Until now, each bull had a single calving difficulty figure (PTA and reliability) derived using farmer-scored records of calving difficulty and genomic data. The resulting PTA percentage provided an indication of the expected incidence of calving difficulty on any female (i.e., heifer or cow).
Such an assumption was not fully reflective of the true incidence of difficulty. For example, where a bull was only used on cows, his PTA and reliability only accurately reflected the on-farm difficulty of cow calvings, not heifer calvings (Figure 1). Therefore, the calving difficulty figures need to be separated for heifers and for cows.
Figure 1. Where a bull was considered ‘easy-calving’ and highly reliable based on the old evaluation his calving difficulty PTA % and reliability might not accurately reflect the on-farm difficulty when mated to all females (heifer vs cow, beef vs dairy) as most of his calving records could have originated from cows (e.g., beef cows in the above example), hence his calving difficulty PTA% and reliability is predominantly reflective of cow difficulty. The new evaluation system will address this issue.
Major Improvements
Farmers will now have a better indication of the expected incidence of calving difficulty on heifers and separately on cows.More data is being used in the evaluation (i.e. calf birth size and calf birth weight).Stricter editing criteria ensure herds with dubiously low levels of data recording do not negatively influence calving difficulty figures.Updated heritability estimates provide a more accurate reflection of the genetic component of calving difficulty on-farm.The cost of calving difficulty in the EBI has been updated.Calving difficulty is now split into two: dairy heifer calving difficulty PTA percentage and dairy cow calving difficulty PTA percentage
Noticeable changes
Presentation: Calving difficulty in the animal search, EBI profiles, sales catalogues etc. will change. Farmers will no longer see a single calving difficulty PTA percentage (and reliability). Calving difficulty is now split into two: dairy heifer calving difficulty PTA percentage and dairy cow calving difficulty PTA percentage. These will reflect the true incidence of on-farm calving difficulty. There will be an additional measure presented with the new calving difficulty figures, which will be called risk of dairy heifer calving difficulty.
Risk of dairy heifer calving
difficulty: This new trait determines how risky a mating is in terms of dairy heifer calving difficulty. A bull will be categorised as either low risk, moderate risk, or high risk. For a bull to be considered low risk, he must be easy calving (low PTA %), have a high reliability, and have a PTA that’s unlikely to move considerably when additional calving records become available (breed dependent). Until now, farmers considered bulls with a calving difficulty PTA of less than ~2.4% and a high reliability as ‘safe’ on heifers. This will no longer be considered best practice. From now on, bulls deemed to be at low risk of dairy heifer calving difficulty are considered the new threshold.
Safe-threshold: The ‘safe-threshold’ for calving difficulty, which farmers have become familiar with, has changed due to the abundance of improvements made to the genetic evaluation. This new development is a considerable change in how the industry has understood the calving difficulty PTA percentage. Initially, this will create an enormous challenge to try gauge a new ‘safe-threshold’, yet ultimately, the new system is more representative of the true incidence of calving difficulty on-farm.Where a bull was only used on cows, his PTA and reliability only accurately reflected the on-farm difficulty of cow calvings, not heifer calvings
To help with the change over, the ICBF has developed a ready reckoner that will aid farmers transition from the single calving difficulty PTA to the two new traits: Dairy Heifer Calving Difficulty PTA percentage and Dairy Cow Calving Difficulty PTA percentage.
The ICBF will be posting this ready reckoner and further information to all dairy farmers over the coming weeks.
Scale: The average calving difficulty PTA is generally the same in the old and new system, where a bull is mated to cows (Figure 2). However, the average calving difficulty PTA has increased where a bull is mated to heifers (Figure 2). The reason for the increase is that that heifers simply have more difficulty calving, and this was not entirely represented in the old evaluation system, as most calving records tend to originate from cows (Figure 1).
Thus, cows tended to dominate the resulting calving difficulty PTA percentage. The average change in calving difficulty PTA differs per breed (Figure 2).
Reliability: For the same number of progeny records, the average calving difficulty reliability has increased, typically by 10 to 15 percentage units, compared to the old evaluation system. This means that a bull will achieve a higher reliability with fewer progeny earlier in life with the new evaluation system, compared to the old system.
Figure 2. Average calving difficulty PTA % for each bull breed for all females based on the old calving difficulty evaluation (blue), for dairy cows with the new evaluation (grey), and for dairy heifers with the new evaluation (orange). Compared to the old evaluation, the average calving difficulty for each bull breed does not change considerably when cows are considered but there is an increase in the average PTA % when heifers are considered.
Advice
When selecting bulls for use on cows, use the dairy cow calving difficulty PTA % (and reliability).
When selecting bulls for use on heifers, use the new trait risk of dairy heifer calving difficulty, which determines how risky a mating is in terms of dairy heifer calving difficulty. A bull will be categorised as either low risk, moderate risk, or high risk.
For further queries, call the ICBF HerdPlus team on 023 882 0452.
Check out your EBI profile (www.icbf.com) to see how the changes will affect your herd.When selecting a bull for use on heifers, use the risk of dairy heifer calving difficulty categories (low/moderate/high risk).When selecting a bull for use on cows, use the dairy cow calving difficulty PTA and reliability.
Until now, each bull had a single calving difficulty figure (PTA and reliability) derived using farmer-scored records of calving difficulty and genomic data. The resulting PTA percentage provided an indication of the expected incidence of calving difficulty on any female (i.e., heifer or cow).
Such an assumption was not fully reflective of the true incidence of difficulty. For example, where a bull was only used on cows, his PTA and reliability only accurately reflected the on-farm difficulty of cow calvings, not heifer calvings (Figure 1). Therefore, the calving difficulty figures need to be separated for heifers and for cows.
Figure 1. Where a bull was considered ‘easy-calving’ and highly reliable based on the old evaluation his calving difficulty PTA % and reliability might not accurately reflect the on-farm difficulty when mated to all females (heifer vs cow, beef vs dairy) as most of his calving records could have originated from cows (e.g., beef cows in the above example), hence his calving difficulty PTA% and reliability is predominantly reflective of cow difficulty. The new evaluation system will address this issue.
Major Improvements
Farmers will now have a better indication of the expected incidence of calving difficulty on heifers and separately on cows.More data is being used in the evaluation (i.e. calf birth size and calf birth weight).Stricter editing criteria ensure herds with dubiously low levels of data recording do not negatively influence calving difficulty figures.Updated heritability estimates provide a more accurate reflection of the genetic component of calving difficulty on-farm.The cost of calving difficulty in the EBI has been updated.Calving difficulty is now split into two: dairy heifer calving difficulty PTA percentage and dairy cow calving difficulty PTA percentage
Noticeable changes
Presentation: Calving difficulty in the animal search, EBI profiles, sales catalogues etc. will change. Farmers will no longer see a single calving difficulty PTA percentage (and reliability). Calving difficulty is now split into two: dairy heifer calving difficulty PTA percentage and dairy cow calving difficulty PTA percentage. These will reflect the true incidence of on-farm calving difficulty. There will be an additional measure presented with the new calving difficulty figures, which will be called risk of dairy heifer calving difficulty.
Risk of dairy heifer calving
difficulty: This new trait determines how risky a mating is in terms of dairy heifer calving difficulty. A bull will be categorised as either low risk, moderate risk, or high risk. For a bull to be considered low risk, he must be easy calving (low PTA %), have a high reliability, and have a PTA that’s unlikely to move considerably when additional calving records become available (breed dependent). Until now, farmers considered bulls with a calving difficulty PTA of less than ~2.4% and a high reliability as ‘safe’ on heifers. This will no longer be considered best practice. From now on, bulls deemed to be at low risk of dairy heifer calving difficulty are considered the new threshold.
Safe-threshold: The ‘safe-threshold’ for calving difficulty, which farmers have become familiar with, has changed due to the abundance of improvements made to the genetic evaluation. This new development is a considerable change in how the industry has understood the calving difficulty PTA percentage. Initially, this will create an enormous challenge to try gauge a new ‘safe-threshold’, yet ultimately, the new system is more representative of the true incidence of calving difficulty on-farm.Where a bull was only used on cows, his PTA and reliability only accurately reflected the on-farm difficulty of cow calvings, not heifer calvings
To help with the change over, the ICBF has developed a ready reckoner that will aid farmers transition from the single calving difficulty PTA to the two new traits: Dairy Heifer Calving Difficulty PTA percentage and Dairy Cow Calving Difficulty PTA percentage.
The ICBF will be posting this ready reckoner and further information to all dairy farmers over the coming weeks.
Scale: The average calving difficulty PTA is generally the same in the old and new system, where a bull is mated to cows (Figure 2). However, the average calving difficulty PTA has increased where a bull is mated to heifers (Figure 2). The reason for the increase is that that heifers simply have more difficulty calving, and this was not entirely represented in the old evaluation system, as most calving records tend to originate from cows (Figure 1).
Thus, cows tended to dominate the resulting calving difficulty PTA percentage. The average change in calving difficulty PTA differs per breed (Figure 2).
Reliability: For the same number of progeny records, the average calving difficulty reliability has increased, typically by 10 to 15 percentage units, compared to the old evaluation system. This means that a bull will achieve a higher reliability with fewer progeny earlier in life with the new evaluation system, compared to the old system.
Figure 2. Average calving difficulty PTA % for each bull breed for all females based on the old calving difficulty evaluation (blue), for dairy cows with the new evaluation (grey), and for dairy heifers with the new evaluation (orange). Compared to the old evaluation, the average calving difficulty for each bull breed does not change considerably when cows are considered but there is an increase in the average PTA % when heifers are considered.
Advice
When selecting bulls for use on cows, use the dairy cow calving difficulty PTA % (and reliability).
When selecting bulls for use on heifers, use the new trait risk of dairy heifer calving difficulty, which determines how risky a mating is in terms of dairy heifer calving difficulty. A bull will be categorised as either low risk, moderate risk, or high risk.
For further queries, call the ICBF HerdPlus team on 023 882 0452.
Check out your EBI profile (www.icbf.com) to see how the changes will affect your herd.When selecting a bull for use on heifers, use the risk of dairy heifer calving difficulty categories (low/moderate/high risk).When selecting a bull for use on cows, use the dairy cow calving difficulty PTA and reliability. 
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