Shannon Cartwright

Like many students who come to the University of Guelph, Shannon Cartwright wanted to be a veterinarian. And then one summer she worked on a dairy farm and the gentle cows won her over.

While studying for her undergraduate degree in science and agriculture, she developed an interest in genetics research and as a master’s student, Prof. Bonnie Mallard, an immunogeneticist in the Department of Pathobiology, encouraged her to follow this path.

“Crossbreeding cows in order that they can live longer, healthier lives is catching on among farmers,” says Cartwright. “There can be misconceptions about how animals are treated on farms, so educating the public about this and animal welfare is a related interest of mine. I would like to help dairy producers as well as the cows. As an undergrad, I did a project on the effects of heat stress on large animals and how to mediate that. Reading articles about breeds of cattle that seem to be heat tolerant made me even more curious about the links between animal health, crossbreeding and genetics.”

Purebred Holsteins, she says, are the most popular breed in the Canadian dairy industry because of the large quantities of milk they can produce. But over time Holsteins were so intensely bred for milk production that infertility and increased susceptibility to diseases like mastitis, an infection of the udder, resulted. The intensive breeding of the Holstein also resulted in inbreeding, which increases the occurrence of harmful recessive genes.

“You need to make smart decisions when you are breeding cows to get the best of both worlds – high milk production in addition to better health. I have been writing reports based on my research findings and some of my articles on the subject have been published in dairy industry magazines.”

Based on official animal registration and pedigree information in its database, the Canadian Dairy Network collects statistics on the level of inbreeding within each dairy breed in Canada to monitor trends.

“It is increasing every year, but we are finding ways to manage it,” says Cartwright. “Why wait for it to become a massive problem before you decide to do something about it? That’s the way I look at it. Most producers have recognized the need to find solutions to the problems stemming from inbreeding. Crossbreeding is one of the most promising solutions currently available.”

A number of studies have shown that crossbreeding dairy cattle is associated with less disease and enhanced antibody response.

The Norwegian Red is an example of a breed that complements the Holstein, offering comparable milk production as well as good health and fertility traits, for which they have been bred specifically over the past 30 years. Holstein health traits have only been studied for about a decade.

Cartwright has been working on a project crossbreeding Norwegian Reds and Holsteins for five years and the overall results show that these crossbred calves have significantly greater survival rates and increased resistance to disease compared with purebred Holstein calves.

“For my study, dairy producers in Ontario were invited to participate and I worked with 26 of them in an area stretching from London to Schomberg,” she says.

Frozen Norwegian Red semen was shipped from Norway. In return for their participation, the Ontario producers were offered a reduced rate for the semen as well as payment for the crossbred daughters and for keeping records. The producers agreed to breed at least 20 crossbred heifers in about four years, after which they could choose to continue or not.

Cartwright studied 150 of each of the crossbred and the purebred Holsteins. She tested the calves for antibody response, which primarily defends against pathogens such as E. coli, and for cell-mediated immunity, which offers protection against intracellular pathogens, including Johne’s disease, a contagious bacterial disease affecting the intestinal tract of ruminants.

“Over the course of 23 days, I took multiple blood samples from each calf, immunized each calf and brought the samples to the lab to evaluate the antibody response in the blood serum. I also performed a skin fold thickness test on the calf’s tail to measure cell-mediated immune response.”

She compared the results from the crossbred calves with an equal number of purebred Holstein calves on each farm to determine which had a better immune response. Cartwright was able to identify cattle as high, average or low immune responders, both safely and accurately. High responders have a more robust response and are capable of defending against the wide variety of pathogens that infect dairy cattle.

“Animals with a stronger immune response are less prone to disease and respond better to vaccinations,” she says. “I found that the crossbred calves in general had a higher response than the purebreds, which suggests they would also have an increased survival rate. It seems to make sense because if an animal is healthier, why would it not survive longer?”

Cartwight’s study ended at the time of the crossbred cows’ first lactation, but she is interested in what happens after that first generation of Norwegian Red-Holstein cattle is born. She also followed 80 of those heifers and tested them again after they had their first calves.

“When I retested them, the answer was the same and my results were confirmed: overall, the crossbreds fared better. Looking at the individual cows, their immune systems were strong and stable before and after pregnancy.”

Many of the producers involved in her research were pleased with the outcome and decided to continue crossbreeding their Holsteins with Norwegian Reds.

“You’re not losing milk production with these crossbreds and the animals are healthier, so in the long run, producers are likely saving money,” says Cartwright. “If a purebred Holstein is producing a large quantity of milk but costing a lot in terms of medical treatment, or it doesn’t live long, that’s a large investment lost.”

Cartwright believes the dairy industry will benefit from healthier cows.

“If a cow is healthier, it’s going to be more comfortable and under less stress. In my opinion, not only do you have a ‘better’ cow, but you also have a happier cow.”