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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.

 year 1960 2015 2015/1960
egg weight (g) 60 62 103%
YFBM grams grams  
Day 11 4.4 4.6 105%
Day 14 10.3 13.6 132%
Day 18 24.5 30.3 124%


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:

year 2015 1960 2015/1960
egg weight (g) 62.4 64  97.5%
heat production Watt/egg Watt/egg  
Day 5 0.006 0.004 128.3%
Day 6 0.008 0.007 120.3%
Day 7 0.011 0.009 127.8%
Day 8 0.016 0.013 118.2%
Day 9 0.022 0.018 122.3%
Day 10 0.031 0.028 109.3%
Day 11 0.045 0.043 104.2%
Day 12 0.065 0.063 103.0%
Day 13 0.088 0.084 105.4%
Day 14 0.113 0.106 106.4%
Day 15 0.131 0.122 107.9%
Day 16 0.141 0.130 108.2%
Day 17 0.143 0.132 109.0%
Day 18 0.150 0.139 107.9%


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.

Literature used:

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