Primary broiler breeding--striking a balance between economic and well-being traits.

Primary breeders are well aware that selecting for better health and well-being along with economic traits such as faster growth rate, higher levels of meat yield, and improved efficiency of feed utilization are critical to balanced long-term genetic progress of their pure lines as well as to increased production efficiency of broiler products for the broiler industry. Cobb collects and selects on over 50 phenotypic observations per pedigree candidate at various ages. Over 50% of these collections are involved with evaluation of each bird's health, welfare, and fitness. Some examples of these traits are various chick defects, various broiler age skeletal and leg abnormalities, feather cover, various physiological measures of heart and lung functions, and specific causes of mortality. Large pedigree populations, massive data collection infrastructure, integration of better technologies in evaluation of phenotypes, and sophisticated data analysis capability have allowed geneticists to perform selections that are balanced for both economic and welfare traits. Cobb's internal as well as worldwide sponsored research has facilitated geneticists to make science-based breeding decisions. Each pedigree line per product available to primary breeders exhibits their own unique characteristics that are enhanced by selective breeding and positioned in special mating schemes to produce the product and welfare performance that our customers demand. Additionally, most if not all primary breeding companies now offer different products for different markets that exhibit varying levels of performance and behavior to fit customer needs. Future expansion of these products and creation of new products by breeding companies will be in large dictated by both our customers and consumers.

Bone mineralization in male commercial broilers and its relationship to gait score.

Broilers selected for increased body size and breast muscle have imposed stress on the skeletal system, resulting in poorer walking ability. Our objectives were to determine the relationship between bone mineralization and gait score in 4 crosses of commercial broilers and to ascertain if mineralization of the toe is correlated to the tibia. Three chickens per pen each with good (gait score of 0 or 1) or poorer (gait score of 3) walking ability were killed and weighed. The left humerus, the left middle toe, and both drumsticks were collected for determination of bone mineral density (BMD), bone mineral content, and bone size traits using dual-energy x-ray absorptiometry. The BMD and bone size traits were similar among the 4 crosses of commercial broilers at 6 wk of age. However, gait scores differed among genotypes, with cross C having better gait scores than crosses A and B, but did not differ from cross D. The bone mineral content and bone size traits did not differ between birds with good walking ability as compared with those broilers of poorer walking ability. However, birds with poorer walking ability had higher BMD (P<0.05) and BW (P<0.001) than males with good walking ability. Within a cross, the correlation between gait score and BMD was not significant except for cross D birds. Broilers of cross D with better walking ability had decreased bone mineralization (r=0.19, P=0.03). The stronger correlation between gait score and BW for all crosses of commercial broilers (r=0.38, P<0.0001) suggests that the low association between gait score and bone mineralization for cross D was mainly due to BW. The BMD of the left toe was correlated to the BMD of the left tibia (r=0.91, P<0.0001) and right tibia (r=0.87, P<0.0001). In conclusion, bone mineralization was similar among crosses of meat-type chickens, and it had little influence on the gait score of male broilers.

Variability in bone mineralization among purebred lines of meat-type chickens.

The variability of bone traits was assessed in purebred lines of meat-type chickens using dual-energy x-ray absorptiometry. Experiment 1 evaluated changes in bone mineralization and size traits of the tibia and humerus in 4 purebred lines from 6 to 24 wk of age. Experiment 2 compared the same traits of the tibia, radius, and ulna of 9 purebred lines at 6 wk of age. Differences in bone traits among purebred lines were apparent in both experiments. Of the 4 purebred lines compared in experiment 1, line C demonstrated the best phenotypic traits relative to bone quality. Even though line C had the longest tibia, one of the largest bone areas, one of the heaviest BW, and one of the highest bone mineral content (BMC) at 24 wk of age, the tibia of line C did not become less dense in mineral as this line of chickens approached sexual maturity as did the tibia of the other purebred lines of chickens. Specifically, its tibial bone mineral density (BMD) showed age-related increases unlike the other purebred lines of chickens, which showed little change in tibial BMD from 6 to 24 wk of age. In experiment 2, all bone traits as well as BW were different among purebred lines (P<0.001). The 2 purebred lines (7 and 8) with the lightest 6-wk-old BW (2,033 and 2,055 g, respectively) had diverse skeletal traits. Birds of line 7 had the lowest BMD (0.1131 g/cm2), BMC (1.05 g), shortest bone length (69.2 mm), and smallest bone area (8.0 cm2); however, the other purebred line low in BW (line 8) showed the opposite trend in that bones from these birds were the highest in BMD (0.1276 g/cm2), BMC (1.38 g), bone length (74.6 mm), and area (9.2 cm2) when compared with all of the other lines. In conclusion, several purebred lines of meat-type chickens expressed large differences in bone traits, suggesting the potential to genetically select birds for increased BMD.

Life cycle changes in bone mineralization and bone size traits of commercial broilers.

Life cycle changes in bone mineralization and bone size traits of the tibia and humerus were evaluated in commercial male and female broilers using dual-energy x-ray absorptiometry (DEXA). Experiment 1 evaluated weekly changes in bone traits from 2 to 7 wk of age, whereas experiment 2 compared the bone traits of 4 strains of commercial meat-type chickens from 4 to 8 wk of age. Birds were restrained without anesthesia, and the humerus and tibia were scanned in vivo. After each scan, individual BW was determined. From the DEXA scans, bone mineral density (BMD), bone mineral content (BMC), as well as bone length, width, and area were determined. Bone mineralization and size traits were analyzed as an analysis of covariance with BW as the covariate. If BW was NS as a covariate, then an ANOVA was used. The BMD reached its peak at 4 wk of age. The BMC of the humerus changed little from 2 to 8 wk of age, whereas tibial BMC increased as the birds aged, especially in males (P < 0.0001). In experiment 1, bone length, width, and area also increased with age (P < 0.0001), with the tibia growing in length at a faster rate than the humerus. In experiment 2, the BMD did not differ among the 4 strains of commercial broilers. Interactions with strain of chicken were also NS, indicating that all strains of chickens responded similarly with respect to age (4, 6, and 8 wk of age), sex, and type of bone (humerus vs. tibia). Coefficients of variation for BMD ranged from 15 to 16%, indicating a potential use of DEXA for selection to improve skeletal integrity. In conclusion, the tibia continued to grow, especially after the initiation of the growth spurt at 3 to 4 wk of age, as indicated by bone length, width, and BMC, but it did not become denser in mineral after 4 wk of age as its surface area increased.

Restricted feeding in early and late-feathering chickens. 1. Growth and physiological responses.

Broiler-breeder females from a parent stock segregating for early and late feathering were fed ad libitum (AL, feed was always available), ad libitum restricted (ALR, feed restricted daily to control body weight), skip-one-day and skip-two-days (SOD and STD, given two or three times ALR allowance on Day 1 and not fed on the next 1 or 2 days, respectively). At 160 days of age, pullets on SOD and STD were changed to ALR feeding, and daily feed allowances were increased to 135 g by Day 180 and 138 g by Day 250. Responses of early and late-feathering females were similar for traits measured. Generally, body weights of AL chickens increased to 4,600 g at 130 days of age and then reached a plateau. Body weights of feed-restricted groups were less than half of that of AL chickens by 160 days of age. Controlled release from feed restriction enabled chickens to reach a weight of approximately 3,200 g by Day 210. Daily feed consumption for AL chickens increased to about 220 g by 140 days of age and then decreased to approximately 150 g by Day 250; it eventually decreased to approximately 120 g by Day 350, where it reached a plateau. Mortality, plasma glucose levels, and surface and cloacal temperatures were lower for females whose feed was restricted than for those fed AL. Plasma protein levels were greater for STD than for AL, ALR, and SOD groups whereas plasma lipids were higher for AL and SOD than for ALR and STD groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Restricted feeding in early and late-feathering chickens. 2. Reproductive responses.

Reproductive traits of females from a broiler-breeder parent stock segregating for early and late feathering were measured under four feeding regimens. Feeding regimens consisted of ad libitum (AL, feed was always available), ad libitum restricted (ALR, feed restricted daily to control body weight), and skip-one-day and skip-two-days (SOD and STD, given two or three times ALR allowance on Day 1 and not fed on the next 1 or 2 days, respectively). At 160 days of age, pullets on SOD and STD were changed to ALR feeding, and daily feed allowances were increased to 135 g by Day 180 and 138 g by Day 250. Early and late feathering chickens responded similarly, except that hen-day ovulation rates, relative egg shell weights, and incidences of extracalcified and compressed eggs were greater for the former than the later. For most traits measured, differences among restricted feeding regimens were minor, but considerable differences were noted between them and the AL regimen. Females fed AL reached sexual maturity approximately 60 days earlier than those whose feed intake was restricted. Also, body weights at onset of lay were about 1,200 g greater for AL than for feed-restricted females. Percentages of hen-day ovulation, normal egg production, and duration of fertility were lower, whereas incidence of defective eggs was higher for AL than for feed-restricted chickens.

Restricted feeding in early and late-feathering chickens. 3. Organ size and carcass composition.

Carcass composition traits and weights and lengths of organs relative to body weight were measured in females from a broiler-breeder parent stock segregating for early and late-feathering alleles maintained under four feeding regimens. Feeding regimens consisted of ad libitum (AL), ad libitum restricted (ALR, feed restricted daily), and skip-one-day and skip-two-days (SOD and STD, given two or three times ALR allowance on Day 1 and not fed on the next 1 or 2 days, respectively). At 160 days of age, pullets on SOD and STD were changed to ALR feeding; daily feed allowances were increased to 135 g by day 180 and to 138 g on day 250. Organ growth and carcass composition were generally similar for early and late-feathering genotypes. Restricted feeding increased relative weights and lengths of segments of the gastrointestinal tract (GIT) and pancreas and reduced relative weights of the abdominal fat depot (AFD) and lipid contents of the liver, AFD, and total carcass. Relative weights of legs, breast, feathers, heart, and liver and carcass protein composition were not affected by feeding regimen. Relative oviduct and ovary weights were not affected, but number of rapidly developing follicles was higher for AL than restricted-fed hens. Relationships of most organs to total body weight changed little after sexual maturity. Relative weights and lengths of GIT decreased whereas breast, legs, and AFD increased. Feed-restricted chickens adapted to fasts by retaining GIT contents longer. Feeding regimen x age interactions were significant for esophagus, proventriculus, gizzard, shank, breast, legs, feathers, and AFD weights; esophagus, duodenum, jejunum, ileum, and shank lengths; and lipid contents of AFD and liver.

Allomorphic relationships from hatching to 56 days in parental lines and F1 crosses of chickens selected 27 generations for high or low body weight.

Allomorphic relationships in chickens selected for high or low juvenile body weight and their reciprocal crosses were examined from hatch to 56 days of age (doa). Those organs whose relationships to whole body weight were most divergent in the parental populations were weights of the gizzard, small intestine, breast, legs, feathers, abdominal fat and lengths of the shank, esophagus and small intestine. Relative weights of feathers, abdominal fat and small intestine were influenced mainly by additive genetic variation, whereas relative weights of breast, legs, and gizzard were influenced by additive genetic variation at younger ages (0 to 24 doa) and by both additive and non-additive genetic variation thereafter. In utilization of resources for high growth velocity and growth rate factor, crosses were 20 to 30% more efficient than their parental populations, demonstrating hybrid vigor. Relationships of rate of development of specific organs to total body weight varied with age and were noted as correlated responses to artificial selection for body weight. Selection for high and for low body weight at 56 days of age resulted in compromises in resource allocations. Specifically, selection for high juvenile body weight favored relatively heavier breasts, legs, abdominal fat depots and small intestines while selection for low juvenile body weight favored relatively heavier feathers and gizzard.

Organ growth of selected lines of chickens and their F1 crosses to a common body weight or age.

Organ growth of male chickens selected for high and low 56-day body weight and their reciprocal F1 crosses was compared at a common age (56 days) or at a common body weight (180 g). Organs that differed at a common body weight included weights of proventriculus, small intestine, lungs, feathers and abdominal fat and length of esophagus. Organ weights that differed at a common age included esophagus, gizzard, heart, liver, lungs, breast, legs, feathers and abdominal fat, and lengths of shank, esophagus and small intestine. Heterosis for most organs was less than 15%. Those exhibiting heterosis greater than 30% included weights of fat depots and feathers, plus lengths of the esophagus, small intestine and shank. Heterosis for these traits, however, varied depending on whether comparisons were made at common body weight or age. These results imply that biological functions of organs at specific ages may not reflect the situations at common body weights and suggest differences in resource allocations among populations.

Prior experience and response of chickens to a streptococcal infection.

Streptococcus fecalis infection was diagnosed in a population of individually caged meat-type pullets. A retrospective analysis showed that the infection occurred in pullets fed ad libitum and not in those whose feed intake was restricted. Among the former, resistance as measured by changes in body weight and blood heterophils and lymphocytes was greater for those from which blood samples had been obtained than from those which had not been bled. These data are consistent with the hypothesis that handling and feeding programs can modify an individual's resistance to infectious agents. Moreover, these experiences do not have to be extreme; rather they may be consistent with routine production practices such as restricted feed intake and removal of blood for serological testing.

Responses of chickens to Streptococcus faecalis: genotype-housing interactions.

Hens from lines selected for high and low juvenile body weight were maintained in single-bird wire cages or in litter-floor flocks and inoculated intravenously with Streptococcus faecalis. Responses to the inoculation, as measured by changes in body weight, plasma xanthophyll levels, heterophil/lymphocyte ratios, and mortality, were more severe in the high-body-weight line than the low-body-weight line. The effect of cage vs. floor housing varied according to genetic line. Responses of individuals to Str. faecalis could not be predicted from either plasma xanthophyll levels or numbers of heterophils and lymphocytes before inoculation. Diagnosis of field outbreaks would be difficult, because autopsies performed 14 days after inoculation revealed no lesions, and liver swabs positive for Str. faecalis were rare.

Response of early and late feathering broilers to various stressors.

Growth and responses to potential stressors were measured in early (k+) and late (K) feathering chicks from a common commercial broiler parent stock. Body weights were not different at 3, 10, or 49 days of age with ad libitum feeding, but K/K cockerels were heavier than k+/k+ ones at 24, 38, and 52 days of age with alternate day feeding. No differences in heterophil:lymphocyte ratios were found between K/K and k+/k+ individuals whether they were moved to a new environment or remained in familiar surroundings. Antibody response to sheep erythrocyte antigens was not different in K/w and k+/w pullets. Typical husbandry procedures that might be considered as mild stressors did not elicit physiological stress responses in these meat-type chickens.