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Good reproductive performance is an essential part of any successful dairy enterprise, and heat detection is an important part of this in herds using artificial insemination. It is commonly accepted that the oestrous cycle of cattle is approximately 21 days, with a normal range of about 18 to 24 days. A more accurate knowledge of normal cycle length may contribute to improved heat detection. The normal range is also the parameter to calculate a variety of fertility parameters used by veterinarians, farmers and other professionals to monitor dairy-herd heat detection. These include first-service success, return-to-service success and analysis of inter-service intervals. Expected cycle length could also affect the interpretation of overall reproductive performance. Furthermore, with increasing global milk yield over time, dairy-herd fertility tended to decline. It is therefore plausible that level of production may have an effect on the length of the inter-service interval. These questions intrigued J.G. Remnant and co-workers. Their study was published in the Journal of Dairy Science, Volume 98 of 2015, pages 889 to 897, with the title: Variation in the inter-service intervals of dairy cows in the United Kingdom
The aim of the study was to assess the inter-service interval in a large number of dairy cows, to explore the variability in oestrous-cycle length and to identify associations between cow factors and cycle length. For that purpose, the time between inseminations in a large number of dairy cows was determined and the possible associations between cow factors and oestrous-cycle length studied.
The study used data from 42,252 cows in 159 herds across England and Wales. Significant differences were observed between different yield groups, parity numbers and the number of inseminations. Significant associations were also found between predicted inter-service interval and insemination number, days in milk, 305-day milk yield and month and year of insemination. Surprisingly, most variation in inter-service interval length occurred between cycles within cows (87%) compared with the variation between cows within a herd (12%). The most common interval between inseminations was 22 days rather than the accepted 21-day normal interval, with a range of 18 to 28 days. A positive result was that only a small association was observed with level of production (measured as 305-day yield).
These results clearly challenge accepted dogma. However, further work is needed to elucidate the physiological mechanisms behind these associations and behind the unexplained within-cow variation in cycle length.