J. Anim. Sci. 2006. 84:221-228
© 2006 American Society of Animal Science
Evaluation of pet food by-product as an alternative feedstuff in weanling pig diets1
E. A. Jablonski,
R. D. Jones and
M. J. Azain2
Animal and Dairy Science Department, University of Georgia, Athens 30602
 |
Abstract
|
|---|
Three experiments were conducted to evaluate pet food by-product (PFB) as a component of nursery starter diets and its effects on pig performance. The PFB used in these studies was a pelleted dog food that contained (as-fed basis) 21% CP, 1.25% total lysine, and 8.3% ether extract. In Exp. 1, 288 early-weaned pigs (5.2 kg at 14 d) were used to determine the effects of replacing animal protein and energy sources with PFB at 0, 10, 30, and 50% (as-fed basis) inclusion levels in phase I (d 0 to 7 after weaning) and phase II (d 7 to 21 after weaning) diets. Phase I diets contained 27.5% whey, 18.75% soybean meal, 1.50% lysine, 0.90% Ca, and 0.80% P, with PFB substituted for corn, fat, plasma protein, fish meal, limestone, and dicalcium phosphate. Phase II diets had a constant 10% whey, 1.35% lysine, and PFB was substituted for blood cells, a portion of the soybean meal, and other ingredients as in phase I diets. In phase I, growth performance by pigs fed PFB-containing diets was similar to that of the control diet. In phase II, ADG (linear; P < 0.05 and quadratic, P < 0.005), ADFI (linear and quadratic, P < 0.01), and G:F (quadratic, P < 0.01) were increased with increasing PFB inclusion. In Exp. 2, 80 weaned pigs (6.7 kg at 21 d) were fed a common phase I diet for 1 wk and used to further evaluate the effect of PFB in phase II diets (same as Exp 1; initial BW = 8.1 kg) on growth performance and apparent total tract nutrient digestibility. There were no differences in ADG, ADFI, or G:F across treatments. Dry matter and energy digestibility did not differ among diets; however, digestibilities of CP (P < 0.05) and the essential AA, arginine (P < 0.02), histidine (P < 0.01), lysine (P < 0.001), threonine (P < 0.01), and valine (P < 0.01), were greater as PFB was increased in the diet. In Exp. 3, the performance by pigs (n = 1 70; 5.5 kg; 21 d of age) fed diets with 0 or 30% PFB in both phases I and II was examined. Growth performance was similar in both diets. These studies demonstrate that pet food by-product can effectively be used as a partial replacement for animal protein sources and grain energy sources in the diets of young nursery pigs.
Key Words: by-product feeding • growth performance • pig
|
INTRODUCTION
|
|---|
Feed costs represent the greatest operating expense in swine production. This is particularly evident in starter diets that rely on high-cost animal protein and energy sources as a means to enhance the transition of young pigs from sow’s milk to grain-based diets (Maxwell and Carter, 2001
). Alternative feedstuffs, such as by-products from the food and feed industries, can decrease the cost of production. As disposal costs increase, opportunities to use by-products in swine diets will likely increase (Myer and Brendemuhl, 2001).
In general, the nutrient requirements of dogs and cats (NRC, 1985, 1986), and thus the chemical composition of commercial pet food diets (Crane et al., 2000), are similar to those of the pig and to the diets typically used in the swine industry. Pet food by-product (PFB) may be a cost-effective replacement for the more typical expensive animal protein sources used in nursery diets, namely spray-dried plasma protein, fish meal, and blood cells. Pet food by-product also may serve as an energy source to replace corn or other ingredients. The term by-product is used loosely to describe pet food that has been rejected because it did not meet quality specifications, was damaged during handling, or was distributed to a retail outlet and not sold before the expiration date. The North American pet food industry is a $19 billion dollar enterprise that continues to expand annually (Campbell et al., 2005). As more pet food is produced each year, more by-product will become available as an alternative feedstuff.
The objective of these studies was to determine the effect of inclusion of PFB, specifically a dog food, in starter diets on the performance of pigs. A secondary objective was to determine the apparent nutrient digestibility of PFB diets in nursery pigs.
|
MATERIALS AND METHODS
|
|---|
General
Experimental protocols were approved by the University of Georgia Institutional Animal Care and Use Committee. Pet food by-product used in all experiments was obtained locally. The product was a premium-brand senior dog food with primary ingredients of rice flour, chicken meal, rice bran, wheat flour, corn gluten meal, and poultry fat. The guaranteed analysis on the label indicated that the product contained a minimum of 22% CP, 10% crude fat, and a maximum of 4% crude fiber and 10% moisture. Chemical analysis data (CP, crude fiber, ether extract, DM, Ca, P) of the PFB are shown in Table 1. Nitrogen content was determined using an N analyzer (LECO FP-528, LECO Corp., St. Joseph, MI). Separation and analysis of AA was performed on an AA analyzer (Beckman 6300, Beckman Coulter, Inc., Palo Alto, CA) following acid hydrolysis (Amos et al., 1976). Norleucine was used as the internal standard. Gross energy determination was performed on a bomb calorimeter (Parr 1261, Parr Instrument Co., Moline, IL). Digestible and metabolizable energies were calculated (Noblet and Perez, 1993). The PFB used in these studies was in the form of extruded pellets and was passed through a roller mill before mixing in experimental diets. The inclusion of PFB (Table 2) increased incrementally (0, 10, 30, and 50% inclusion rates) at the expense of corn, fat, fish meal, plasma protein (in phase I diets), blood cells (in phase II diets), and soybean meal (in phase II diets). Because of the well-established role of whey as a source of AA and lactose (Tokach et al., 1989; Mahan, 1992; Hansen et al., 1993; Nessmith et al., 1997) and the effect of soybean meal as a gut antigen in weanling pig diets (Li et al., 1991), the dried whey and soybean meal contents were maintained at 27.50 and 18.75%, respectively, in phase I diets. Dried whey also was maintained in phase II diets at 10.00%. The PFB used in these studies did not list soybean meal or any milk products as ingredients. The analyzed nutrient content of the PFB was used in the formulation of experimental diets. Diets were formulated to specific lysine requirements (1.50 and 1.35% for phases I and II, respectively) and to maintain relatively constant lysine:energy; however, CP increased with higher inclusion levels because of the nature of the PFB. All nutrient levels met or exceeded those suggested by the NRC (1998). It should be noted that the label on the whey used in these studies indicated that it contained 6.35% salt; therefore, additional salt was not included in the diet. All experimental diets were fed in meal form.
Experiment 1
The study was conducted at the University of Georgia Swine Center in 2 trials using pigs from consecutive farrowing groups. Two hundred eighty-eight terminal crossbred pigs [DRU sire (International Boar Semen, Eldora, IA) and Hampshire x Landrace x Large White females] were weaned at approximately 14 d of age (average BW = 5.2 kg). Pigs were housed in pens with dimensions of 1.22 x 2.84 m. Temperature was maintained at 27.8°C. In trial 1, pigs were allotted on basis of gender, ancestry, and weaning weight to 16 pens and fed 1 of 4 dietary treatments (4 replicate pens per diet) with 8 pigs per pen; barrows and gilts were penned separately. In trial 2, pigs were allotted similarly to trial 1, but with 10 pigs per pen. At weaning, pigs were given 0.5 kg/pig (as-fed basis) of a commercial creep feed (20% CP and 1.60% lysine; Akey Pig 3000, Akey, Inc., Lewisburg, OH), which was followed with phase I diets from d 0 through 7. At d 7, pig BW were measured and feeders were emptied to measure feed intake. Phase II diets were fed from d 7 through 21. At d 21, pig BW and feed intake were measured, and pigs were then placed on a common phase III diet (Table 2
) until d 31, at which time final nursery BW and feed consumption were measured.
On d 14, blood samples were drawn from 4 average-weight pigs per pen (based on previous weight) in replicate 1 and from 2 average-weight pigs per pen in replicate 2. Blood was drawn again on d 28 from the same pigs. These dates were selected to assay serum urea N (SUN) concentration as an index of protein nutrition (Coma et al., 1995) in pigs fed the test diets for 2 wk and after returning to a common diet for 7 d, respectively. After centrifugation (1,200 x g at 4°C for 20 min), blood serum was removed and frozen for later analysis. A commercial kit was used to measure end point SUN concentration indirectly by coupled enzyme reactions involving urease and glutamate dehydrogenase (Sigma BUN End point Kit; Sigma Diagnostics, Inc., St. Louis, MO).
Experiment 2
The study was conducted at the University of Georgia Large Animal Research Unit using the same phase II dietary treatments as in Exp. 1 and included both a growth trial and a digestibility trial. Eighty crossbred pigs [DRU sire (International Boar Semen, Eldora, IA) and Hampshire x Landrace x Large White females] from the same farrowing group were weaned at approximately 21 d of age (average BW = 6.7 kg). Pigs were housed in a nursery growth room in pens with dimensions of 0.94 x 1.83 m. Temperature was maintained at 27.2°C. Pigs were allotted on the basis of gender, ancestry, and weaning weight to 16 pens and 1 of 4 dietary treatments (as in Exp. 1), with 4 pigs per pen (mixed gender). Four additional pens were allotted with 4 barrows each, which were later used in the digestibility trial. At weaning, piglets were placed on a common phase I diet (control diet; Table 2), which was fed for 6 d. The same (0, 10, 30, 50% PFB) experimental phase II diets fed in Exp. 1 (Table 2) were then fed for 14 d. Pigs were then placed on a common phase III diet (Table 2) for 7 d, when final BW and feed consumption were measured. Body weight and feed intake were measured at weekly intervals.
The barrows for the digestibility trial were moved to a metabolism room for a single 10-d period that consisted of 6 d of adaptation and 4 d of collection. Pigs were housed individually in stainless-steel metabolism crates measuring 0.71 m high x 0.59 m wide x 0.81 m deep. Temperature was maintained at 27.2°C. Pigs in the metabolism trial were fed the same phase II nursery diets as those in the growth trial, with the exception that 0.25% chromic oxide was added as an indigestible marker for use in determining apparent nutrient digestibility. Pigs were fed ad libitum twice daily during the 6-d adjustment period, and water was freely accessible at all times. During the 4-d collection period, pigs were taken off feed at 1700, and fecal screens and trays were cleaned; this was done to prevent contamination of fecal samples with feed. At 0800, feces were collected and frozen. At the end of the collection period, fecal samples were composited for each pig across the 4-d collection. Samples were freeze-dried, finely ground, and kept in a desiccator under constant vacuum until analysis.
On d 13, blood samples were drawn from 2 average-weight pigs per pen in the growth trial and from all 16 pigs housed in the digestibility trial. After centrifugation (1,200 x g at 4°C for 20 min), blood serum was removed and frozen for later analysis of SUN (Coma et al., 1995).
Chromic oxide content of feed and fecal samples was determined colorimetrically (Fenton and Fenton, 1979). In addition, energy, protein, and individual AA digestibility were determined for diet and fecal samples using procedures described previously.
Experiment 3
The study was conducted at the University of Georgia Swine Center in a single replicate. One hundred seventy terminal crossbred pigs [DRU sire (International Boar Semen, Eldora, IA) and Hampshire x Landrace x Large White females] from the same farrowing group were weaned at approximately 21 d of age (average BW = 5.5 kg). Pigs were housed in pens with dimensions of 1.22 x 2.84 m. Temperature was maintained at 27.8°C. In this study, pigs were maintained as litters when they were transferred to the nursery. Pens of pigs were assigned randomly to either the control (n = 9 pens; 0% PFB) or 30% PFB (n = 8 pens) diets as in Exp. 1. The average and range for the number of pigs per pen was similar in each dietary treatment. The phase I diet (Table 2) was fed for d 0 to 7; phase II (Table 2) diet was fed for d 7 to 21; and a common phase III diet (Table 2) was fed for d 21 to 35 after weaning.
Statistical Analyses
Data were analyzed using the GLM procedure of SAS (SAS Inst., Inc., Cary, NC). Experiment 1 was analyzed as a 4 x 2 factorial, with dietary treatment and gender as main effects. Replicate, block, and their interactions were the effects included in the model. The growth performance portion of Exp. 2, where pens were of mixed gender, was a randomized block design, with 4 dietary treatments in a single replicate. The experimental unit in both trials was pen. Orthogonal polynomials were used to compare the linear and quadratic effects of diet on performance. Coefficients were generated based on an unequally spaced independent variable (Robson, 1959). The individual pig was considered the experimental unit in the digestibility portion of Exp. 2, which also was analyzed using the GLM procedure and contrast statements to test for linear and quadratic effects. Experiment 3 was analyzed using 2-way ANOVA; pen was the experimental unit. Least squares means, probabilities of differences, and SEM were obtained to evaluate differences among treatment means. Differences were considered significant at P < 0.05, whereas P < 0.10 was considered a trend.
|
RESULTS
|
|---|
Pet Food By-product Analysis
Proximate and AA analysis of PFB indicated its potential as a high-quality feedstuff (Table 1). The CP value of 20.94% was slightly below the guaranteed analysis listed on the label of the product (minimum 22% CP). Fat content, as estimated by ether extract value, was 8.29%, which again was below the label value (minimum 10%). Gross energy and chemical analysis values were used to calculate an ME value of 3,980 kcal/kg (Noblet and Perez, 1993). Calcium and P contents were 0.82 and 0.84%, respectively.
Experiment 1
There was no effect of gender and no interaction between gender and dietary treatment for any variable. From d 0 through 7 after weaning (phase I), no differences in any measure of pig performance were observed for pigs fed the experimental diets (Table 3). Feed intake in phase I included 0.5 kg/pig of a common creep feed that was added to the feeder before addition of the experimental diets. From d 7 through 21 (phase II), ADG (linear, P < 0.03 and quadratic, P < 0.005, Table 3) and ADFI (linear, P < 0.0 and quadratic, P < 0.01) improved with increasing dietary inclusion of PFB. On d 21, pigs fed any level of PFB were 0.7 to 1.0 kg heavier than the control pigs (linear, P = 0.06 and quadratic, P < 0.005). There was a quadratic effect for improved feed efficiency (P < 0.01) with increasing PFB inclusion level.
View this table:
[in this window]
[in a new window]
|
Table 3. Effect of pet food by-product (PFB; as-fed basis) in phase I and II diets on nursery pig performance in Exp. 11
|
|
When all pigs were fed a common diet (d 21 through 31 after weaning), no differences in growth rate, intake, or efficiency were observed. Similarly, there were no differences in overall nursery performance from weaning through d 31.
Serum urea N concentrations did not differ across treatments after the pigs had been fed experimental diets for 2 wk (d 14; Table 3) or after being fed a common diet (d 21 to 28).
Experiment 2
Given the results of Exp. 1, we focused on phase II diets in Exp. 2. In this study, pigs were fed a common phase I diet for 6 d after weaning, at which time they were assigned to the same phase II diets used in Exp. 1. As in Exp. 1, there were no effects of gender and no interactions between gender and treatment for any variable. Growth performance for d 0 to 14 was similar across dietary treatments (Table 4). Overall nursery performance did not differ across treatment groups. Serum urea N concentrations increased with increasing dietary PFB inclusion (linear, P < 0.02).
View this table:
[in this window]
[in a new window]
|
Table 4. Effect of pet food by-product (PFB; as-fed basis) in phase II diets on nursery pig performance in Exp. 21
|
|
All digestibility values are reported on a DM basis (Table 5). There were no statistical differences (P > 0.20) among diets for DM or energy digestibility; however, pigs fed PFB-containing diets had increased digestibility of CP and several of the essential AA. Digestibility of arginine (P < 0.02), histidine (P < 0.01), lysine (P < 0.001), threonine (P < 0.01), and valine (P < 0.01) increased as PFB increased in the diet. There also was a trend (P < 0.10) for increased digestibility of methionine as PFB was increased. Tyrosine digestibility is not reported because of high experimental error. There also was a linear increase in digestibility of most nonessential AA as PFB level in the diet increased (P < 0.03).
View this table:
[in this window]
[in a new window]
|
Table 5. Apparent total tract digestibility (DM basis) of phase II diets containing pet food by-product (PFB, as-fed basis; Exp. 2)1
|
|
Experiment 3
Performance by pigs fed the control or 30% PFB diet in both phase I and II was examined. Although there was no difference in overall performance (Table 6), pigs fed the 30% PFB diet had greater ADG (P < 0.05) and improved feed efficiency (P < 0.01) on the phase I diet (d 0 to 7). Pigs fed the PFB diet had a numerical improvement in intake during both phase I (11.5%; P < 0.25) and phase II (11.9%; P < 0.10). Conversely, G:F of pigs fed the PFB diet was (P < 0.05) less than for the control during phase II, and there was a trend (P < 0.08) for poorer performance when fed the common diet in phase III.
|
DISCUSSION
|
|---|
Pet food by-product was an effective replacement for more expensive animal protein sources in nursery diets. The results of Exp. 1 showed an increase in BW, ADG, and ADFI for pigs fed PFB compared with the control diet during the phase II period. This advantage was not maintained after feeding a common phase III diet, and it was less evident in Exp. 2 and 3. Nevertheless, the combined results of these experiments suggest that when available, PFB is a viable alternative to other protein and energy sources.
In Exp. 1, pigs were weaned at approximately 2 wk of age, whereas pigs in Exp. 2 and 3 were 3 wk old at weaning. The early weaning age may have accounted for the lack of a dietary effect in phase I. It may be necessary to feed phase I diets for a longer period of time to detect performance differences. Combined across the 3 levels of PFB, pigs fed these diets in phase II gained 31% faster than those fed the control diet. During phase III, when pigs were fed a common diet, this advantage was lost, and pigs gained at a 7.7% slower rate than controls. Because soybean meal was present in all diets, it is unlikely that this relative drop in performance was due to a hypersensitivity reaction (Li et al., 1991), and it may simply be explained by a decrease in palatability. Increased growth rate in pigs fed PFB also was observed in phase I of Exp. 3 but not in phase II. Pigs in Exp. 2 were fed a common phase I diet and showed a numerical improvement in gain (P < 0.20) when introduced to PFB in the phase II diet.
Intake of phase II diets containing PFB in Exp. 1 (linear, P < 0.01 and quadratic, P < 0.01) and Exp. 3 (P < 0.10) was greater than that of the control diet, suggesting improved palatability of diets with this ingredient.
Overall feed efficiency values did not differ statistically among treatments in any of the experiments. Efficiency was improved in Exp. 3 in phase I but decreased in phase II in pigs fed PFB. Serum urea N concentrations can be used as an indicator of protein nutrition (Coma et al., 1995), and they are inversely related to growth rate or lean tissue accretion (Whang and Easter, 2000). Pigs fed higher CP levels have greater SUN values (Lopez et al., 1994). Because PFB diets had greater CP than the control diet, SUN was determined. Although there was no difference in concentrations in pigs in Exp. 1, a linear increase in SUN was observed in Exp 2; this was likely indicative of excess protein consumption.
In these studies, we selected levels of PFB to include and then supplemented diets with lysine and methionine to meet the requirements. An alternative to the greater CP concentrations would have been to use greater amounts of crystalline AA. In practice, other modifications to diets with PFB might include decreasing the quantity of vitamin and trace mineral mix and salt because the PFB contains these nutrients.
Significant improvements in digestibility of selected AA and numerical improvements in DM, CP, and energy digestibility were observed in diets that contained PFB. This finding perhaps can be accounted for by the small increase in energy content of the 30 and 50% PFB diets, associated with a higher fat content. Increased fat intake tends to increase digestibility by decreasing passage rate through the digestive tract (Mateos et al., 1982).
Another possible explanation for the increased digestibility of the PFB-containing diets is the additional processing that pet food undergoes. Extrusion processing, which is used in more than 95% of dry-type pet food (Campbell et al., 2005), improves energy and N digestibility in young pigs fed sorghum grain, and increases ME of soybeans (Noland et al., 1976). Herkelman et al. (1990) found no change in protein digestibility when young pigs were fed extruded vs. nonextruded yellow corn; however, DE and ME values were greater in diets containing extruded corn (Herkelman et al., 1990).
Based on the results of these studies, we concluded that feeding PFB resulted in performance that was similar to that of pigs fed conventional diets. Pigs in these experiments were weaned at 14 or 21 d of age and fed PFB in phase I or II diets. In none of these experiments was performance inferior to the control groups. It should be emphasized that the PFB used in these experiments was a premium brand with rice and chicken meal as the primary ingredients. Performance by pigs fed other sources of PFB may differ depending on the composition and ingredient content of PFB.
|
Footnotes
|
|---|
1 The authors gratefully acknowledge T. Glaze and S. Hulsey for animal care and technical assistance and A. Nelson for input on statistical evaluation of the results. 
2 Corresponding author: mazain{at}arches.uga.edu
Received for publication April 7, 2005.
Accepted for publication August 30, 2005.
|
LITERATURE CITED
|
|---|
Amos, H. E., J. Evans, and D. Burdick. 1976. Abomasal protein recovery and microbial protein synthesis in wethers fed high and low quality forage diets. J. Anim. Sci. 42:970–976.[Abstract/Free Full Text]
Campbell, K. L., J. E. Corbin, and J. R. Campbell. 2005. Companion animal industry. Chapter 2 in Companion Animals: Their Biology, Care, Health and Management. Pearson Prentice Hall, Upper Saddle River, NJ.
Coma, J., D. R. Zimmerman, and D. Carrion. 1995. Relationship of rate of lean tissue growth and other factors to concentration of urea in plasma of pigs. J. Anim. Sci. 73:3649–3656.[Abstract]
Crane, S. W., R. W. Griffin, and P. R. Messent. 2000. Introduction to commercial pet foods. Chapter 3 in Small Animal Clinical Nutrition 4th ed. M. S. Hand, C. D. Thatcher, R. L. Remillard, and P. Roudebush, ed. Mark Morris Institute, Topeka, KS.
Fenton, T. W., and M. Fenton. 1979. An improved method for the determination of chromic oxide in feed and feces. Can. J. Anim. Sci. 59:631–634.
Hansen, J. A., J. L. Nelssen, R. D. Goodband, and T. L. Weeden. 1993. Evaluation of animal protein supplements in diets of early-weaned pigs. J. Anim. Sci. 71:1853–1862.[Abstract]
Herkelman, K. L., S. L. Rodhouse, T. L. Veum, and M. R. Ellersieck. 1990. Effect of extrusion on the ileal and fecal digestibilities of lysine in yellow corn in diets for young pigs. J. Anim. Sci. 68:2414–2424.[Abstract]
Li, D. F., J. L. Nelssen, P. G. Reedy, F. Blecha, R. Klemm, and R. D. Goodband. 1991. Interrelationship between hypersensitivity to soybean proteins and growth performance in early-weaned pigs. J. Anim. Sci. 69:4062–4069.[Abstract]
Lopez, J., R. D. Goodband, G. L. Allee, G. W. Jesse, J. L. Nelssen, M. D. Tokach, D. Spiers, and B. A. Becker. 1994. The effects of diets formulated on an ideal protein basis on growth performance, carcass characteristics, and thermal balance of finishing gilts housed in a hot, diurnal environment. J. Anim. Sci. 72:367–379.[Abstract]
Mahan, D. C. 1992. Efficacy of dried whey and its lactalbumin and lactose components at two dietary lysine levels on postweaning pig performance and nitrogen balance. J. Anim. Sci. 70:2182–2187.[Abstract]
Mateos, G. G., J. L. Sell, and J. A. Eastwood. 1982. Rate of food passage (transit time) as influenced by level of supplemental fat. Poult. Sci. 61:94–100.[Medline]
Maxwell, C. V., and S. D. Carter. 2001. Feeding the weaned pig. Pages 691–715 in Swine Nutrition. 2nd ed. A. J. Lewis and L. L. Southern, ed. CRC Press, Boca Raton, FL.
Myer, R. O., and J. H. Brendemuhl. 2001. Miscellaneous feedstuffs. Pages 839–864 in Swine Nutrition. 2nd ed. A. J. Lewis and L. L. Southern, ed. CRC Press, Boca Raton, FL.
Nessmith, W. B., J. L. Nelssen, M. D. Tokach, R. D. Goodband, J. R. Bergstrom, S. S. Dritz, and B. T. Richert. 1997. Evaluation of the interrelationships among lactose and protein sources in diets for segregated early-weaned pigs. J. Anim. Sci. 75:3214–3221.[Abstract/Free Full Text]
Noblet, J., and J. M. Perez. 1993. Prediction of digestibility of nutrients and energy values of pig diets from chemical analysis. J. Anim. Sci. 71:3389–3398.[Abstract]
Noland, P. R., D. R. Campbell, R. K. Gage, R. N. Sharp, and Z. B. Johnson. 1976. Evaluation of processed soybeans and grains in diets for young pigs. J. Anim. Sci. 43:763–769.[Abstract/Free Full Text]
NRC. 1985. Nutrient Requirements of Dogs. Natl. Acad. Press, Washington, DC.
NRC. 1986. Nutrient Requirements of Cats. Natl. Acad. Press, Washington, DC.
NRC. 1998. Nutrient Requirements of Swine. 10th ed. Natl. Acad. Press, Washington, DC.
Robson, D. S. 1959. A simple method for constructing orthogonal polynomials when the independent variable is unequally spaced. Biometrics 15:187–191.
Tokach, M. D., J. L. Nelssen, and G. L. Allee. 1989. Effect of protein and (or) carbohydrate fractions of dried whey on performance and nutrient digestibility of early weaned pigs. J. Anim. Sci. 67:1307–1312.[Abstract/Free Full Text]
Whang, K. Y., and R. A. Easter. 2000. Blood urea nitrogen as an index of feed efficiency and lean growth potential in growing-finishing swine. Asian-Aust. J. Anim. Sci. 13:811–816.
Top
Abstract
INTRODUCTION
