The average number of lactations of a U.S. dairy cow currently rests at about 2.8, or around 5 years of age.
That’s a fairly shocking statistic, considering a cow’s natural lifespan can be up to 20 years or more. And, on average, it takes about two full lactations before heifers begin to generate return on investment for their rearing or purchase cost. Given today’s robust heifer values, that time before young cows begin to pay the bills may be even greater.
So, how can we help cows live longer, more productive lives? Researchers at the University of Florida assessed one factor: birth season. They predicted that cows that entered the world during seasons of heat stress would have shorter lifespans. And they were right.
The study, led by researcher Izabella Toledo and published in the Journal of Dairy Science, examined the DairyComp records of more than 10,000 cows in Florida and 8,000 in California that remained alive and productive for more than 5 lactations.
The data were sorted to identify animals born over a period of 10 years (2012-2022) in the cool season (December, January, February, and March) and the hot season (June, July, August, and September). Cows born in the more temperate months of April, May, October, and November were not included in the dataset.
In Florida, 14.5% of cows (1,567) born in the test months were still alive and milking after 5 lactations. Of them, more than double (1,129, or 72%) were born in the cool months compared to the hot months (438, or 28%). In California, 20.4% (1,669) of the dataset made it 5 lactations or longer, with 56.% of them born in the cool months, versus 44% born in the hot months.
The Florida dataset also was analyzed for the number of cows born in the tests months that were dead or sold for beef in the first 4 lactations, and the reasons why. A total of 1,454 were sold and another 238 died. Selling reasons included breeding, foot and leg, digestive, and respiratory issues, along with mastitis.
Significantly more cows born in the hot season (53%) compared to the cool season (47%) were sold for beef.On-farm deaths also were significantly higher for cows born in the hot (54%) versus cool (46%) season.
Toledo and her team concluded that the results give even more credence to the deleterious impacts of heat stress on dairy productivity. Previous studies – many also conducted by University of Florida researchers – have shown that heat stress during late pregnancy affects dams’ milk production in the next lactation, immune function, and calf birth weight.
Further, they have found that calves exposed to heat stress in late gestation had 19% lower milk production in their own first lactations, and even passed that lower milk production potential on to their offspring.
Toledo said the results of the current study suggest a potential two-pronged approach to protecting the productive life integrity of newborn heifers: (1) implement heat-stress abatement measures for dams, including shade, fans, soakers, and misters; and (2) alter breeding decisions to avoid births in seasons of peak heat.
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