It is well known that over the last decades there has been an enormous development in the genetic potential of broilers. Where in 1950 the average body weight of a broiler after 56 days of growing was approximately 900 grams, in 2005 it was already more than 4 kg, and since then the genetic progress has only continued.
The question is often raised if this genetic modification has influenced the embryo during incubation, if the genetic progress has made the embryo grow faster as well. When we look at the incubation process the embryo still needs 21 days, and a day old chick is on average still approximatley 67% of the initial egg weight. However, when we look at the incubation process day by day, it seems to be that embryos from 1960 are not completely comparable with modern embryos.
A. L. Romanoff published data in 1960 about the embryo weigth without the residual yolk, (Yolk Free Body Mass, YFBM) at different moments in incubation. When we compare these results with data from 2015 (Nangsuay et al), we can see in table 1 that the embryos from 2015 seems to be slightly bigger, especially in the second part of incubation. This will be partly due to a slightly bigger egg weigth, but that difference was relatively small.
|egg weight (g)||60||62||103%|
Table 1: Egg weight and yolk free body mass (YFBM) in 1960 and 2015
Romijn and Lokhorst published in the same year (1960) a very detailed report of the embryonic heat production during incubation. When we compare these data with data from 2015 (Nangsuay et al), we see that also the heat production has slightly increased (table 2), although the egg weight of the 1960 eggs was even slightly higher:
|egg weight (g)||62.4||64||97.5%|
Table 2: Heat production in 1960 and 2015 (percentages might differ slightly from the calculation as the original heat production figures have been used for the calculation, where in the table only 3 digits are presented)
When we compare the two tables, we see that the heat production of modern broilers seem to be higher especially in the first part of incubation, where the embryo mass is higher in the second part. This seems strange but it might (partly) reflects the metabolic activity in different stages. At the end of incubation the oxygen availaibility will be the limiting factor for the metabolic rate, so regardless of the genetic potential the embryo cannot grow faster due to limitations in oxygen supply. However, the embryo usually reaches a limitation in oxygen supply around day 14, where the difference in heat production seems to get lower earlier in incubation.
Comparing datasets from experiments done more than half a century apart is always questionable, as instruments, procedures etc will have changed. For heat production the same procedures are used nowadays as in 1960, but of course the materials, instruments etc have seen an enormous development. But when we compare the data, it seems that there might be a slight difference in embryos from past and present. However, this is by far not of the same magnitude as the genetic progress in growth in broilers in the same period.
Nangsuay, A., R. Molenaar, R. Meijerhof, I. van den Anker, M.J.W. Heetkamp, B. Kemp and H. van den Brand (2015). Differences in egg nutrient availability, development, and nutrient metabolism between broiler and layer embryos. Poultry Science 94: 415-423
Romanoff, A.L. (1960). The avian embryo. Structural and functional development. MacMillan Company, New York
Romijn, C. and W. Lokhorst (1960). Foetal heat production in the fowl. J. Physiol 150: 239-249