The animals and the circumstances within the research studies being considered should be as similar as possible to those animal(s) being treated.
To have an accurate understanding of the relevance of a study to your patient(s) you should use studies conducted with animals that are similar to them. Studies based on other species, with other diseases, in other husbandry conditions, at different ages, or in some cases just different breeds may not be generalisable to your situation.
Furthermore, it is important to assess whether the outcomes measured in the study are important in the context of your patient. For example, if a lactational intramammary antibiotic is being assessed for its ability to cure a contagious pathogen (e.g., Staphylococcus aureus) but your client mainly has issues with environmental pathogens (e.g., Streptococcus uberis, Escherichia coli) then you must bear this difference in mind when making decisions and when advising the farmer.
It is not always possible to find studies that perfectly mirror your situation. This does not mean you cannot consult scientific evidence from situations different from your case, but you and your client must be aware that there is some uncertainty surrounding the decisions you are making.
REMEMBER: Ask whether the participants in the studies are similar to your patient(s).