the chick can sustain pipping efforts, often leading to mortality. 5. Higher Residual Yolk Sack Weights Elevated temperatures during late incubation disrupt embryonic development by increasing metabolic stress and impairing yolk nutrient absorption. Under heat stress, embryos prioritise thermoregulation, reducing their ability to efficiently process yolk-derived lipids, proteins, and antibodies. This often results in higher residual yolk sac weights at hatch—up to 15 % of chick body weight compared to the ideal range of 6 to 10 %— which correlates with slower early growth rates and compromised immunity. Residual yolk sac percentages naturally decline post- hatch as chicks metabolise remaining nutrients. While immediate post-hatch weights may be higher due to retained yolk, the efficiency of yolk absorption depends on several factors: • Hatcher Conditions: Poor ventilation (elevated CO₂ levels) and humidity extremes can slow yolk absorption. Higher eggshell temperatures (EST) accelerate hatching but increase the risk of incomplete yolk use if chicks are processed prematurely. Additionally, late-stage heat stress reduces the transfer of yolk-derived carotenoids, causing pale down feathers. • Feed Access Delay: Chicks held for more than 24 hours post-hatch deplete their glycogen reserves, increasing reliance on residual yolk and potentially exacerbating growth delays. The first four chicks seen in the Table 1 fall within the ideal range. In contrast, the last two chicks display high residual yolk levels, exceeding the optimal 6 to 10 % range. Chicks exhibiting elevated yolk sac percentages and black button navels, indicative of late-stage incubation issues. The accompanying navel photograph illustrates severe cases where the navel is prematurely forced closed around the yolk sac before complete absorption.
Image 2: Black button navel
Practical Tips for Managing Late-Stage Temperature 1. Measure EST Directly • Use an, ear thermometer or probe logger on confirmed living embryos • Sample eggs systematically from top, middle, and bottom trays, front to back • Monitor temperature variations across positions - wide variations may indicate airflow issues • Continue EST measurements until approximately 20 days and 12 hours of incubation
Table 1: Chick weight and yolk sac percentage
Chick Weight (g)
Yolk Weight (g)
Yolk Sacks as % of body weight
39.7
3.5
8.8 %
40.5
3
7.4 %
40.5
3.5
8.6 %
41.5
4
9.6 %
46.4
6
12.9 %
44.1
7.1
16.1 %
Image 3: Well healed navel
Technical Focus - Optimising Egg Collection and Hatchability, Copyright © 2025 Cobb-Vantress, LLC. All Rights Reserved.
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