Demand Driven Productivity in the Egg Business

Prof. Dr. Dietmar K. Flock Dr. Flock and Dr. Havenstein came from Iowa State University (Animal Bree- ding) and the University of Wisconsin (Poultry Science) when they first met as colleagues at Heisdorf & Nelson Farms, where they were introduced to modern poultry breeding. The authors have followed developments in different parts of the world for more than 50 years and review how progress in genetic potential was combined with improved labor efficiency, disease control and nutrition to produce eggs at least cost for the changing global demand.

To contact the author: Mail at [email protected]

Demand Driven Productivity in the Egg Business: Combining Advances in Genetics, Health Control and Nutrition to Meet Changing Consumer Preferences

Abstract feed efficiency and carcass value in meat- poultry diseases more effectively, based Since seven decades, poultry meat and type chickens, while heterosis effects were on diagnostics, eradication and prophyl- egg consumption in many countries has exploited to maximize egg production, actic vaccination. Modern poultry nutriti- been accelerating at a faster rate than feed efficiency and egg quality in egg- on is based on least cost feed formulation global population growth. In this review, type chickens. A small number of primary for individual flocks to minimize feed cost we will focus on key contributions of ap- poultry breeders continue to improve the per egg in layer stock, and to minimize plied science and technology which exp- genetic potential for efficient production feed cost per unit of meat produced in lain the efficiency of today’s production: of poultry meat and eggs and supply li- broilers, making best possible use of lo- the change from hatch seasons and egg censed hatcheries in many countries with cally available resources, while minimizing production in open houses to hatching parent stock to multiply the improved ef- the environmental impact of production. throughout the year and egg production ficiency. Electronic data management is in environment-controlled houses to sup- used to optimize logistics and the flow of Introduction ply fresh eggs for urban consumers on de- information in a global network of produc- Some developments in our life are easier mand. Initially, simple mass selection was tion and marketing. Results of research in to understand if we look at them in terms applied to maximize juvenile growth rate, poultry pathology are applied to control of evolution. Since Darwin shocked the

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establishment of his time with his theo- the practice of growing about 90 percent rily in small flocks to assure self-sufficiency ry of the descent of man, many excellent of the males for human food slowly de- for eggs and chicken meat for the family books have been published on evolution veloped over time. The next step was to who owned them, and possibly for a small to confirm that evolution is not only the develop management procedures to dis- local market. With increasing urbanization most plausible explanation for past deve- courage broodiness, and to hatch large of societies, some farmers began speciali- lopments, but a continuing process (Wei- numbers of chicks in hatcheries with con- zing in egg production, others in hatching ner, 1994). The same principle holds not trolled temperature and humidity. Initially chicks and selling ready-to-lay pullets, and only for all species of plants and animals the males were generally raised and sold flock size tended to increase when eggs (including man), but also for ecosystems, for human meat consumption, and the could be sold at an attractive price. During human societies, ideas and technical in- females were kept to produce eggs for the 19thand early 20thcenturies, poultry ventions: every mutation or invention human consumption. When egg produc- organizations were founded to promote has to be “better” than previous models tion dwindled to a point where the hens information on the husbandry and pro- to compete successfully against tradition were no longer profitable, they were also duction of poultry meat and eggs as a and the establishment. slaughtered for food production. means of increasing farm income. During the late 1800s and early 1900s, Poultry de- Evidence for the domestication of chi- As documented in recent FAO Statistics, partments were established at agricultural ckens has been reported from China as chickens in recent years have become schools, colleges and universities, often early as 6000 B.C.. Initially, people proba- the most successful domesticated animal combining research with teaching and bly enjoyed seeing chickens near their species in terms of the numbers of ani- outreach or extension programs. dwellings and attracted them with some mals grown (25 billion), as well as in terms feed or left-over food. Shelter and defense of their contribution to human nutrition The USA became the leading country in against predators was not yet essential as (67.3 million tons of eggs/yr and 107.5 mil- efficient poultry production, based on long as they could find an elevated place lion tons of poultry meat/yr). These pro- novel research and the application of to roost at night. In the spring, as the days duction figures may serve as a reference theory to practice. Initially, some breeds grew longer and warmer, each hen would for governments and NGOs concerned were placed under a program, called the produce a “clutch” of eggs. She would then about adequate human nutrition of peo- Registry of Production (ROP) that utilized become broody and set on them. Three ple currently suffering from starvation or individual bird production records obtai- weeks later the chicks would hatch. If peo- malnutrition. The main obstacles to increa- ned by trap-nesting. ROP breeders then ple (or other predators) found and consu- sed poultry consumption are: (1) traditions selected the best performing individuals med the eggs before the chicks hatched, and limited purchasing power in develo- from their trap-nest records to reproduce the hen would try to find a better hiding ping countries; (2) lack of information on the next generation. ROP breeders (like place and lay another clutch of eggs, until the nutritional value of poultry meat and poultry breeders in Europe) were also the end of the season. Centuries later, peo- eggs, compared to other sources of food; required to follow breed standards that ple noticed that hens continue to lay eggs (3) objection to products from intensive required that any birds used for breeding in the same nest, as long as one or more animal farming by consumers who associ- had to be totally free of any defects such eggs are left. Apparently birds can “count” ate low food prices with poor animal wel- as crooked toes, cross-beaks, the wrong and try to lay a typical number of eggs be- fare and excessive use of antibiotics. This color of earlobes, the wrong color of the fore they go broody. review will try to explain key factors which shanks, feathers on the shanks, etc., all of have contributed to and accelerated the which caused some potentially very pro- We will never know for sure, whether peo- development of today’s efficient poultry mising breeders to be discarded rather ple started to eat eggs before chicken industry. than kept to improve the breed’s produc- meat, but they must have known that the tivity. Selection for phenotype did not sex ratio at hatch is about 1:1, and if too How broiler breeding star- increase productivity significantly. Thus, many males survive to sexual maturity, ted on the East Coast of the progress on improving the productivity they will start to fight and kill each other, USA of a breed under that system was extre- until only the most dominant is left. Thus, For many years, chickens were kept prima- mely slow, and when they were making

31 Demand Driven Productivity in the Egg Business

their selections they were not taking into introduce RRS in his White Leghorn lines. pure white feathers and a much nicer ap- account negative genetic correlations bet- Results presented at the European Poultry pearing carcass for the meat-type breeds. ween some of the important traits, such as Conference in Hamburg (Flock, 1980) indi- Eventually, the broiler market also moved egg production, egg size and shell quality. cated that the reciprocal crosses exceeded toward the use of dominant white female For example, improvement in egg number the average of the parent lines by 20% in line breeders and Cornish dominant white generally resulted in hens that produced hen-housed egg production, while the F2 male breeders. The Cornish lines produced smaller eggs. and back-cross combinations were inter- broader breasted and faster growing birds mediate, as expected from theory. Today, that were very attractive to consumers. During the 1940s, Jay L. Lush, I.M. Lerner all commercial egg-type chickens are cros- These white feathered meat-type breeds and a number of their students developed ses between two or more lines, and they rapidly replaced all of the brown-feathered statistical approaches to animal breeding. are vastly superior to the strains that were breeds during the late 1960s. Those statistical systems, combined with used in the first half of the 20th century. the development of computers that could Reducing the number of days to reach quickly analyze thousands of records mo- For a number of reasons, modern meat market weight (3 lbs or 1350 grams) was ved the science of animal breeding for- type chickens have a somewhat different simple and effective with mass selec- ward, and the poultry industry responded history than do egg type chickens. The tion. All birds were individually weighed with the development of a number of ma- primary trait was body weight or juvenile and graded for body conformation, the jor poultry breeding organizations. Kimber growth rate. Other important traits such heaviest males and females retained to Farms in Niles, California, was the first to as feed conversion, and breast shape and reproduce the next generation, and the hire trained geneticists to develop and size were generally improved by selecting majority sold as broilers. Broiler growth run their commercial breeding programs. for body weight. Body weight at a given rate has a high heritability, and mass selec- Initially those programs concentrated on age was highly heritable, so one could tion reduced age at market weight by one finding the best strains of white egg or simply choose the fastest growing males day per year. In subsequent years, primary brown egg chicks, and the breeding stock and females for reproducing the line. Thus broiler breeders invested more into their for those “pure” strains were then selected simple mass selection was initially used to breeding programs: large numbers of fully by utilizing the heritability for each trait, improve most of the meat lines. pedigreed chicks were reared to estimate the genetic correlations among the traits, heritabilities and genetic correlations, and and the economic value of each trait to Initially, males from some of the brown-egg family information was used to estimate improve all of the most economically im- breeds were used for meat production, be- breeding values more accurately. Eventu- portant traits simultaneously. cause those breeds were heavier and faster ally specialized male and female lines were growing than the white egg breeds. Seve- selected on different selection indices to It was not until 1947, when the Heterosis ral breeders from the New England area combine divergent traits in the best pos- Conference was held at Iowa State Uni- of the USA rapidly became the suppliers sible cross. versity in Ames, Iowa, USA, that crossbree- of brown feathered meat-type chickens. ding came into the limelight. Some of the Nichols Poultry in New Hampshire had When negative genetic correlations bet- breeding companies took the information nearly 90 percent of the meat bird mar- ween juvenile growth rate and reproduc- from that conference to begin breeding ket from the mid-1950s to the early 1960s. tive efficiency became a limiting factor for programs to utilize the improved perfor- Most of the brown-egg breeds also had further progress, specialized sire and dam mance of strain crosses. As the authors brown feathers, and some of the brown lines were selected for different perfor- learned during their training at Heisdorf & pigment stayed in the skin when the fea- mance profiles, combining acceptable egg Nelson Farms, Art Heisdorf was among the thers were removed. Because of that, the production of the female parent with maxi- participants of the Conference in Ames, whole dressed birds that were being sold mum growth rate and carcass value of the and he became fascinated by a paper on were not very attractive. Therefore, during male parent. During the 1970’s the broiler recurrent reciprocal selection (RRS), which the 1950s, geneticists began developing breeding industry had to respond to three was subsequently published by Comstock synthetic breeds that carried the domi- different customers: hatchery managers et al. (1950). The theory convinced him to nant white feather gene which produced wanted to get as many day-old chicks as

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From seasonal to year- round supply of eggs In North America and Europe, egg production used to be more or less seasonal: chicks would hatch in spring, pullets start to lay 5-6 months after hatch, and winter pauses of variable length and intensity resulted in shortage of eggs and correspon- dingly high egg prices. Producing more eggs around Christmas time was highly desirable. To extend the availability of eggs to full-year, egg producers started to introduce lighting programs and placed flocks Figure 1: Average production index by years in the UNIVERSITY OF CALIFORNIA production- twice a year, making double use of their bred flock. the flock had an increase in incidence of lymphomatosis in 1937-38. The slope of rearing facilities. With unlimited availability the least squares` regression line is 5.6 eggs per year. Source: Lerner and Hazel (1947) and less fluctuation in egg prices, people bought eggs more regularly and included possible out of the parent flocks, at least Meanwhile, Mendel’s Laws of inheritance them in their diet as a convenient and 140 within 40 weeks of production; proces- had been rediscovered, and geneticists attractive part of their diet year round. sing plant managers wanted uniform body at the University of California used the Research into the mechanisms of light sti- weight and the highest possible breast experimental White Leghorn flock of the mulation and the response of chickens to meat yield; and broiler growers wanted ra- Poultry Science Department to demons- increasing, constant and decreasing day pid weight gain, efficient feed conversion trate in a series of selection experiments length and light intensity led to the deve- and minimal mortality. Instead of trying that egg production can be improved by lopment of specific recommendations for to combine all these goals in one superior selection. Egg production was a perfect lighting programs to optimize onset of lay cross, breeding companies started to de- example to teach students decision ma- and persistent egg production. veloped different “models” simultaneously. king when applying quantitative genetic Results of cumulative selection have been theory for breed improvement: the ma- From eggs “as laid” on the documented by Havenstein (2006), details jor trait to be improved was full-year egg farm to graded eggs and of modern broiler breeding by Laughlin production, which could be measured egg products (2007) in Lohmann Information. only in females and required trap-nes- Modern consumers like “one-stop-shop- ting for many months; progeny-testing ping,” and they buy eggs increasingly How modern breeding of of males would require pedigree ma- from discounters. Discounters try to maxi- laying hens started in Cali- tings and extend the generation interval; mize their profit with a strategy of buying fornia egg production of the hens would be af- a large volume of eggs from a few sup- Before broilers were “invented” as a more fected by broodiness, winter pauses and pliers at the lowest possible price. They attractive source of poultry meat, it was mortality; disease risks and variable feed then optimize logistics to keep all outlets common to grow cockerels from dual quality affected egg production and supplied with quality eggs at amazingly purpose breeds for meat, killing them at made comparisons across years difficult. low prices. Consumers asking how eggs an age when they reached a desirable Despite these difficulties, the selection can be produced so cheap without com- weight. Separating male and female chicks experiment at the University of California promising hen welfare and/or egg quality at hatch (chick sexing) became important (Fig. 1) produced convincing results (Ler- are surprised if they are told that egg pro- when interest focused on pullets for egg ner and Hazel, 1947), and poultry com- ducers seldom get more than half of the production, while cockerels became “by- panies invested in large-scale breeding retail price – unless they invest additional products”, which no longer could recover programs to benefit from the growing time to sell a small volume on the open the feed cost if grown for meat. demand for eggs. market. Discounters also like “one-stop-

33 Demand Driven Productivity in the Egg Business

shopping” when they load their big trucks not always materialize. In a free market, initially, the success of the vaccine resulted at packing stations. egg prices drop below production cost, if in a great reduction in mortality and a lar- production exceeds demand. The industry ge increase in numbers of healthy layers Getting eggs efficiently from the farm to may try to get help from the government, resulting in overproduction of eggs and the consumer is a logistic challenge, in- e.g. in the form of subsidies for export, very low egg prices. It took several years cluding the collection, grading and stam- but if diseases threaten the industry, avian for the industry to scale back the number ping of each egg to identify the country, veterinarians and producers have to join of layers so that production was back in production system (cage, barn, free range, forces to utilize the latest scientific informa- line with the demand for eggs. Disease organic) and a farm code. Most shell eggs tion to safeguard the industry in practice. risks are minimized today with a combina- are sold in the intermediate weight range, tion of bio-security, prophylactic vaccinati- for example grades M (53-63 g) and L (63- Major problems for both the poultry bree- on and monitoring of the health status of 73 g) in EU countries. At the beginning of ding industry and for commercial egg our industry flocks. Of major concern to- the laying period, most pullets start with producers in California included respira- day, due to animal welfare requirements, an egg weight below 53 g, and egg size tory diseases (CRD) and, mortality due to is the control of feather pecking and can- increases with age and toward the end of Marek’s disease during pullet rearing and nibalism without beak treatment and/or the laying period the shell quality of those Leucosis during the laying period. During reduction of light intensity. XL eggs tends to become a problem. the 1960s, several breeding companies tried to reduce Marek’s mortality by selec- Combined efforts of nutri- Eggs that are outside of the “marketable” tion, using genetic markers (B21 haploty- tionists and geneticists to weight range are generally sold to brea- pes) or exposure of pullets to infection on reduce feed cost king plants, along with stained eggs and farms with previous history of high morta- Feed accounts for about 60% of egg pro- eggs with shell defects, but they generally lity (Beaumont et al., 2003) along with se- duction cost and is getting major attention will not recover their feed cost. Breaking lection for the most resistant families. With whenever focus is on profitability. Based on plants are a special part of the egg indus- the development of Marek’s vaccines and scientific studies on nutrient requirements try, focused on growing demands by the widespread use in commercial hatcheries of different species of poultry, nutritionists food processing industry, e.g. bakeries, during the 1970s, these major risks for egg can formulate “least cost” feed for any flock restaurants, food service units, etc.. Some producers were pretty well controlled. But of laying hens, taking the prices of available modern breaking plants are connected directly to large egg production units that process eggs from millions of laying hens “online”. Countries like Brazil, with relatively low energy prices, use the egg breaking industry to their advantage to increase their share of the world market for dried egg products, whereas the trade of shell eggs depends more on regional feed prices and differences in poultry welfare requirements.

Risk management with contributions from avian research The growing demand for eggs and poultry meat encourages investment in large units with labor-saving equipment, but the expected return on investment does Fig. 2: Combined effects of genetic selection and management to improve feed efficiency

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feed components and the production level gulations against beak treatment, primary riation in commercial lines to select for of the hens into account. For many years, breeders have selected against these be- adaptability to different requirements, and it had been assumed that laying hens fed haviors since many years. Egg producers management recommendations “from a balanced ration ad libitum will eat just appreciate the experience and advice hatch to end of lay” help to translate gene- enough to satisfy her nutrient needs for from independent sources like random tic potential into efficient egg production. maintenance and egg production. If this sample tests. were true, nutritionists could focus on feed A continuing challenge for poultry bree- formulation to meet nutrient requirements Sustainable breeding of ders in Europe is the strong position of at minimal feed cost per ton. In the 1970s, egg-type chickens: co-evo- ethicists and animal welfare organizations. when egg prices were low and feed prices lution with human societies When national or regional politicians use high, geneticists started to select more Several authors have addressed sustaina- the term “sustainability” in the context of intensely for improved feed efficiency: bility in this publication before: Hodges agricultural policies, they are more likely to instead of simply weighing hens to estima- (2007) called attention to the importance focus on emotions of potential voters than te maintenance requirements, individual of family farming in developing countries on the global food market. Poultry bree- feed intake was measured to determine “re- and criticized negative effects of intensi- ders in Europe have the “benefit” of getting sidual” feed consumption and “egg income ve animal farming on rural employment; this public pressure first-hand, while inter- over feed cost”. Analysis of records from Eberz (2009) looked at the increasing national customers of parent stock expect pedigreed hens confirmed that hens with demand for food, feed and energy of a continuous supply of genetic potential comparable body weight and egg mass growing world population, with focus on for economic egg production under their production differ more in daily feed intake research and development to safeguard conditions. than previously assumed. Industry data yields, produce high-quality crops and and results of random sample tests con- manage resources efficiently; and Flock Genetic theory is applied in large popula- firm that the efficiency of feed conversion (2009) recalled the history of breeding tions to maximize genetic progress, and has been substantially improved since the egg-type chickens, with increasing atten- changes in the performance profile of spe- 1970s: from about 3 kg to 2 kg feed per kg tion to sustainability in terms of “quality cific crosses can be predicted with high egg mass (Flock and Heil, 2002; Anderson of life for egg consumers and producers accuracy. The productivity of commercial et al., 2013). At the same time, we have also worldwide, with appropriate attention to layers has been doubled within less than learned a lot more about the importance bird welfare and natural resources”. 50 years, from about 150 to more than 300 of house temperature, water intake, feather eggs per year; and the efficiency of feed cover, feed structure and changing nutrient Geneticists in the poultry industry, trained conversion has been improved from 3 to 2 requirements with age and time of the day. to think in terms of long-term response to kg feed per kg egg mass. Most of this im- selection, review the breeding goals perio- provement has been generated with the As recently documented by Damme and dically, ideally before each selection cycle, introduction of systematic cross-breeding, Hildebrand (2015), based on results from to respond to short-term requirements. followed by within-line selection. Large random sample tests at Kitzingen, feed ef- For example, selection on feed efficiency populations assure that the inevitable loss ficiency has been continuously improved started in response to suddenly rising feed of genetic variation due to intensive selec- during the last 50 years, but also depends prices in the mid-1970s.;With single cage tion is kept to a minimum. also on the management system (Fig. 2):. management of pedigreed populations this selection was apparently successful Experimental results to With floor management, behavior traits and contributed to the increasing market quantify effects of hetero- (nesting behavior, tendency to develop share of brown-egg layers in Europe. Floor sis and inbreeding feather pecking and cannibalism) have management, on the other hand, requires Primary breeding companies have a become more important. Comparisons sufficient “feed intake capacity” to meet wealth of internal data on pure-line and across years confirm that strains differ in the nutritional needs in the critical period cross-line performance of their commer- their tendency to develop feather pecking from sexual maturity to peak production. cial and experimental lines. To illustrate and cannibalism. In anticipation of EU re- Fortunately, there is sufficient genetic va- heterosis effects which may be found in a

35 Demand Driven Productivity in the Egg Business

ly during the last 50 years and will continue to change. The migration of rural people to cities continues in most countries, and there will be more people looking for eggs as a relatively cheap source of high quality food. According to FAO statistics, 107,500 Mio t of poultry meat and 67,300 Mio t of eggs were consumed in 2013. It seems reasonable to assume that the de- Table 1: Estimates of heterosis (h, h’), recombination (r) and selection (s) from a population mand for eggs will continue to increase at under long-term RRS (Flock, 1980) a faster rate than the growing world po- superior cross, the results in Table 1 were lowing table 2 indicate a “loss” of heterosis pulation. Eggs could contribute to better presented at the European Poultry Con- compared to the earlier study, which was nutrition of millions of people currently ference in Hamburg, 35 years ago. In this explained as a result of improved pure-line suffering from starvation or malnutrition. case, the two parent lines (P) and recipro- performance. The poultry industry could quickly provide cal single crosses (F1) had been directly all necessary input for efficient local egg compared with samples of all four double To remain successful in an ever changing production, if governments or NGOs pro- crosses (F2) and all eight backcrosses (BC) and highly competitive world market, pri- vide the necessary funding. Governments under identical conditions, in single cages. mary breeders continuously monitor the in developing countries trying to impro- Highly significant heterosis effects were genetic variance in their lines. The focus ve the nutrition of pre-school and school found for hen-housed egg production and of genetic improvement will remain on children should be aware of the fact that egg mass. As expected from theory, about efficient production of “saleable” eggs, but eggs would be a relatively cheap source of one-half of the superiority of the cross was details of the performance profile have valuable nutrients (Yalçin and Yalçin, 2013). lost in the F2 and BC combinations. The changed and will continue to change. The loss of heterosis in subsequent crosses application of genomic selection is expec- Summary and Conclusions assures that franchize hatcheries buy new ted to contribute significantly to continued In this review, we recall key factors which parents every year instead of breeding progress in all traits of economic signifi- helped the egg industry to achieve its their own replacements. cance, and primary breeders of egg-type strong position in the global food market: chickens can offer a wider range of different (1) the use of lighting programs in environ- The rate of inbreeding in these lines was crosses to meet specific regional demand. ment-controlled houses had two advanta- analyzed from 24 generations of pedigree ges: egg consumption increased, when records, and inbreeding effects were com- Changing lifestyles fresh eggs became available throughout pared with heterosis by regression analysis The lifestyles and preferences for different the year, and egg production cost decrea- (Flock et al. 1991). The results in the fol- sources of food have changed considerab- sed due higher annual egg production per hen and better utilization of facilities; (2) separate selection of meat-type chickens with focus on rapid juvenile growth and egg-type chickens with focus on high egg production accelerated genetic progress in opposite directions, for meat vs. eggs; (3) family selection in pedigreed populations confirmed quantitative genetics theory and generated predictable progress in egg production; (4) theories to explain the

Table 2: Effects of inbreeding and heterosis on reproductive traits and production to 44 phenomenon of hybrid vigor (heterosis) weeks of age in commercial lines under long-term RRS (Flock et al. 1991) stimulated the development and applica-

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tion of cross-breeding schemes to maxi- Flock, D.K. (2009) A history of layer breeding mize the utilization of general and specific in Cuxhaven: from serendipity to sustaina- combining ability of two or more lines; bility. Lohmann Information 44(2): 9-15. (5) based on research in avian pathology, common poultry diseases were controlled Flock, D.K. (1980) Estimates of heterosis with improved diagnostic tools, eradica- in a population of White Leghorns under tion of egg-transmitted disease agents long-term RRS (in German). Proc. European and prophylactic vaccination; (6) based on Poultry Conference, Hamburg, Vol. II, 57-63. nutrition research and least cost feed formulation from available components, the Flock, D.K., H. Ameli and P. Glodek (1991) impact of rising feed cost has been mini- Inbreeding and heterosis effects on quan- mized; and (7) global communication and titative traits in a White Leghorn popula- logistics assure the supply of parent stock tion under long-term reciprocal recurrent and exchange of information between pri- selection. Brit. Poultry Sci. 32: 451-462. mary breeders, multipliers and egg producers to satisfy world-wide demand. Flock, D.K. and G. Heil (2002) A long-term analysis of time trends in the performance Literature profile of white-egg and brown-egg hy- Anderson, K., G.B. Havenstein, P.K. Jenkins brid laying strains based on results of of- and J. Osborne (2013) Changes in com- ficial German random sample tests from mercial laying stock performance, 1958- 1974/75 to 1997/99. European Poultry Sci- 2011: thirty-seven flocks of the North ence 66: 1-20. Carolina random sample and subsequent layer performance and management tests. Havenstein, G.B. (2006) Performance chan- World Poultry Sci. J. 69: 489-513. ges in poultry and livestock following 50 years of genetic selection. Lohmann Infor- Beaumont, C., G. Dambrine, A.M. Chaus- mation 41 (2): 30-37. see and D.K. Flock (2003) Poultry Genetics, Breeding and Biotechnology. Chapter 20 Hodges, J. (2007) Sustainable agriculture in W.M. Muir and S.E. Aggrey (Ed.). CABI and food at risk. Lohmann Information Publishing. 42(2): 3-9.

Comstock, R.E., H.F. Robinson and P.H. Har- Laughlin, K. (2007) Poultry genetics – an- vey (1949) A breeding procedure desig- ticipating the industry requirements. Loh- ned to make maximum use of both gene- mann information 42(2): 10-13. ral and specific combining ability. Agron. J. 41: 360-367. Lerner, I.M. and L.N. Hazel (1947) Populati- on genetics of a poultry flock under artifi- Damme, K. und R.A. Hildebrand (2015) Lege- cial selection. Genetics 32: 325-339. hennenhaltung und Eierproduktion. Eugen Weiner, J. (1994) The Beak of the Finch. Evo- Ulmer, Stuttgart. ISBN 978-3-8001-8066-0. lution in Real Time. A.A. Knopf, New York.

Eberz, G. (2009) Agriculture needs inno- Yalçin, Songül and S. Yalçin (2013) Poultry vation and a sense of responsibility: chal- eggs and child health – a review. Lohmann lenges facing sustainable agriculture. Loh- Information 48(1): 3-14. mann Information 44(2): 3-8.

37 Notes