Phytoestrogens are plant estrogens. When consumed by livestock they
exhibit estrogenic activity and may hamper reproductive performance. Clovers
such as subterranean clover (Trifolium subterran) and red clover
(Trifolium pratense) generally contain isoflavones. Alfalfa (Medicago
sativa)contains coumestrans.
Subterranean Clover
Subterranean clover is a winter annual grown in the northwest regions
of the U.S. as a winter and spring pasture crop. It pushes its seeds down
into the ground upon maturity and thus reseeds itself. Its seeds can remain
dormant for many years making it difficult to eradicate once established
in a pasture.
Red Clover
Certain water soluble glucosides in plants bear a close resemblence
to estradiol and are known as plant estrogens (click
here to see the similarities). Clovers synthesize isofavones such as
biochanin A, daidzein, formononetin, and genistein from phenylalanine,
while alfalfa synthesizes coumestans from cinnamic acid. Although formononetin
has about half, one third, and one quarter the estrogenic activity of biochanin
A, diadzein, and genistein, respectively, negative estrogenic effects on
sheep appear to be directly correlated to the formononetin content of the
clover forage. This is because rumen microorganisms
degrade genistein and biochanin A to p-ethylphenol and phenolic
acids, thus destroying their estrogenic effects. However, rumen microorganisms
demethylate formononetin to daidzein, and then together with any daidzein
directly consumed from the clover, these minor phytoestrogens
are metabolized to equol which is a much more potent form of estrogen.
Absorbed equol is rapidily excreted in the urine of cattle, but not in
sheep. Thus sheep are much more susceptible to the estrogenic effects of
isoflavones. Isoflavines alter the responsiveness of the animal tissue
to endogenous estrogen. High serum levels of phytoestrogens may also inhibit
the release of gonadotropic hormones from the pituitary gland and fill
receptor sites in the cervix and uterus meant for endogenous estrogens.
The cervix of a ewe exposed to high formononetin clover cultivars undergoes morphological changes. The cervical folds fuse together so that the cervical tissue resembles uterine tissue. During breeding season, the cervix and vagina fail to to respond to endogenous estrogen stimulation and temporary infertility results because of reduced ability of the sperm to penetrate the oviduct. This is because the cervical mucus does not undergo its normal changes or "priming"to aid sperm longitivity in the cervix. After 24 hours, the amount of viable sperm present in the cervix is only about 5% of that found in a healthy cervix. Ovulation generally occurs following standing heat so this reduced longevity of sperm can have serious consequences on conception rates. The condition may become permanent if the ewe remains on these forages for prolonged periods of time.
Cattle that are exposed to alfalfa cultivars with high contents of
coumestans may experience cystic ovaries and irregular heat cycles. Dairy
heifers may exhibit precocious mammary and genital development.
[Index] [Introduction][Clovers][How poisoning occurs][Symptoms of poisoning]
This series of web pages was created by an undergraduate student at Cornell University for the AS625 class. All comments and suggestions are welcome.
WARNING: These web pages are only meant to be informative. Neither Cornell University nor the author of this site endorse or recommend the use of these plants.
