Responses of broiler breeder strains to a thermal challenge late in incubation

Currently we have several strains of broilers that have been developed with the object of increasing percentage meat yield and effective protein accretion. Geneticists have been very effective in reduction of the number of days to a particular market weight, increasing the percentage of breast meat on the carcass, and improvement of the feed efficiency in each generation. The improvement in carcass characteristics have been accomplished because of economic and consumer demands. One of the byproducts has been a decrease in the fertility and hatchability parameters. Qualls (2002) reported that US broiler WOG (Carcass Yield, without giblets) yield has increased from 68.5 to 70.5% in the past five years while hatchability has decreased by one percent. He reported that hatching and incubation cost had increased from 2.6 to 3.0 cents per lb, which is fairly significant. In analysis of problems associated with hatchability, it was found that increased temperature late in incubation has been a common diagnosis for these higher meat yielding broilers while other classic broilers grown for lower weights and not increased meat yield do not seem to show some of the traditional (Hill, 2000). Just as differences in performance and heat stress have been noted for different strain and strain crosses, it was decided to determine the hatchability performance for various broiler stains selected for increased breast meat. Many recent presentations have documented the lesions associated with high heat during incubation or hatching for our modern strains. Meijerhof & van Beek (1993) and French (1997) showed the effects of air flow and egg position in the incubator on increase of embryonic temperature and hatchability. Wineland et al. (2000) showed the effects on chick yield was decreased when incubator or hatcher temperature was increased. Another significant finding was that heart weight as a percentage of chick weight (heart yield) was significantly decreased when heat in the incubator or hatcher is increased. Gladys et al. (2000) found that chicks that were either too hot (103.5°F shell temperature) or too cold (99.5°F shell temperature) during the last five days of incubation had reduced growth at 44 days of age when compared to chicks incubated at a shell temperature of 101.5°F. Birds hatched under the higher temperature did not immediately start eating for the first 12 hours in the brooder house unlike the birds incubated at 101.5°F that immediately Table 1 Effects of incubation temperature (last 5 days) on hatch of fertile eggs (%) of high yielding broiler strains Strain A Strain B Strain C Strain D Control-101°F 87.27 84.12 84.30 72.05 High-104°F 73.94 65.09 67.77 62.73 Difference 13.33 19.03 16.53 9.32 went to feed and water. This delay in activity was thought to be part of the reason for decreased growth performance. Lourens & van Middelkoop (2000) found similar effects on hatchability and grow out performance. Commercial data (personal information, 2000) that compared a hatchery that controlled incubation temperature with one that was unable to completely decrease higher temperature or uniformity of temperature within the hatcher had five points better feed conversion for the a yearly comparison of broiler performance. When shell temperatures during incubation of 103°F and 101°F were compared for eggs from Cobb breeders, no difference in hatchability was found but a difference in growth performance was found. Recently, eggs from two high yielding strains Hubbard Ultra Yield and Cobb were tested for effects on hatchability of incubating for the last 5 days at shell temperatures of either 100.5°F or 103.5°F. Differences in hatchability of 2.9 and 3.9% were found. Other lesions observed were a increased chick weight, decreased heart and heart yield, and increased yolk sac yield (% of chick) for the chicks hatched under the higher egg shell temperature. Another hatcher temperature trial was attempted with eggs from four different strains at temperatures of 104.0°F during the last six days. Differences in hatchability between the control and treatments were between 9.32 and 19.03% hatch (Table 1). The lowest difference also had the lowest hatchability, which would have meant least heat production. More studies will be conducted to determine heat production and other hatch related characteristics are related to genetic background.