Unexplained abortion and abnormal ageing in cattle

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Cattle ranchers are facing some puzzling – and, at times, economically devastating – problems with pregnant cows and calves.

At some facilities, high numbers of fetuses are aborting for no apparent reason. Other farmers successfully raise what look to be normal young cattle, only to learn when the animals are butchered that their carcasses appear old and, therefore, less valuable. As a result, farmers are making only a fraction of the anticipated profit from their livestock.

At present, no one knows what is behind either syndrome, notes Michael J. Fields, an animal scientist at the University of Florida in Gainesville. He suspects, however, that both trace to a common problem, and proposes that the feed or water at these ranches is tainted with hormone-mimicking pollutants.

Fields has been investigating environmental factors that might be triggering the spontaneous abortions. The sporadic problem is so bad both in the United States and abroad that in some herds around 40 to 50 percent of pregnancies are being lost.

These fetal losses occur early, at the time of embryo implantation, he says.

The problem may be new, or just newly recognised now that cattle producers have begun making wide use of ultrasound. They employ the technology to identify pregnant animals within a month of when they conceive. It is when farmers check on the progress of those pregnancies a month later, again with ultrasound, that they discover a problem, Fields notes. Many cows prove no longer pregnant.

Fields notes that a colleague, Mordechai Shemesh of Chemron Veterinary Institute in Bet Dagan, Israel, recently conducted an experiment in which he grazed pregnant cows on grass or a legume. Those foraging on the legume had twice the spontaneous-abortion rate of animals eating grass. One of the big differences in those two plant types, Fields observes, is that legumes can contain high concentrations of phytoestrogens – natural compounds that in mammals mimic the biological activity of estrogens. Ordinary grasses don’t. Such environmental estrogens may explain what’s happening in the cows’ reproductive tract.

Oxytocin is a pituitary hormone that stimulates uterine contractions during the natural birthing process. It also stimulates the secretion of milk. However, Fields explains, when oxytocin is released in the early stages of pregnancy, it prompts the uterus to secrete a substance that kills off the ovarian tissue nurturing a fertilised egg.

What normally triggers the secretion of oxytocin is a hormone-like substance called prostaglandin-F2alpha. Interestingly, Fields notes, estrogen can sensitise the animals’ reproductive tracts in ways that can enhance their production of that prostaglandin.

Legumes provide cattle with good nutrition, Fields is quick to note. However, he adds, if cows forage heavily on these and other plants rich in natural estrogen-like substances during a critical window of susceptibility – such as embryo implantation – their pregnancies may fail.

Moreover, he notes, many pesticides and industrial pollutants also possess a hormonal alter ego. Lactating animals excrete estrogens, as do feedlot cattle that are receiving growth-stimulating hormone implants. Even some of the molds that contaminate silage can produce estrogenic products. If such hormonally active agents taint feed, forage, or a herd’s drinking water supply, they might also foster fetal losses, Fields proposes.

He is currently in the process of testing these hypotheses under a grant from the US Department of Agriculture.

Hard bone
On ranches where there is no problem of abortion, livestock producers sometimes raise calves that bear the look and vigor of health but have a hidden problem. As yearlings, these animals will usually be shipped to a feedlot where they will be provided with growth-boosting feed and sometimes drugs. However, when these animals are later slaughtered as 2-year-olds, some carcasses will give every appearance of coming from cattle twice their age. This condition is described as ‘hard bone.’

Fields explains that ordinarily the carcass of a 2-year old will have nice, red-coloured bone and cartilage. But in animals with hard bone, the carcass looks overly mature, with dried out white bone and minimal cartilage. Meat graders who inspect these carcasses can see the difference easily.

Because those graders will not know the real age of the animal, they will assume it is a much older one and devalue the carcass accordingly – by up to 60 percent.

Although the problem crops up only sporadically, it generally occurs in clusters – many of the animals from one herd and none of those from another. Moreover, a farm or feedlot hit hard one year may experience no problems the next. Because animals with and without the syndrome will have experienced similar feedlot conditions, Fields now suspects the problem traces back to the cattle’s natal farms.

Here again, he suspects environmental estrogens may be to blame. Certainly, natural estrogen can slow bone growth in older animals, he notes, leaving that tissue with a hard, flinty appearance – the same hard-bone structure now afflicting some young animals.

Today, the United States is the world’s largest producer of beef, primarily high-quality grain-fed beef. Indeed, the US cattle industry is the country’s single largest agricultural sector, with approximately 1 million cattle producers generating some $30 billion in revenues annually. These cattlemen contribute significantly not only to rural economies but also to the nation’s export earnings.

However, the viability of this important industry is threatened, according to Montana cattleman Dennis McDonald.

Speaking in Washington, DC, last July, before the Senate Agriculture Committee, he noted that cattle producers over the past 3 decades have received an ever-shrinking share of the retail dollar that consumers pay for beef. The cattlemens’ share of the beef retail dollar has fallen from some 70 percent in the 1970s to less than 50 percent in 1996, noted McDonald, speaking on behalf of the Ranchers-Cattlemen Action Legal Fund.

Not surprisingly, ranchers and feedlot operators are looking to maximise the return on their investments in calf production and carcass sales. High rates of fetal loss and hard-bone, where they occur, can devastate those economics, Fields says. And until the cause of each is nailed down, producers have no way of identifying measures they might take to limit their risks.

References:
McDonald, D. (2001) Testimony of the Ranchers-Cattlemen Action Legal Fund, United Stockgrowers of America, to the Senate Agriculture Committee hearing on livestock issues for the New Farm Bill. July 24. Available at: www.rcalf.com.

Raloff, J. (2002) Hormones: Here’s the beef. Science News 161(Jan. 5):10. Available at: www.sciencenews.org.

Raloff, J. (1994) The gender benders. Science News 145(Jan. 8):24. Available at: www.sciencenews.org.

Science News Online
6 April 2002

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