Cattle that graze toxic endophyte-infected tall fescue can have a vulnerability to severe heat stress under moderate air temperatures, rough hair coats in the summer to add to the heat stress, reduced dry matter intake, and generally poor performance and thriftiness. Conception/calving percentages and weaning weights can be reduced, and poor post-weaning weight gains have caused toxic endophyte tall fescue to be minimally used for stocker production and heifer development.
There are several options for livestock producers in dealing with fescue toxicosis. Toxic endophyte tall fescue can be replaced with endophyte-free cultivars, but once the endophyte is removed from fescue, the plants lack the persistence to have long-term reliability. Heavy grazing combined with dry weather and the stand is gone!
The only option to alleviate fescue toxicosis is by replacing toxic endophyte tall fescue with a nontoxic, novel endophyte fescue (see pages 24 and 25 of the March issue of Hay & Forage Grower). These nontoxic endophyte tall fescue may require more attention than toxic endophyte fescue, but the extra management can reduce a risk of stand loss. Remember, all grass stands can be lost, including toxic endophyte fescue, when heavy grazing is combined with drought conditions!
What if your farm has steep, shallow, and rocky hillsides that make you hesitate to replant? You might consider that you have enough other grasses in your pastures to dilute the toxic endophyte fescue, but what is the maximum percentage of tall fescue before the toxic endophyte can have a subtle effect on livestock performance? Research done at the University of Tennessee over 30 years ago indicated that the toxic endophyte affects animal physiology and performance when toxic endophyte fescue is over 20 percent of the pasture stand.
There are management options to mitigate the adverse effects of tall fescue on livestock physiology and performance: 1) chemical suppression of tall fescue seedheads, 2) feeding of soy hulls, or 3) overseeding with red clovers. Each has advantages and disadvantages that will be discussed.
Cattle readily consume seedheads of tall fescue, primarily when the seedheads are immature and not fully developed. The seedheads offer an excellent source of soluble carbohydrates but, unfortunately, are highly concentrated with the toxic ergot alkaloids that cause the toxicosis. Our research has demonstrated that treatment of endophyte-infected tall fescue with metsulfuron-methyl, an active ingredient in Chaparral herbicide (Dow AgroSciences; Indianapolis, Ind.) can suppress the emergence of seedheads.
A two-year grazing experiment with steers grazing toxic endophyte-infected fescue found a 39 percent increase in average daily weight gain with seedhead suppression as compared to those on unsuppressed pastures. Prolactin, the hormone that is consistently low in cattle exhibiting signs of fescue toxicosis, was twofold greater in the serum of steers grazing seedhead suppressed fescue pastures. Furthermore, maintaining the fescue in a vegetative stage of growth resulted in higher crude protein and digestibility being maintained through the two grazing seasons.
Another grazing experiment with endophyte-free tall fescue grazed with light and moderate grazing intensities determined that seedhead suppression provided a 19 percent bump in average daily weight gain, which indicated that approximately half of the improvement in steer performance with seedhead suppression could be linked to maintaining the plants in a vegetative stage of growth.
Use rotational stocking
The research we had done was with pastures that were continuously stocked, but it was apparent the cattle selectively grazed the tall fescue and no other cool-season grasses, such as Kentucky bluegrass and orchardgrass, that were in the mixture with fescue and allowed to accumulate and mature. The intensive grazing of fescue and lack of uniform grazing across pastures was a concern, so another grazing experiment was conducted to compare between rotational and continuous stocking of seedhead-suppressed toxic endophyte tall fescue.
The thought was that higher stocking densities (stocking rate at a given point in time when a rotationally stocked paddock is grazed) would force the cattle to graze other grasses. With rotational stocking of seedhead-suppressed pastures, the cattle in the experiment grazed all forage grasses, and the uniform grazing was indicated to provide higher individual performance and weight gain per acre. Therefore, rotational stocking is highly recommended when using chemical seedhead suppression.
What about controlling seedheads of tall fescue by mowing? Seedheads can be mowed, but this must be done at or just prior to the boot stage because cattle will graze the seedheads as they emerge out of the boot. You could find a pasture of barren stems with no seedheads if you wait too long to mow. A second mowing will also likely be needed to remove seedheads of late developing plants. Another consideration should be made if weed control is needed. Weed control by mowing will be best with close mowing, something you may not want to do when you have good, lush vegetative growth.
For maximum suppression, spraying should be done according to the label and in the late vegetative stage of growth as close to the boot stage as possible. Spraying too early can cause excessive yellowing and dampening of growth; Chaparral will also kill any clovers in the stand.
Pastures of toxic endophyte tall fescue also can be overseeded with clovers to dilute the ergot alkaloids in the cattle diets to mitigate the adverse effects they have on cattle performance; however, our research has shown there could be more than a dilution effect on mitigating fescue toxicosis. We conducted grazing research to evaluate the use of ear implantation with steroid hormones (estradiol-progesterone) and feeding soy hulls for increasing average daily weight gain of 8- to 10-month-old steers on toxic endophyte fescue.
Compared to the pasture-only control treatment, the implants improved average daily gain by 13 percent, and the soy hulls, fed at approximately 0.8 percent of body weight, increased average daily gain by 31 percent. However, by combining the two treatments we found a 70 percent increase in average daily weight gain. Feeding soy hulls also increased prolactin concentrations in the blood, and a higher percentage of these cattle shed their hair coats. It was not understood why there was a synergistic effect of combining ear implantation with soy hulls, but we now know that soy hulls contain phytoestrogens, isoflavones, that could have the same activity as estradiol in promoting animal growth.
It appears that ear implantation of estradiol can be combined with the isoflavones of soy hulls to boost calf weight gain in a synergistic manner. Further, published results from a pen experiment with goats demonstrated that an isoflavone produced by red clover, biochanin A, can reverse the reduction in blood flow to the peripheral tissues caused by ergot alkaloids that lead to severe heat stress and fescue foot. More research is needed to verify this work but, thus far, it appears that red clover can provide good benefits to the cattle when it is overseeded into toxic endophyte-infected tall fescue.
These technologies provide methods to mitigate the adverse effects of ergot alkaloids on cattle performance and physiology. The use of seedhead suppression, feeding soy hulls, or overseeding with red clover are methods to manage around fescue toxicosis, but it must be understood the cattle are still consuming some ergot alkaloids with these strategies. Nonetheless, these practices have shown to increase cattle growth performance and provide enough mitigation of fescue toxicosis to warrant their use by cattle operations that cannot justify replacement of their toxic endophyte fescue with a non-toxic endophyte fescue. •
This is the second of a two-part series on strategies to alleviate the effects of fescue toxicosis.
This article appeared in the April/May 2016 issue of Hay & Forage Grower on pages 6 and 7.
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