Michael Wellington and Karissa de Belle, Bush AgriBusiness Pty Ltd.
With the current lack of stock across the country, many herd or flock managers will not have the luxury of culling that they normally would. This raises the question of how genetic progress is affected by, and should be managed in, different seasonal conditions. We have heard from some producers and advisors that genetic progress will slow during commercial herd/flock rebuilds. However, we are not entirely convinced that this is necessarily the case.
In the equation for genetic progress (see footnote 1), selection intensity (i) is affected by selection itself (how different are the new sires and breeders coming into the breeding herd from the current herd average?) and culling (how different is the herd average after culling, from the original herd average?).
During the sell-down phase, producers have the opportunity to increase i by ensuring that culled animals are, on average, those with inferior genetic merit. Conversely, culling opportunities are limited during herd or flock build-up phases, and i will be almost completely determined by the selection of new genetics to be added to the herd/flock. However, we don’t think this necessarily means i will be different in different seasons.
To demonstrate this, we’ve developed the hypothetical case study of Parched Creek and Ocean Lake Stations over the forthcoming three years. On average, both stations carry similar sized herds of around 1,000 breeders. However, Parched Creek has experienced a series of failed wet seasons and is progressively selling down breeders, whereas Ocean Lake is in a steady herd rebuild following recent favourable conditions.
This year, Parched Creek has to sell 200 breeders. We’ll assume that if not sold, these breeders would’ve weaned calves at a rate of 75%, giving 450 calves over the next three years. On the other hand, Ocean Lake is in the market for three new bulls which we’ll assume will have 50 calves per year for the next three years, also giving 450 calves.
Let’s suppose both stations are interested in the hypothetical trait ‘agility’ and have the same genetic profile for this; a mean value of 200 and standard deviation of 50. The 3 bulls purchased onto Ocean Lake have a mean agility of 214. This gives an i value of 14 (difference between mean of selected and mean of population, 214-200). The normal distribution below illustrates that this puts the selected bulls very slightly above the current herd average.
At Parched Creek, they have been able to identify the bottom 20%, or 200 breeders, based on agility. This group is identified in the illustration below. The average agility score for the culled mob is measured as 128, and they are removed from the herd. After they are removed, the average of the remaining 800 breeders’ agility score is 214. As the remaining 800 are the ‘selected’ herd, this culling exercise also gives a selection intensity (i) of 14 (214-200).
As i is the same in this hypothetical comparison, it would be reasonable to expect that genetic progress would be similar for Parched Creek and Ocean Lake over the ensuing three years.
We think there are several key messages from this case study:
- The addition of three bulls, which are very slightly above current herd average, has the same effect on genetic progress as correctly identifying and culling the most inferior 20% of breeders. This is a testament to the importance of sires compared with females in determining genetic progress, and to the significance of astute sire selection.
- Despite losing the luxury of culling during the herd rebuild phase, Ocean Lake still drives genetic progress through sourcing of superior external genetics. It would be reasonable to expect that bulls of greater merit relative to the existing herd could be sourced, than those shown in this example.
- Therefore, genetic progress does not slow due to lack of culling. In fact, a herd rebuild provides an excellent opportunity to introduce superior genetics and drive progress through judicious selection of externally sourced stock.
Parched Creek were able to identify their genetically inferior animals and use them as part of their sell-down strategy in the dry time. This takes planning and a good understanding of the herd’s genetic performance. Preparing for the dry and knowing your sell-down strategy is the best way to preserve the current genetic makeup of the herd. Knowing what individuals make up your nucleus herd or “A team” is imperative to being able to plan. The A team should be made up of your top performing cattle with an emphasis on fertility, as this will make up the core of your breeders when the dry breaks. With a wool flock, the ranking should be based on fleece value.
Ensuring good quality bulls are retained is even more important, it was clear that Ocean Lake was able to have a significant impact on genetics by the addition of just three bulls. It is important to realise that the bull will have the greatest impact on the herd, and therefore his selection is most fundamental. When it comes to bulls, knowing who is in the A team is imperative to make quality decisions that will have lasting impacts on the herd. The proof of this is in the example whereby 3 bulls were able to have the same genetic impact as 200 breeder cows. This shows the relativity of importance in decision making. Genetic decisions made now will affect your performance for the next decade and beyond.
The equation for genetic progress is as follows:
- ∆G = Change in genetic merit, or genetic progress.
- H2 = Heritability; the proportion of total variation in a given trait explainable by genetic variation.
- i = Selection intensity; the degree of difference between the merit of selected (or non-culled) individuals from the current average.
- σp = Phenotypic standard deviation; the variation observed in a given trait.
- gi = Generation interval; how quickly new generations are added to the population.