO [
5]. These byproducts are regarded as potential alternative protein sources for swine diets [
6]. Hatchery byproducts are distinguished from poultry byproducts. However, in some cases, hatchery products are incorporated into poultry byproducts [
7]. Currently, these products are often disposed of in landfills, resulting in environmental pollution and disposal costs.
An accurate nutritional evaluation of feed ingredients is required for precise feed formulation [
8,
9]. To the best of our knowledge, however, information on energy and phosphorus (P) utilization of hatchery byproducts by nursery pigs is currently unavailable. Therefore, the objective of this study was to determine digestible energy (DE), metabolizable energy (ME), and standardized total tract digestibility (STTD) of P in hatchery byproducts fed to nursery pigs.
3. Results
Crude protein and GE contents in hatchery byproducts ranged from 32.2 to 67.5% and 4206 to 5884 kcal/kg on an as-is basis, respectively (
Table 3). Infertile eggs (31.9%) and unhatched eggs (35.6%) had greater ash contents than culled chicks (8.7%).
In Experiment 1, two pigs fed the mixture diet had diarrhea during the collection period and these two observations were excluded from the data. The energy digestibility of the diet containing infertile eggs was lower (
p < 0.05) than that of other experimental diets (
Table 4). The diet containing culled chicks had the greatest DE and ME among all the experimental diets, whereas DE and ME of the diet containing infertile eggs were the lowest (
p < 0.05). The DE in the mixture diet was greater (
p < 0.05) than that in the unhatched egg diet.
The energy values of ingredients showed similar patterns as those in experimental diets (
Table 5). Culled chicks had the greatest DE and ME, whereas infertile eggs had the lowest energy values among the test ingredients (
p < 0.05). The DE in the mixture was greater (
p < 0.05) than that in unhatched eggs. However, ME concentrations did not differ between unhatched eggs and the mixture. Infertile eggs had the lowest DE:GE and ME:GE ratios among the three ingredients and their mixture (
p < 0.05). The ME:DE ratio was greater in unhatched eggs compared with that in culled chicks and the mixture (
p < 0.05).
In Experiment 2, three observations were excluded due to diarrhea and three additional observations were excluded due to feed refusal. Total feed intake and DM intake did not differ between the experimental diets, but the P intake of pigs fed culled chicks and the mixture diet was greater than that of pigs fed infertile eggs and unhatched eggs (
p < 0.05;
Table 6). Total dry feces output was not different between treatments, while the P concentration in feces and P output were greater in pigs fed culled chicks and the mixture compared with those fed infertile eggs (
p < 0.05). No difference was observed in ATTD of DM. However, the ATTD and STTD of P in infertile eggs was the greatest (
p < 0.05) among all hatchery byproducts. The ATTD and STTD of P did not differ between unhatched eggs, culled chicks, and the mixture.
4. Discussion
Hatchery byproducts are routinely discarded from hatchery facilities. A total of 6335 tons were discarded in 2015 in the Republic of Korea [
18]. Hatchery byproducts are potential substitutes for animal protein sources in swine diets, particularly during the nursery period [
6]. To the best of our knowledge, however, nutritional information on hatchery byproducts as a swine feed ingredient is very limited. In the present work, the energy concentrations and P digestibility of hatchery byproducts as feed ingredients for nursery pigs were determined.
In hatchery facilities, hatchery byproducts are pooled and then discarded together. To mimic a natural product in the layer facility where we obtained the ingredients, a mixture of the three ingredients (20% dried infertile eggs, 20% dried unhatched eggs, and 60% dried culled chicks) was prepared in this study.
Values of CP, EE, and ash in unhatched eggs and culled chicks agreed with previously reported values [
19]. Dhaliwal et al. [
6] summarized chemical compositions of hatchery byproducts reported in various studies. However, because various types of hatchery byproducts were pooled into a mixture and the inclusion rate of each ingredient was not declared in previous reports, the comparison of chemical components in each ingredient in the current study with reported values is difficult. When comparing the mixture in the present study with reported values, the chemical components of the mixture were within the ranges given in previous studies [
6,
19,
20,
21].
The ATTD of GE was not different between the experimental diets except that the infertile egg diet had less GE digestibility than other diets. However, DE in diets differed between the experimental diets mainly due to differences in diet GE contents. Considering that GE concentrations in infertile eggs and unhatched eggs were comparable, the lower DE in infertile eggs compared with that in unhatched eggs was most likely attributed to the lower DE:GE ratio. The low energy digestibility of unhatched eggs may be associated with the drying temperature and duration. The reason for the low energy digestibility of infertile eggs is currently unclear. On day 8 post-fertilization, eggs identified as infertile by inspection were separated from the incubator. The microbial analysis of infertile eggs used in the present study showed no evidence of spoilage [
18]. In contrast to the present pig study, interestingly, true ME concentration and energy metabolizability in infertile eggs were found to be greater than those in unhatched eggs when fed to 50-day-old chickens [
22]. Further research is warranted to identify the reason for the low energy digestibility of infertile eggs fed to pigs and to search for a method to improve energy digestibility.
Urinary GE output was not different between the unhatched egg diet and the mixture diet, although the value in the unhatched egg diet was numerically lower than that in the mixture diet (56 vs. 71 kcal/d). Due to this numerical difference in urinary GE output, the ME:DE ratio was greater in the mixture than in unhatched eggs, resulting in no difference in ME between unhatched eggs and the mixture, even though DE was greater in the mixture. This is because urinary GE output is the major factor determining the proportion of DE converted to ME [
12]. The DE and ME of the mixture were very close to the values calculated based on the inclusion rate of infertile eggs, unhatched eggs, and culled chicks in the mixture (4203 and 3990 kcal/kg as is, respectively). This indicates that energy values in the hatchery byproducts are additive.
In the P digestibility trial, the ATTD of DM was expected to be lower in the diet containing infertile eggs and unhatched eggs, as the greater ash concentration in egg shells would have a lower digestibility compared with other components of DM. In the ATTD of DM, however, no difference was found between experimental diets. The lower ATTD and STTD of P in unhatched eggs compared with infertile eggs are likely due to indigestible parts in unhatched eggs including feathers, beaks, and claws, which are mainly composed of keratins [
23]. Interestingly, the energy digestibility of unhatched eggs was not largely compromised in Experiment 1 in spite of these indigestible parts. Perhaps this result may have been partially due to a large quantity of ash in the indigestible parts of unhatched eggs. A wider Ca to P ratio and an excessively high Ca concentration in diets have been reported to negatively affect P digestibility and growth performance in pigs, particularly when the P concentration in diets is below the requirement [
24,
25]. The Ca to P ratio in the diets containing infertile eggs, unhatched eggs, culled chicks, and the mixture was 15.2, 14.4, 2.0, and 4.4, respectively. Except for the culled chick diet, all experimental diets had extremely high Ca to P ratios. However, ATTD and STTD of P in the culled chick diet did not differ from those in the unhatched egg diet or the mixture diet. The reason for this unexpected result is unclear.