Alternative indicators for pregnancy success

Genetics editor Alastair Rayner, 05/09/2023

ACHIEVING successful pregnancies, particularly among the replacement female cohort in any herd requires a degree of pre-planning.

Recommendations for producers to consider ahead of joining focus largely on liveweight of heifers. One of the most often cited production targets is that of Critical Mating Weight.

While this term is often cited by producers and at breeding workshops, there can be some confusion around what it actually means, and also if it is a useful indicator of pregnancy success on its own.

The (non-region specific) definition of Critical Mating Weight is the target weight for maiden heifers to achieve an 84pc pregnancy rate in a six week joining period (or two complete reproductive cycles).

This is based on the expectation that 60pc of these females will conceive in the first cycle of three weeks, and then 60pc of the remaining portion will conceive in the second cycle. This assumption is often calculated as (60pc cycle +1 (24pc cycle 2) = 84pc).

The challenge with this recommendation for many producers is setting an actual target or measure for their own herd. The challenge lies in the fact that there is significant variation between breeds and within breeds. This can be magnified by variation that also exists within herds.

To bring this target back to a practical goal for producers, the general recommendation is for their replacement females to achieve a weight of approximately 60-65pc of the average mature weight of the cow herd at joining.

Source: M.P. Davis and R.P. Wettemann Oklahoma Ag Experiment Station. Click on image to enlarge

At these weights, heifers should have achieved puberty and as a result are more likely to successfully conceive. So as a guide, these liveweight targets are helpful for producers seeking to achieve the conception rates outlined within the definition of Critical Mating Weight.

Achieving these weights has focused many producers’ attention to their selection for traits such as growth as well as on management of nutrition in order to maintain suitable levels of daily gain to achieve their target weights.

Considering Body Mass Index

As a production indicator, weight at puberty remains one of the most commonly recorded data sets in commercial and seedstock herds.

However, there are other production records available to producers, which may actually be a better indicator of pregnancy success.

During the recent NABRAC Conference in Darwin, Dr Kieran McCosker from the University of Queensland presented a paper outlining the use of Body Mass Index as a viable option for producers to use when determining pregnancy success.

Kieren McCosker

The work undertaken at the University of Queensland involved a trial of 40 maiden Brahman heifers, with records taken pre-joining and at pregnancy detection, 40 days after a ten-day round of Fixed Time AI.

All the heifers were recorded for liveweight and for hip height. Their Average Daily Gain (ADG) was also calculated. As a further assessment, the Body Mass Index (BMI) was calculated for each heifer by dividing their pre-mating liveweight by their recorded hip height.

The results of the project offer some interesting insights that might be valuable for producers who are looking to achieve better pregnancy success in their maiden heifers.

In this trial, after a ten-day Fixed time AI program, 52.2pc of heifers were confirmed pregnant. Looking at the data, Dr McCosker and the team were unable to determine a significant relationship between pregnancy and the parameters of premating liveweight, hip height and Average Daily Gain.

However, they did identify a strong relationship between pregnancy and Body Mass Index.

Predicted probability of pregnancy for body mass index

Source: K. McCosker, A. Boulton, M Dekkers, E. Ross, N Hudson, M Fortes the University of Queensland, Gatton

The value of this work is the demonstration that calculating Body Mass Index is a stronger predictor for pregnancy success than relying on the single measures of either liveweight or Average Daily Gain.

For producers seeking to join groups of similarly-aged heifers, that may have variation in liveweight, recording hip height and calculating BMI may actually assist in identifying these females that are more likely to conceive.

This could be useful where some females may fall below the arbitrary rule of thumb for critical mating weight, but when assessed for BMI are likely to conceive.

At the very least, this study does suggest a practical and useful measure for producers looking to refine their selection and joining of replacement heifers.


Alastair Rayner is the Principal of RaynerAg, an agricultural advisory service based in NSW.  RaynerAg is affiliated with BJA Stock & Station Agents.  He regularly lists and sell cattle for clients as well attending bull sales to support client purchases.  Alastair provides pre-sale selections and classifications for seedstock producers in NSW, Qld, and Victoria.  He can be contacted here or through his website













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  1. Marina, 12/09/2023

    Hi, it should be mentioned who is the team and that this is part of the LESTR project

  2. Stephen Carpenter, 06/09/2023

    One thing we have found when doing embryo transfers was that maintaining a steady level of blood sugar prior to , during and after implant nearly doubled results. If a cow has a drop in blood sugar for even an hour during the crucial time she will cycle but not ovulate. This was addressed with a daily ration of copra meal . Easy in a small group I know but food for thought. ps this is not my idea a past dean of Vet science proved this many years ago.

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