----- Original Message -----
Sent: Tuesday, July 09, 2002 6:51
AM
Subject: Re: [RC] The skinny on tall
fescue and horses --was Pasture
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)
