Using the example 400 cow herd from above:
Using the same number of cows, pregnancy rates, AB length and pregnancy loss/death and assigning sex of calf randomly with equal chance of heifer or bull for the equivalent of 1000 years (see figure below).
In most years the number of heifer calves born alive was around 124, however, some years as few as 88 and other years as many as 164 were born.
The difference is simply due to random chance.
Most years (2/3 of the time) the number of heifers born was between 114-134.
On 25 occasions the number of heifers born was less than 104.
With more than 10,000 dairy herds in NZ, due to chance, approximately 250 herds would be expected to generate less than 25% replacement heifers from 6 weeks AB and 67% 6 week in calf rate.
Authors: Dr. Christine Couldrey, Dr. Lorna McNaughton, Joyce Voogt.