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Long return to service intervals are being recognized as an important source of loss to dairymen. It was earlier believed that abnormally long estrous cycles. were due to missed heats or "silent" heats, but it is now being recognized that embryonic death is partially responsible. Early embryonic mortality could be caused by an imperfect zygote lacking capacity for sustained mitosis, by hormonal imbalances causing improper uterine environment, by localized infections, of the female reproductive tract, or by characteristics or contaminants of semen. Doubtless all these factors and others are involved.
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Erb et al. (1954) studied this question.
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