Why do some heifers fail to achieve anticipated levels of milk production?
Genetic indices have been used for many years to predict milk production capability of replacement animals. While we all understand that these are average figures for a group of animals, it is still somewhat disappointing when an individual heifer from a good bull fails to produce to the level of its dam.
Research from a trial in Ontario, Canada has identified one reason why some heifers will never perform to their genetic capability.
215 female calves from 3 dairy herds were enrolled in a trial to assess if lung condition in young dairy calves influenced first-lactation milk production. Rather than just depend on clinical signs of BRD (Bovine Respiratory Disease), the researchers used ultrasonic lung scanning to create a more objective diagnosis of lung condition.
92(43%) of the 215 calves in the trial displayed clinical signs of BRD within 56 days of birth, which would be consistent with many farms. However, a further 31 (approx. 14%) calves had suffered lung damage visible to ultrasound without displaying any clinical signs. In total 57% of the calves suffered lung damage before they were 8 weeks old.
Roll on to 3 years later to the end of first lactation.
5% of healthy calves, for different reasons, did not complete their first lactation. 9% of the calves that suffered lung damage did not complete first lactation. Completed lactation data was collected for 140 animals, average milk production was 8,626kg. The heifers who had been diagnosed with lung damage as calves averaged 525kg less milk in the first lactation than the calves that did not have lung damage. This trial only quantified the effect over the first lactation, but it would be reasonable to assume that this lung damage would also negatively affect performance in subsequent lactations.
Week 2-3 were the peak periods for diagnosis of lung consolidation. Lung consolidation can be thought of as scar tissue within the lung, reducing both lung capacity and the ability to properly oxygenate blood. Once the lung has been damaged, consolidation cannot typically be reversed.
Pre-weaned calves are especially vulnerable to environmental pathogens as their passive immunity is declining and their endogenous humoral immunity has not yet compensated for this decline. The peak frequency of diagnosis of lung damage in the trial (2-3 weeks old) coincided with the decline of passive immunity.
There are a few lessons to be learnt from this trial.
1) Pre-weaned calves are especially vulnerable to BRD.
2) Calves with BRD will suffer lung damage without displaying clinical signs.
3) Calves can survive BRD but their lifetime profitability has been seriously compromised.