Abstract
PICO question
 In dairy cattle, which months should producers target age at first calving in order to maximise milk yield, minimize risk of non-voluntary culling and optimize fertility?
 
 Clinical bottom line
 Category of research question
 Risk
 The number and type of study designs reviewed
 Seventeen papers were critically reviewed: 15 sets of case series, one review of case series and cohort studies and one randomised control trial, summarising over 2.4 million individual cow records
 Strength of evidence
 Strong
 Outcomes reported
 An optimum range of age at first calving (AFC) on dairy farms appears to be 22–25 months inclusive. Lower or higher than this figure can bring lower first lactation 305 day and lifetime milk yields, lower fertility and lower chances of surviving to a second lactation. Achieving an AFC of 22–25 months can bring the highest economic return to dairies
 Conclusion
 Age at first calving is a useful and key performance parameter to measure in dairy cattle. Achieving a range of 22–25 months at first calving can help to optimise both long term milk yield, fertility and longevity within the herd
 
 How to apply this evidence in practice
 The application of evidence into practice should take into account multiple factors, not limited to: individual clinical expertise, patient’s circumstances and owners’ values, country, location or clinic where you work, the individual case in front of you, the availability of therapies and resources.
 Knowledge Summaries are a resource to help reinforce or inform decision making. They do not override the responsibility or judgement of the practitioner to do what is best for the animal in their care.
 
Highlights
Optimum range of AFC The optimum range of AFC varies according to paper, outcome studied and geographical region but a consensus can be seen in overlapping months:
Summary of the evidence Of the articles discussed in this Knowledge Summary, the reported range of AFC is from months old to 42 months old
As mentioned in the methodology section, this Knowledge Summary has reflected the PICO and is not focused on the influencing factors of AFC but it is worth noting that major inputs to AFC include diet and insemination procedures
Summary
Collection of data over 6 years and other factors could influence the outcomes studied such as herd improvement programs and environment. Many of these include farms on DHI programs (Elahi Torshizi, 2016 and Ettema & Santos, 2004) and cover data spreading over [2,3,4,5] years (Berry & Cromie, 2009; Eastham et al, 2018 and Nilforooshan & Edriss, 2004) This technique gives large sample population numbers, it may create confounding factors: farms that use computer records and that participate in herd improvement schemes may not be representative of all dairy farms and are already skewing data towards the more advanced farm management systems. As the effects of AFC (yield, fertility and culling) are influenced by so many other factors, the model is required to have many inputs, from labour, feed price, veterinary costs, rent, energy costs, disease rates, etc When this is calculated, it is easy for some groups to miss some inputs and without considerable detail, avoid double-counting. Further research in the interaction of body weight at birth and AFC on first lactation yield would provide more clarity on the individual effects of both on production
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