RESEARCH
Beefing up options By Samantha Tennent
Research shows there are various factors to consider when choosing bulls to use over the herd and should be selected based on traits the farmer is targeting.
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esearch has shown that not every dairy cow needs to be mated to a genetically superior dairy sire, since roughly only 25% of calves born each season are required as replacements. And although dairy farmers have been using beef bulls for decades, there has been little work to identify which bulls will perform the best. Dr Lucy Coleman has spent the past five years trying to identify what makes a good beef bull for a dairy cow. “The dairy farmer and beef farmer have different considerations so, in my research, I was looking at it from both perspectives,” Coleman explains. “Dairy farmers want a bull that won’t impact the health and production of the cow or heifer, so they are interested in the calving difficulty and birth weight of the calf, as well as the gestation length. “Where the beef producer is interested in, is how the calf will perform in a beef rearing system.” Beef bull estimated breeding values are only relative within breeds, so it can be hard to predict the likely impacts when they are used across dairy cows. Coleman’s work with Massey University has been part of the Beef + Lamb NZ Genetics Dairy Beef Progeny Test. “A lot of the earlier work on beef bulls and what people bred beef bulls for focuses on the beef production system, and there are plenty of dairy farmers who can be cautious because they have heard about some of the risks and that has put them off,” she says. “But as we know with dairy bulls, there are good bulls within breeds; one bull isn’t equal to another.” She looked at a comparison of birth weight, gestation length and preweaning growth of calves from mixed-aged dairy cows that were artificially bred to a selection of Angus and Hereford bulls from the lighter end of the scale. And she found there was little impact from the variation in birth weight on calving difficulty.
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Lucy Coleman’s research has shown that dairy farmers look for certain traits when selecting bulls so their progeny carry desirable traits.
“But as we know with dairy bulls, there are good bulls within breeds; one bull isn’t equal to another.” “Less than 1% of the cows needed assistance at calving, although there was a negative correlation between birth weight and gestation length with age at weaning,” she says. “So, when choosing a bull with very light birth weight there may be a tradeoff to the growth of the calf.” This means when choosing Angus and Hereford bulls to use over dairy cows, farmers should firstly be looking for bulls with low enough birth weights to reduce the risk of calving difficulty, but also emphasise gestation length and preweaning growth. “They do need to be conscious of the
tradeoffs between calving traits and growth traits, but should relate them specifically to the objectives of the herd and how important the growth traits are to their system,” she says. “The goal should be to produce calves that are born without assistance that strike the right balance between income from calf sales and income from extra days in milk.” She also uncovered there were no negative effects on milk production or rebreeding success of the cow bred to the different Angus and Hereford bulls. Another novel trait she looked at in her research was tongue colour, hoping to determine whether it could be a useful predictor of the breed of newborn Angus-cross-dairy and dairy crossbred calves, when their similar coat colour makes it difficult. “Holstein-Friesian cattle have a white spotting gene that causes nonpigmentation in their coat colour and
DAIRY FARMER
September 2021
