Fifty years ago in Australia, soon after subterranean clover (Trifolium subterraneum) became a popular forage there, farmers and scientists observed a dramatic decline in the fertility of ewes grazing subclover pastures. The longer the sheep grazed these fields, the lower their fertility. Farmers also reported seeing occasional udder development and aberrant lactation in non-pregnant ewes and even in wethers. The Australians called this problem “clover disease” and concluded it was caused by a type of estrogen. But who would put estrogens in the diet of grazing sheep? It turns out it was the plants themselves: They contained hormone-active chemicals called phytoestrogens.
Trifolium subterraneum and Medicago sativa. Illustrations by Kristen Phillips.
“Phyto” = derived from or associated with plants. “Estrogen” = a steroid hormone with powerful physiological effects, especially on the reproductive organs. Some plants, particularly legumes, can contain phytoestrogens – sometimes at high levels – and these can have profound effects when consumed by livestock.
Before everyone runs out and pulls their cattle and sheep off perfectly good legume pastures, I must add that most legumes are safe most of the time. Only some species of legumes contain enough phytoestrogens to cause problems. Also, some perfectly good legumes that are usually quite safe for livestock produce phytoestrogens only when they suffer from certain kinds of stress. But read on.
Phytoestrogens in legumes come in two chemical flavors: isoflavones and coumestans. These two families of estrogen-like compounds are synthesized in different biochemical pathways. The isoflavones are usually found in the true clovers: subclover, red clover, berseem clover and occasionally white clover. Specific isoflavones are formononetin, genistein and biochanin A. As Australian farmers observed, subterranean clover can be the biggest culprit. Its isoflavone levels can sometimes reach nearly 5% of its dry weight. However – and this is a big however – isoflavone levels are heavily influenced by genetics.
Some varieties of subclover (and red clover, etc.) produce very high levels of isoflavones, like the classic Australian subclover varieties of Yarloop, Dinninup and Tallarook. In contrast, some varieties produce very low levels. For example, we grow lots of subclover here in western Oregon – but our local varieties, Mount Barker and Nangeela, are low in phytoestrogens so we don’t worry about our cattle or sheep grazing them. In Australia, as fewer acres are now planted to high-isoflavone varieties, the incidence of clover disease is going down.
The other family of legume phytoestrogens, coumestans, can occasionally occur in alfalfa (Medicago sativa) and the annual medics (other medicago species). But I need to emphasize the word “occasionally” because coumestans in these plants seem to be a response to leaf disease, unlike isoflavone levels in clovers that are highly influenced by genetics. These coumestans (specific compounds such as coumestrol, sativol and others) only appear in these plants after their leaves suffer serious stress, like fungus infections or attacks by aphids. Otherwise, these forages don’t contain enough phytoestrogens to cause problems in livestock. This helps explain the sporadic phytoestrogen reports with alfalfa – because the problem only occurs sporadically.
Phytoestrogens, particularly the isoflavones, create their reproductive havoc by affecting the cervix. In ewes after mating, sperm cells usually remain in the cervical area for a brief period before moving into the uterus. Normal cervical cells produce a mucus that helps support these sperm cells. Phytoestrogens alter this cervical mucus so that within 24 hours, more than 95% of the sperm is no longer viable. Phytoestrogens also modify cervical cell growth, so that under a microscope the cervical tissue looks more like a uterus than a cervix.
Cattle are also affected by phytoestrogens, but not quite in the same way. Fertility indeed declines, but cows show different symptoms, such as cystic ovaries, irregular estrous cycles, etc.
Half the population of cattle and sheep, however, can breathe a sigh of relief because these phytoestrogens have no effect on male reproduction. Bulls and rams remain fertile and virile, although a farmer may become a bit unnerved to see udder development in a wether.
A point of interest, however, about the potency of these phytoestrogens: Molecule for molecule, mammalian hormones such as estrogen are far more potent than these plant phytoestrogens. For example, mammalian estrogen is nearly 200 times more potent then coumestrol, 6,900 times more potent than genistein and 26,500 times more potent then formononetin. Livestock are affected by these phytoestrogens because they can consume quite a lot of phytoestrogen molecules by eating many pounds of forage.
But here’s a logical question: Why do plants even make phytoestrogens? A clover plant doesn’t have a cervix or any need to regulate an estrous cycle. So why waste precious biochemical energy manufacturing useless compounds? But the concept of regulation may be key to this riddle. Regulation entails chemical communication – i.e., biological tissues communicating with each other. Cell-to-cell communication. When we look at the whole picture, a broader picture emerges. It seems that the real role of estrogen is to communicate information (instructions) among cells.
Which forage plants produce isoflavone phytoestrogens? Only legumes. Aha! Not grasses or trees or thistles. Legumes, of course, have that wonderful ability to create a symbiotic relationship with specialized bacteria (rhizobia species) that can extract, or “fix,” nitrogen from the air. These rhizobia bacteria form nodules on the roots of legumes, where they create bacterial colonies and fix atmospheric nitrogen into bacterial protein. The legume plant gains nitrogen from this relationship, giving it a great competitive advantage in the struggle for nutrients.
Each legume species needs its own unique rhizobia species. So how, in that dark world in the soil, can a plant get in touch with the appropriate bacteria? By communication. Possibly by using phytoestrogens. In fact, rhizobia bacteria contain receptor proteins that seem precisely designed for these phytoestrogens. This research is new, and it may not explain all the phytoestrogens, but scientists are beginning to speculate that when these bacterial proteins encounter the appropriate phytoestrogens, the resulting reaction tells the bacteria to initiate other biochemical processes, causing the formation of root nodules. In other words, legume phytoestrogens may be the plant equivalent of signal lamps in a darkened room. “One if by land; two if by sea …”
The effects of phytoestrogens on livestock – clover disease, irregular estrus, lowered fertility – well, these problems may only be collateral damage from the main event, a chemical battle among plants for nutrients and livelihood. Our cattle and sheep are just innocent bystanders.
