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A little history on fescue, since there seems to be a lot of misinformation
floating around (and my apologies if it gets more technical than usual--this is
my area of specialty and I tend to get carried away)--
Tall fescue was discovered growing on the Suiter farm in Kentucky in the
late 20's, and produced such a nice stand of grass that handled grazing
pressure, the climate, etc. that when the seed was released as Kentucky 31 in
the 30's it sold like hotcakes. This explains why it now covers
approximately 35 million acres of pasture in the US alone. However, in the
40's, they started to correlate fescue foot and summer syndrome in cattle with
this grass, and in the early 50's found that it contained the endophyte.
It was not until the mid-80's that my major advisor in college proved
definitively that the presence of the endophyte was connected to the problems in
mares--until then it was accepted that it was a selenium deficiency, a fallacy
that was promoted by a poorly designed study in Missouri where they compared
mare response to Se/Vit. E injections in mares grazing fescue with mares grazing
orchardgrass. His graduate student, Julie Monroe, published the first
paper showing this in 1988. Tall fescue was never specifically bred for
the purpose of maintaining the presence of the endophyte--the majority of fescue
pastures were established long before anyone discovered the endophyte or started
making connections between it's presence and the problems in cattle and
horses.
This endophyte produces about 9 different classes of alkaloids, several of
which have been demonstrated to be the causative agents of fescue
toxicosis. The ones of most concern are the ergot alkaloids (related to
the ones produced by ergot, which explains the similarities between fescue foot
and ergot poisoning) and the lolines and perlolines. The first group is
the one which has the most impact on horses, as they act like a
neurotransmitter called dopamine by preventing prolactin secretion from the
pituitary gland. This hormone is the main trigger in starting the
development of the udder and stimulating milk secretion.
Besides the primary effect of depressing prolactin, the ergot alkaloids
also have a secondary effect of depressing progesterone and increasing
estrogen. As I pointed out in an earlier post, the problems associated
with the altered steroid profiles is compounded by the fact that the ratio
between the two is altered--this creates problems with uterine contractions,
placenta expulsion, etc. We believe that the effects on estrogen also
demonstrate that the alkaloids are crossing the placenta and affecting the foal,
and this is supported by research that shows levels of ergocryptine in lamb
fetuses in sheep injected with that alkaloid. We also know that there are
effects on thyroxine and cortisol levels. The release of cortisol,
thyroxine, estrogen, and progesterone are all under the control of the
pituitary, so it's not surprising that the toxins affect those
hormones.
Why do we keep grazing tall fescue if it's so bad? Except for the
toxins, the nutritive quality is great for a grass. The presence of the
endophyte also confers insect and disease resistance, drought tolerance,
tolerance of high grazing pressure and high stocking rates (possibly becuase the
endophyte seems to affect the growth pattern). In the southeast, it's the
best cool season forage going as far as stand life and persistence.
Endophyte-free varieties just don't stand up to the stresses of being a pasture
grass in the fescue belt. We are also finding ways to cope with or prevent
the problems. I was very fortunate to discover the effectiveness of the
drug now called Equidone during my doctoral research, and am thrilled that it's
soon to be released commercially. Although I would prefer to find ways
nutritionally to cope with the problem, in this case a drug is the best solution
until an endophyte-free fescue is developed that performs as well or better than
the infested varieties.
Obviously, I can't reprint my dissertation for you here, and it would
probably bore or confuse everyone but Susan and Heidi and Charlotte
anyway. However, if you want to see the article on the original study, or
see a review that was created from my lit review and that of a colleague, Dr.
Jim Strickland, go to www.equitox.com and
go to the research link. My article is the one about domperidone in
horses, and the review is the one called Equine Fescue Toxicosis, Signs and
Solutions. Whatever happened to the dose titration study I did, I don't
know....Dr. C. keeps promising to send me galley proofs....and has been for
years! LOL
Lisa (Fescue Fungus Fighter Extraordinaire)
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