Heifer Size at First Breeding 

Background 

The weight and growth of dairy animals is a critical component of achieving goals for lactating animals.  After first partition, the growth yet to be done (difference between the size of the animal and mature) before the cow is mature impacts energy partitioning and milk production.  

One of the practical difficulties of managing animals with weights is both in the analysis and extrapolation of previous weights.  Growth curves give the ability to take limited data and extrapolate to other points in time.  BoviSync utilizes the Gompertz growth curve as described in France et al. (1996); using 0.056 as the ratio of birth weight to mature body weight and 0.003826 as the coefficient to days of age.  For mature body weight (ExpMatureWt), which is calculated from breed average is adjusted based on the genomic or pedigree (BWC) of each animal. Using the curve, we are able to extrapolate the current expected weight (WeightExp) and the expected weight at the time of a weight measurement.  The difference between measured and expected weight is the phenotypic difference (WghtPhnDif).  That phenotypic difference is carried forward for the animal to adjust its expected current weight from the standard curve. 

These functions allow us to estimate the age of an animal at a certain weight.  The animal’s age at 55% of mature body weight (AGE55MBW) is a common threshold for animal weight for first breeding.  Heifers come into puberty after 50% of mature body weight (DCRC, 2017), and should be bred after they have attained 55% mature body weight and 90% of structural growth.  These thresholds allow the animals to get to a sufficient size at first and second calving.  

The desired size at first and second calving is a balance of either paying too much for growth in non-lactating heifers, or in getting milk from these animals earlier but suffering additional energy partitioning to growth of the animal and away from milk reducing the complete lactation yield. Analysis of this within the farm’s heifer growing is the key to understanding this economic trade-off.  Additionally, the translation of the difference in economics between growth and age is the value of the higher/lower plane of nutrition the animals are kept on. 

Analysis of Lactation Production 

The key to this evaluation is to look at the age and % mature bodyweight of animals, compared to their lactational performance in first and second lactation.  This is a report that can help you in this evaluation: Lactational Performance Report- BoviSync 

Implementation 

To implement a system of heifer breeding that utilizes animal size, the first step is to determine the rules for the earliest eligibility for breeding.  Using a combination of age and size; the VWP for the heifer could be defined as the greater of 390 days of age or date of achieving 55% of mature body weight.  The most accurate estimated weight will be achieved by obtaining a weight as close to that as practical.  Ideally a weight is taken after 9-10 months of age.   

In BoviSync, setup a protocol to record the VWP in 21 days with the criteria being 369d and less than 21 days to mature body weight.  This will cause a VWP event to be recorded at 390d or delayed later until the animal achieves 55% mature body weight.  The 21d is chosen to give opportunity to get animals moved into pre-breeding pens and managed in preparation for breeding.  Movements into the pre-breeding would then be setup based on days until VWP instead of days of age.  This can be fully automated or semi-automated to allow manually adjustments based on variability in cattle flows. 

Practically, this can have the impact of moving up the earliest age on all animals, while using exception criteria for animals growing at a slower rate.  Getting more animals to the right balance of growth before and after first calving can have significant economic impacts by optimizing animals individually, while keeping group management simple. 

Analysis of Distribution of Age at 55% Mature Body Weight

References

France J., Dijkstra J., Dhanoa M.S. Growth functions and their application in animal science. Anim. Res., 45 (Suppl. 1) (1996), pp. 165-174 

DCRC, Focus on Heifer Size to Determine Age at Breeding, (2017) Focus-on-Heifer-Size-to-Determine-Age-at-Breeding.pdf (dcrcouncil.org) 

Example Reports