Return Interval Analysis and Non-Returns


The non return rate (NNR) is the percentage of inseminations that occur early in the AB period where the cow did not return to heat within the maximum normal estrous cycle period (24 days). Cows that do not return to heat are either pregnant or phantom ( a non return that is not pregnant).

NRR can be an accurate estimation of conception rate (CR) if heat detection is accurate.

A low NNR (<64%) may be an early warning sign of poor conception rate. This could be due to a number of factors including inaccurate heat detection, poor insemination practice, disease, low body condition at calving or excessive loss of condition, a high number of late calvers or high number of treated non-cyclers.

Return interval analysis

A return interval analysis looks at the spread of returns to heat for cows that have been mated in the early AB period.

A cow will normally cycle every 18-24 days if she does not become pregnant. Reasons for short (<18 days) or longer (>24 days) returns include: inaccurate heat detection (missed or invented heats) and early embryonic loss.

By monitoring the NNR and return interval analysis for your herd from about 4 weeks after the planned start of mating there are some actions that can be taken to reduce the impact of poor heat detection and conception rate on the final mating result if a problem is detected. These may include early pregnancy testing, hormonal intervention, additional heat detection aides, changes to insemination practice or altering the length of mating period. Actions to improve results for next year can also be started early.

Please get in contact if you would like to do some early scanning to identify and treat phantom cows. We can scan cows from 35 days – phantom cows can get enrolled in a CIDR programme. The other advantage of early scanning is to determine an accurate conception rate for those cows.