The Potential of Brahman Genetics to Mitigate Fescue Toxicosis in Beef Cattle

In an earlier issue of the Brahman Journal, Dr. Mike Brown provided an introduction to the problem of fescue toxicosis and the potential benefits of using Brahman genetics to offset reproductive and preweaning performance problems that may occur in cow-calf herds grazing endophyte-infected tall fescue. The current article details some of the basics of fescue toxicosis, extends the concept of Brahman influence to post-weaning growth, and takes a brief look at the limited data on basic physiological responses to ergot alkaloids in Brahman-influenced steers.

Tall fescue (Festuca arundinacea) is the most commonly used cultivated grass for grazing beef cattle in the United States. Most tall fescue are infected with an endophyte (the fungus Neotyphodium coenophialum) that is beneficial to grass performance but detrimental to cattle performance. This cool-season perennial grass is one that many cattle producers ‘can’t live with, but can’t live without’ because of its satisfactory forage yields, but negative effects on cattle yields. Relationships between tall fescue endophyte toxicosis and cattle performance have been well documented (Paterson et al., 1995; Browning, 2003). The three general conditions associated with endophyte-infected tall fescue consumption by cattle are fescue foot, fat necrosis, and fescue toxicosis. Fescue foot and fat necrosis are relatively infrequent occurrences. Fescue toxicosis is a pervasive problem in fescue-based beef production systems across the Southeast and Midwest, extending west to eastern regions of the southern Great Plains. Cattle experiencing fescue toxicosis may exhibit rough hair coats, heat stress, suppressed appetite, poor growth, or reduced calving rates. Fescue toxicosis costs the U.S. beef industry an estimated $500 million to $1 billion annually in lost revenue because of reduced reproductive and growth rates in cattle herds. Seedstock, commercial cow-calf, and yearling/stocker segments have a financial incentive for seeking ways to minimizing or eliminating the problem.

Ergot alkaloids are the generally accepted toxic agents produced by the fescue endophyte. Ergovaline is one of the most abundant ergot alkaloids detected in endophyte-infected tall fescue. Testing of fescue samples for endophyte infection rate and(or) ergovaline concentration is recommended to indicate the toxic potential of tall fescue pasture or hay to be fed. Ergotamine is an ergot alkaloid found in endophyte-infected tall fescue at lower levels than ergovaline (Yates et al., 1985). Ergotamine and ergovaline have similar structures and pharmacodynamic properties (Porter, 1994; Larson et al., 1999; McLeay et al., 2002). The effects of purified ergot alkaloids such as ergotamine on cattle physiological status are studied to gain a better understanding of how endophyte-infected tall fescue reduces cattle well-being and performance (Osborn, 1992; Browning and Leite-Browning, 1997).

Researchers have sought to devise methods of alleviating fescue toxicosis on two fronts, forage management and animal management. The pasture management approach is aimed at reducing or eliminating dietary ergot alkaloid exposure. The most recent development is the use of ‘novel’ endophytes that produce altered profiles of ergot alkaloid production (Gunter and Beck, 2004). Novel endophytes produce alkaloids that provide stress resistance to the fescue, but do not produce alkaloids responsible for fescue toxicosis in cattle. The time and expense involved in pasture renovation, the vast number of acres covered in endophyte-infected tall fescue, difficulty in cultivating some terrains, and general reluctance to eradicate long-established, healthy tall fescue stands may limit widespread use of novel endophyte varieties and other pasture management approaches. Less research attention has been directed towards animal management procedures to help cattle overcome exposure to ergot alkaloids. Unlike the recent emergence of novel endophytes, there have not been developments of similar magnitude on the animal side. However, like in the plant research effort where recent advancements were made by exploiting genetic variability in endophyte populations, genetic variation in cattle populations may be utilized to manage against fescue toxicosis.

An animal genetics approach would be to identify and select animals less responsive to ergot alkaloids. Studies on within-breed or within-herd selection have not yielded promising results for timely genetic improvement in beef cattle tolerance of ergot alkaloids consumed on fescue diets. Cattle grazing endophyte-infected tall fescue often exhibit exaggerated signs of summer heat stress. Classic indications of fescue toxicosis include high respiration rates and open-mouth panting, increased time spent under shade, standing in water sources, creation of and lying in mud wallows, and decreased daytime grazing. These behaviors are attempts to dissipate excess body heat created by physiological responses to ergot alkaloids. Heat stress in cattle experiencing fescue toxicosis has led to experimentation on the potential of heat-tolerant germplasm for cattle on endophyte-infected tall fescue, such as the work presented earlier by Dr. Mike Brown for cow-calf performance.

Effects of Fescue ToxicosisThe effects of fescue toxicosis go beyond cow-calf performance to weaning. A very limited number of studies have investigated the effect of Brahman-influence on average daily growth rates (ADG) in steers exposed to endophyte-infected fescue (Table 1). Goetsch et al. (1988) tested British breed cross and Brahman cross steers from April to July and from August to November in Arkansas. Reductions in ADG over 12 weeks by endophyte-infection were deemed similar for the breeds in the spring and fall as breed-by-diet interactions were not statistically significant. An exception was during the first half of the fall season when the ADG of Brahman crosses was statistically less affected by the endophyte. Angus, Brahman x Angus, and Simmental x (Brahman x Angus) steers were examined from November to May by McMurphy et al. (1990) in Oklahoma. A breed-by-diet interaction was statistically significant for post-weaning ADG as half-blood Brahman steers were less affected by high endophyte levels than straight Angus or quarter-blood Brahman steers. Cole and coworkers (2001) did not detect a statistically significant breed-by-diet interaction for the growth of Brahman-cross and Angus steers when grazing fescue pastures with high or low endophyte infection levels from April to August in Georgia.

The lack of statistically significant breed-by-diet interactions in individual studies may be a function of limited observations. Collectively, a consistent trend is apparent. High endophyte levels in the tall fescue diets invariably reduced ADG of Brahman-cross steers to a lesser degree than when steers did not possess the Brahman influence. Brahman genetics reduced the adverse effects of endophyte-infected tall fescue on post-weaning ADG by 26% on average (range = 10 to 65%) across the studies. Actual growth rates of Brahman-cross steers on high endophyte pastures were ‘equal to’ or ‘greater than’ non-Brahman-influenced steers that grazed low or non-infected pastures, with the one exception being quarter-blood Brahman steers (Goetsch et al., 1988; McMurphy et al., 1990; Cole et al., 2001). Data from Table 1 suggest that Brahman germplasm would be a useful animal management option to lessen the impact of fescue toxicosis in stocker programs.

McMurphy et al. (1990) and Cole et al. (2001) also recorded rectal temperatures. In both reports, high endophyte-infected tall fescue diets caused rectal temperatures to rise in Angus steers by the end of the grazing period, whereas rectal temperatures Brahman x Angus steers were unaffected by diet. Rectal temperatures were not affected by diet in any breed during cooler observation periods, although breed-by-diet interactions showed the ADG of Brahman x Angus to be less inhibited by high endophyte tall fescue during some of those same cool season intervals (McMurphy et al., 1990).

Subsequent research in Tennessee assessed the comparative physiological responses of Brahman to ergot alkaloids associated with the tall fescue endophyte. In one experiment, fullblood Red Brahman and Hereford steers were similar in 90-minute cardiovascular and peripheral skin temperature responses to ergotamine administered intravenously (Browning, 2000). The same steers were observed for 240 minutes in a second study during which ergotamine increased respiration rates and plasma thyroid hormone concentrations in the Hereford but not in the Brahman steers (Browning and Thompson, 2002). For several other hormones and blood metabolites, Brahman steers were more reactive to ergot alkaloid treatment. The ergotamine work involving Brahman steers and the data in Table 1 agree in suggesting that Brahman-influenced cattle differ in their responsiveness to ergot alkaloids compared to cattle not carrying Brahman genetics.

Infusion of Brahman genetics seems to offer an alternative means of reducing the impact of fescue toxicosis on both cow-calf and stocker performance. Use of Brahman genetics to overcome challenging environmental conditions is not a new concept. Somewhat overlooked, however, may be the potential of Brahman germplasm to enhancing cattle performance in another challenging production environment, the high endophyte-infected tall fescue pasture.

This article was written by
Richard Browning, Jr., Ph.D.
Associate Professor at
Tennessee State Univerrsity

Dr. Browning, Jr. has been an Assistant Professorat Tennessee State University since the Fall of 1994. He focused on physiological mechanisms and heat-tolerant cattle genetics in relation to fescue toxicosis. In 2002 he began a comprehensive meat goat reaerch effort at TSU focusing on fitness and performance traits among various breeds. He received his B.S. degree from Prarie View A&M University in 1989, and his M.S  and Ph.D from Texas A&M University. Dr. Browning’s graduate research focused on the reproductive performance of postpartum Brahman cattle and pre-weaning calf performance of tropically-adapted genotypes under the guidance of Dr. Ron Randel at Texas A&M – Overton. Dr. Browning was raised in Raywood, Texas where he was active in FFA and 4H through raising and showing Red Brahman cattle.