Effects of genotype on nutrient utilisation in boilers

Transcription

1 130 Effects of genotype on nutrient utilisation in boilers R.A.E. Pym Department of Farm Animal Medicine and Production, University of Queensland, St Lucia, Qld, 4072 Australia Summary The present rapid change in broiler genotypes in Australia as a result of importation by the major meat chicken companies has created both problems and opportunities for the industry. The unquestionably greater egg production of the imported broiler dam lines is countered to some extent by problems with Marek s Disease and ascites susceptibility in the breeders and broiler progeny, and more needs to be learned about their nutrient requirements in the Australian climatic, nutritional and disease environment. The paper presents data from a series of studies aimed at evaluating nutrient requirements in both imported and Australian-bred commercial broiler lines using information on the genotypes obtained by growth modelling. Studies of experimental lines have shown that protein turnover and hence the net efficiency of protein utilisation, can be influenced by selection, and other studies have revealed differential response of the commercial broiler genotypes to dietary amino acid allowances. Dietary amino acid allowance, particularly in the latter part of the growth period, would appear to have a significant influence on breast meat yield. This has ramifications for closely following growth-related requirements for amino acids from hatch to slaughter. Introduction It is generally acknowledged that Robin Cumming has made an enormous contribution to the Australian Poultry industry during a career spanning more than 35 years. What has made Rob s contribution unique has been his great depth and breadth of understanding of poultry science across a wide range of disciplines including all aspects of bird health and disease, nutrition, physiology, husbandry and management, his commitment to application of his research by industry, and his impressive capacity for problem solving within industry. His earlier ground breaking work on Infectious Bronchitis, resulting in the development of effective vaccines and in effective therapeutic measures for infected flocks, has resulted in huge benefits to industry, and Rob is acknowledged as one of the world authorities in the area of IB-nephritis. Additional to this core research work, Rob has been involved on his own and with his postraduates in a wide range of studies including protein supplements, pullet management, mycotoxins, biotin and FLKS, competetive exclusion and salmonella infections, polypeepers and behaviour, nesting behaviour, choice feeding, nutritional control of coccidiosis, and control of Newcastle Disease in village chickens in South East Asia. It is thus a great honour for me to have been asked to present this paper to pay tribute to Rob s contribution to Recent Advances meetings over the years. Apart corn pleasure at the request, I was initially somewhat bemused as to the choice of a now disenfranchised poultry geneticist for the role. I can only assume that I was chosen because of my somewhat similar, though less spectacular involvement in poultry science across a number of disciplines. In a somewhat similar role to Rob s past involvement, I cover a number of disciplines in my lectures to Veterinary Science and Agricultural Science students and in my own and my postgraduates research. Poultry seem to be unusual in this regard, most other appointments in Animal Science Departments tend to be discipline based, often occupied by people with little expertise, or for that matter, interest, in chooks. As a result, an across-discipline chook-person appointment is indicated. The real challenge (for the appointee) is to gain the interest of the undergraduate students who traditionally regard chickens as insignificant, well down the peck order of the livestock species and generally a source of jokes rather than a species or industry deserving of serious consideration. In this regard, I was no exception, as Rob will testify, and he took an amount of vicarious pleasure in my appointment Recent Advances in Animal Nutrition in Australia 1997 University of Ne w England, Armidale NSW 235 I, Australia

2 Effects of genotype on nutrient utilisation in broilers 131 on graduation as a poultry genetics researcher in the NSW Dept ofagriculture. In choosing a topic for this paper, I was concerned to choose one that reflected our mutual interests and concerns relating to the poultry industry and, on reflection, decided to address the issue of the recent large increase in importation of overseas layer and broiler genetic stock into the country, the consequent effects of such on our egg and chicken meat industries and the need for research support.. Importation of broiler and layer genetic lines The recent decision by the major poultry companies to, in many cases, discontinue breeding programs and import genetic stock from overseas, has had a number of major effects. The uptake of these new strains by the egg and chicken meat industry has been rapid and has gone ahead before a thorough evaluation has been made of the potential and problems of the new genotypes under Australian conditions. In the layer industry, it is now evident that despite their high egg laying potential, many of the new strains have a much greater susceptibility to Mareks Disease and some are more prone to cannibalism than the Australian strains they are replacing (Nolan et al. 1997). There is an indication that these strains are generally more susceptible to stress from a wide range of causes than their Australian-bred counterparts. Egg size is also too large for the Australian market and much needs to be learned about their nutritional requirements in the climatic, feed ingredient, management and housing environments here. Prior to importation of overseas stock, the juvenile growth performance in terms of growth rate, feed efficiency and leanness of the top Australian commercial strains of broilers was among the best in the world. This arose from the inclusion of feed efficiency as a selection trait in some Australian breeding programs much earlier than in overseas programs. Reproductive performance was, however, generally neglected, and overseas strains were demonstrably better in this regard and this was probably the main initial incentive for importation. It also became apparent that some overseas strains at least had significantly higher breast meat yield than the best of the Australian strains. Given the relative price of breast meat in comparison with other cuts, there was considerable incentive to produce birds with high breast yield. The Australian-bred meat chicken at present is rapidly being replaced with overseas-bred stock. The Cobb bird has been used exclusively by Biaida for some years, the Steggles bird is now either a Steggles male X Ross female crossbred or a Ross X Ross bird, Inghams have stated their intent to discontinue breeding and instead import the Cobb bird., and Bartters now use the Lohmann broiler. Problems are now being experienced in these new strains with increased Marek s Disease susceptibility in the breeders, and with increased ascites susceptibility and less than optimal feed efficiency and carcass leanness in the broiler progeny. The cost of breeding programs is high, and needs to be defrayed by the sale of large numbers of commercial broilers. The Australian domestic market is very small in world terms and with shrinking profit margins, four or five local breeders, and essentially no export market, most companies over a period of time made the decision to discontinue breeding. However, given the outstanding performance of some of the Australian nucleus lines in juvenile growth performance, it is perhaps surprising that an export market for breeder stock was not pursued more vigorously. For some time now Rob has publicly expressed concerns about the curtailment of poultry breeding in this country and the rapid replacement of Australian stock with overseas-bred birds before thorough evaluation of their performance and problems. I share his concerns, more particularly about the loss of unique genetic material, for which, on the basis of past practice, cornrnercial industry has demonstrated scant regard. The down side to all of this is the inexorable contraction of the world s poultry genetic base. If and when problems are encountered in future with undesirable correlated responses in disease susceptibility or physiological adaptation, it will be harder and harder to find suitable alternative genotypes for replacement or incorporation. We have, however, essentially been presented with a fait accompli by the poultry companies here in Australia, and it serves no purpose to rail against the decisions taken regarding importation. We must assume that these have been based on good biological and economic information, at least from their perspective. What is required is a proper evaluation of the available strains so that bird performance can be optimised and problems identified and, where possible, overcome. There is work presently underway at UNE and at QPRDC to evaluate the responses and requirements of the different layer genotypes, and some work is also underway with broiler genotypes at UQ. It is about the latter work that I would like to direct the discussion of this paper. Before so doing, it would be appropriate to provide some background to past selection practices in broilers both here and overseas. Selection in commercial broiler breeding programs Selection in commercial broiler breeding programs continues to put pressure on growth rate, but in more recent times greater emphasis has been placed on feed efficiency and body composition in the growing bird

3 732 Pym, R.A.E. and on reproductive performance in the breeder. In selecting for feed efficiency, birds in the sire lines pre-selected on growth rate to about 21 days are placed in single cages with individual feeders, and growth rate and feed efficiency are measured to typical slaughter weight. A selection index approach is frequently used which optimises economic response with respect to liveweight at a given age and food intake, both measured and estimated. One desirable correlated response to such selection is a decrease in body fatness. Birds pre-selected on growth and efficiency are then subjected to selection against leg and skeletal problems and for increased breast meat yield by a range of techniques. Selection for ascites resistance may also be applied at this point, although there is some question at this stage as to the efficacy of the different available techniques, particularly in light of the antagonistic effect of selection for increased breast meat yield. Dam lines are typically selected for growth rate in the young birds and reproductive performance in the adult breeders, using part-period selection. Selection in the young birds can also incorporate some emphasis on leanness using either abdominal fat calipers or plasma very-low-density lipoprotein concentration as the selection trait. In both sire and dam lines, selection is also exercised for general fitness, deformities, conformation and feather colour etc., using either individual or family selection. A comprehensive listing of selection criteria in commercial broiler breeding programs was given recently by Hardiman (1996). Genotype differences in nutrient utilisation It is now well recognised that there are differences in the nutrient requirements of different commercial broiler stock. In some cases such differences can simply be ascribed to gross differences in growth potential between the strains, but evidence is now emerging of strain differences in the net utilisation of nutrients. Feed utilisation efficiency is the summation of the efficiency with which all the nutrients are utilised. When expressed in its usual form as the feed conversion ratio or FCR, it is a gross measure of efficiency and selection for this trait places emphasis on the efficient use of food for both maintenance and growth. This was nicely demonstrated in studies of energy metabolism and protein turnover of lines produced in a selection experiment where selection was exercised for high growthrate (line W), food intake (line F) or feed efficiency (line E). An unselected control line was maintained (line C) (Pym, 1985; 1990). As shown in Table 1 the E line selected for high feed efficiency (low FCR) had the lowest FCR, the least amount of body fat, the highest metabolisability of dietary energy, the highest net availability of metabolisable energy for gain, and the lowest fractional rate of protein breakdown of all the strains, and along with the W line, the lowest maintenance energy requirement,. The finding relative to protein breakdown is important since the protein breakdown rate determines the net efficiency of protein utilisation (Tomas et al ). Thus selection for feed efficiency has profound effects on nutritional physiology that are not achieved through selection for increased growth rate alone. For this reason commercial broiler breeders began to include direct selection for feed efficiency during the 1980 s, and some quite remarkable responses have been achieved in some lines. It is now commonplace for commercial broilers to achieve an FCR of less than 1.7 at an average liveweight of 1.8 kg, and for selected lines under good conditions to achieve efficiencies of better than 1.5 to the same Iiveweight. Because of genetic differences in founder populations and dlffererent selection histories in the lines, however, there is significant variation in growth body composition and conformation, and nutrient utilisation between the different commercial strains of broiler. Table 1 Components of energy and nitrogen metabolism in lines of chickens selected for increased growth rate (line W), increased appetite (line F), increased feed efficiency (line E), or at random (line C). Standard errors in Parenthesis (from Pym, 1990).

4 Effects of genofype on nutrient utilisation in broilers 133 Broiler growth models In order to achieve excellent growth and feed efficiency and to do so in a cost-effective manner, it is important to know the nutrient requirements of the strain of bird in question and to meet those requirements in an appropriate balance. Apart from genotype, the physical environment influences nutritional requirements and the complexity of interacting factors demands the use of sophisticated growth models to accurately identify requirements under different specified conditions. In 1995 in association with Professor Rob Gous from the University of Natal, we conducted a study to characterise the major Australian broiler genotypes for purposes of performance prediction using the Gous growth model (Gous et al. 1996), and are presently undertaking further experiments to validate the predictions of the model. Growth-related changes in nutrient requirements One of the outputs of the Gous growth model identifies the predicted amino acid requirements of the strain and sex in question under the specified conditions. For most amino acids there is a very rapid decline in requirements, expressed as a proportion of the diet, from hatching through to slaughter weight. Whilst account is taken of this general trend in the use of starter, grower and finisher diets, such allowance only crudely approximates the birds actual requirements, and nutrients are in under- or over-supply for much of the time. To effectively accommodate this, the diet must change almost daily, but it is essential that the growth related nutrient requirements are accurately known and supplied. Industry presently is looking at the Flockrnan system where a starter diet is progressively diluted with grain. One of the problems with this is that the amino acid profile changes towards that of the grain, which is unlikely to be ideal. A summit-dilution approach is unquestionably indicated, but one in which both the summit and dilution diet truely reflects the nutrient requirement of the birds at day-old and slaughter weight respectively. Early in 1996 we undertook a study with two commercial broiler strains in which the birds were offered 2-, 4- or 8-diet regimens from hatching to 40 d of age. In the case of the 2-diet regimen the flust diet given to them from hatching to 20 d of age was formulated corn information from the Gous model to meet their 10 d old requirement for amino acids (i.e. midway). The second diet given from 20 to 40 d of age was formulated to meet their 30 d old requirement. In both cases the diets were compounded by mixing appropriate proportions of of a summit (day-old) diet and a dilution (40-day-old) diet. In a similar manner the diets for the 4- and 8- dietary regimens were prepared. In the latter case the diets were changed every five days. The experimental design is illustrated schematically in Figure 1. Birds were reared to 20 days in replicated line X sex X dietary regimen groups in brooder cages and transferred to single bird cages at 20 days of age. Unfortunately, growth rates were depressed in all groups on transfer to the single cages, presumably in response to the physical environment, and subsequent daily gains did not exceed 60 g per day in the males and 50 g per day in the females, well short of their potential. As a possible result of this there were no significant differences in growth rate or FCR between the dietary regimens, but differences between the dietary regimens were observed in 4O-day breast meat yield. As shown

5 134 Pym, R.A.E. in Table 2, in three out of the fourline X sex groups, the of 140,160 and 180 g crude protein/kg for the males and birds on the 2-diet regimen had significantly greater 120,140 and 160 g/kg for the females. The crude protein breast meat yield than those on the eightdiet regimen. levels are included as being indicative of the relative A likely explanation for this is that the birds on the concentration of amino acids in the dilution diets. The 2-diet regimen were receiving considerably greater day-old surnmit diet used in each regimen contained amounts of dietary amino acids in the last lo-days 247 g crude protein /kg (16.8 g lysine/kg). The growth prior to slaughter than those on the 8-diet regimen, response of the males in the two strains was quite and that this was reflected in a significantly greater different, as shown in Figure 2. deposition of breast muscle. This fmding requires In strain A, there was essentially a linear response validation in a study where the birds are allowed to to increasing amino acid levels in the dilution diet, but grow at their potential, and we will shortly be repeating in the strain B males there was essentially no the study for this purpose. improvement in growth response f+om the 140 to the In late 1996 in a preliminary experiment designed 180g CP/kg diets. This requires validation but to look at predictions of the Gous model based on the indicates that growth-related nutrient requirements earlier strain characterisation exercise, we included may differ very significantly between the different two commercial strains in a deep litter pen study commercial genotypes. The response of strain B in involving three dietary regimens wherein amino acid this study did not accord with predictions based on the levels were reduced weekly to arrive at 42-day levels Gous model. We are conducting further studies to determine growth-related nutrient requirements of the different genotypes and the accuracy of prediction of the model. The need for this sort of work is made more Table 2 Breast meat yield (g/kg) in birds from the two apparent by the fact that there has been, or shortly will strains given 2-, 4-or &dietary regimens. Standard errors in be, some major changes in the genotypes used here by parenthesis. the large companies. Free choice feeding No discussion about Rob Cumming and poultry nutrition would be complete without reference to free choice feeding. Rob has extensively researched the area in both layer and broiler feeding and has been a c... strong advocate... of the commercial application of the technique in both industries and also as an experimental tool in many studies wherein the bird defmes its own nutrient requirements. The classical Figure 2 Livewight gain to 42 days of age in male chickens from the two strains on three dietary regimes.

6 Effects of genotype on nutrient utilisation in broilers 135 application of the technique where the bird is provided with essentially a source of energy, usually a whole grain, a high protein supplement and a source of granular calcium, either in the same or separate feed troughs, has merit even if performance and nutrient utilisation are not improved, due to the cost saving of provision of the un-milled and unmixed grain component. The varied biological response to free choice feeding across numerous experiments suggests that the bird undoubtedly possesses the ability to meet its nutrient requirements, but that there are many factors at play which may impair this. The cost of feed mixing and transport aspect is undoubtedly an important factor in determining the commercial merits of the technique. Free choice feeding has great potential as a research tool in any study attempting to define nutrient requirements. In our studies, we have used a somewhat less purist approach to the exercise than the above, in providing the birds with free access concurrently to both the starter and finisher diet in the strain characterisation study with Rob Gous, or both the summit and dilution diets as an additional treatment in the above comparison of the 2-, 4- and 8-dietar-y regimens. Unfortunately, in the latter study, the vitamin premix was omitted from the summit and dilution diets used in the free choice treatments, which rendered the comparison useless. It is our intention to include a free choice treatment wherever design space permits, in all future studies. References Gous, R.M., Pym, R.A.E., Mannion, P.F. and Wu, J.X. (1996). An evaluation of the parameters of the Gompertz growth equation that describe the growth of eight strains of broiler. Proceedings ofthe 1996 Australian Poultry Science Symposium 8,174-l 77. Hardiman, J. W. (1996). Genetic advancement technology for the primary broiler breeder of the year Proceedings of the 10m Australian Poultry and Feed Convention. Melbourne, October pp Australian Poultry Industries Association. Nolan, J.V., Roberts, J.R., Thomson, E., Ball, W. and Cumming, R.B. (1997). Comparison of performance of three imported and two Australian layer strains on three diets. Proceedings of the 1996 Australian PouZby Science Symposium 9, 161-l 65. Pym, R.A.E. (1985). Direct and correlated responses to selection for food efficiency. In: Poultry Genetics and Breeding, pp. 97-l 12 (Eds. W.G Hill, J.M. Manson and D. Hewitt.). British Poultry Science Ltd. Pym, R.A.E. (1990). Nutritional genetics. In Poultry Breeding and Genetics. pp (Ed. R.D. Crawford). Elsevier: Amsterdam. Tomas, F.M., Pym, R.A.E. and Johnson, R.J. (1991). Muscle protein turnover in chickens selected for increased growth rate, food consumption or efficiency of food utilisation: Effects of genotype and relationship to plasma IGF-I and growth hormone. British Poultry Science 32, Future studies There will be a continuing need to effectively identify nutrient and general management requirements of the newly imported strains of layers and broilers. Undoubtedly an amount of the the broiler work will be conducted in house by the companies themselves, but there is a real need to properly evaluate the capacity of growth models to accurately predict both the nutrient requirements of the different genotypes and the performance outcomes of the stock under specified management and nutritional inputs. The complexity and scope of a study of this nature suggests the need for public sector research inputs, to properly address the issue. As indicated from the outcomes of a recent Egg Industry Research and Development committee sponsored workshop, industry is very keen for the nutritional and management requirements of imported layer genotypes to be comprehensively studied, to build on work already conducted at UNE and at QPRDC.