Cattle genetics and heritability

Cattle genetics and heritability

Dairy cow breeding has become highly complex as progress is made into the understanding of which genes are responsible for which traits and how well a particular trait is passed from one generation to the next. Most work in dairy breeding has obviously centred around those characteristics associated with milk yield and productivity, but an increased appreciation for the value of genetics in breeding livestock that suffer fewer mobility problems, have greater resistance to parasites and are generally healthier and longer-lived has led to work in defining which genes are responsible for conferring resistance to mastitis causing pathogens as well as the traits most likely to infer reduced susceptibility to the disease.

The heritability for Somatic Cell Counts (SCC) and clinical mastitis is relatively low. SCCs have been genetically linked to mastitis incidence, and thus selecting for a low SCC will inevitably lower the incidence of mastitis. However, there is evidence for an unfavourable correlation between resistance to mastitis and those traits associated with milk production, which means that, in simple terms, selecting for general resistance to mastitis is likely to result in lower-yielding cows.

There has been a great deal of research work to define the heritability for the various genes responsible for resistance or susceptibility to specific mastitis-causing pathogens but at farm level this information is not of particular use as dairy breeders are already selecting for a large number of traits.

Many udder and teat traits or characteristics associated with a susceptibility to mastitis have a relatively high heritability. At this level, breeders have the possibility of rejecting herd replacements from cows with undesirable traits; these characteristics include short teat canals with a wide cross-section diameter that have been identified as being prone to mastitis. Unfortunately, cows with these particular traits also tend to be faster milkers due to the ease with which milk can be drawn from their teats, and their offspring may indeed be chosen for these very traits of milking speed. Furthermore, milking speed and milk yield tend to co-relate, so many cows chosen as breeders because they milk quickly and yield well may well be those that are more prone to mastitis due to teat canal length.

Other physical traits such as udder suspension can also be selected for; breeding from cows with good long-lasting udder suspension promises longevity and avoids the large, pendulous udders that result in awkward teat angles which can be prone to liner slippage and the risk teat damage and infection with environmental mastitis pathogens.