The basics of heritability – Part 2

Selecting for disease and parasite resistance in cattle can help improve production, says Prof Cheryl McCrindle of the University of Pretoria.

The late Prof Jan Bonsma’s life work was focused on the theory that cattle could be selected and bred to be disease- and parasite-resistant in South Africa. Recent work on genetic markers that could be selected to increase disease resistance have, however, raised more questions than answers.

Unlike smallstock, where specific markers for meat and wool production and worm resistance have been identified and are forming part of selection criteria, heritability data are still being calculated from production records in cattle.

A good example is a recent paper in the South African Journal of Animal Science (Imbayarwo-Chikosi I. et al, 2015), which suggests that the number of years a cow remains functional (in essence, disease-free) can be estimated from the bull progeny records in BLUP (Best Linear Unbiased Prediction).

Bonsma always maintained that resistance to ticks was the most important genetic trait when selecting for disease resistance in beef cattle. Certainly, on extensive and communal grazing, the reality is that only those cattle that are resistant to ticks will pass their genetic characteristics on to the next generation.

Bonsma suggested that skin thickness was the key to tick resistance. Although subsequent studies have questioned the link between skin thickness and tick resistance, it’s certainly true that thin-skinned dairy animals are overwhelmed and die rapidly if put onto veld with many long-mouthed ticks.

Slowly developing resistance

Although it is not well-documented in the literature, cattle farmers with cattle grazing out in the veld know that calves gradually become resistant to ticks and that, as Bonsma suggested, selecting breeding animals from heifers with the fewest ticks eventually results in a herd that needs dipping less frequently.

Resistance to ticks goes hand in hand with ‘endemic stability’ of tick-borne diseases as a regular small dose of this organism keeps the animal’s immunity high. Inherited conformation traits also contribute to the health of breeding cows and fertility of bulls. Selecting for wider hips in cows and bulls that sire smaller calves result in fewer calving complications.

The size and shape of the udder and the length and spacing of teats are highly heritable. This factor not only decreases the chance of mastitis, but also makes it easier for calves to drink easily. Low-hanging udders with long teats attract more ticks and are easily damaged by thorns. A low hanging, fleshy prepuce in a bull is also highly heritable and is linked to poor fertility in bulls, as it is easily damaged.

Jaws and legs
Other conformation traits that are highly heritable are crooked jaws and legs. Poorly structured legs and hooves prevent a cow from walking to good grazing and water, while strong, straight jaws are needed to chew fibrous grass during winter. Bonsma also mentioned that a good medium-size cow is better than an oversized cow as she uses less energy in moving around and puts the same amount of food into keeping body condition in winter.

Coat colour is another heritable trait that can improve health and disease resistance. White faces and eyelids are susceptible to sunburn and cancer, while red-brown cattle seem to keep cooler in summer and warmer in winter.