The focus for graziers has always been on forage quality in meeting the nutrient requirements of pastured cattle. The content of crude protein, neutral- and acid-detergent fiber (structural carbohydrates), water-soluble carbohydrates, crude fiber and minerals in a specific forage has been used for estimating forage quality and for formulating supplemental concentrate rations.
Forage nutritive values have been the cornerstone of research conducted by ruminant nutritionists and forage agronomists in developing managements to maximize animal performance for both confinement and grazing operations.
Another group of compounds have been ignored, but now there is interest in determining if these compounds are consumed by ruminants and if they can benefit the health, performance and well-being of animals.
These compounds are broadly classified as plant-secondary metabolites. Protein, carbohydrates and lipids in plants are considered the primary metabolites because plants require them for growth, development and maintenance. Therefore, the primary metabolites are required for plants to survive.
Understanding secondary metabolites – the good and bad
Plant-secondary metabolites are produced from primary metabolites and are not required for growth and survival but typically serve a role of improving a plant’s defenses against disease, environmental stresses and grazing.
Some plant-secondary metabolites are toxic to grazing animals. For example, cyanic glucosides produced by many of the sorghums and other plants can cause prussic acid poisoning in ruminants in dry weather conditions or early freezes. The most recognized secondary metabolites are the lethal toxins known to occur in numerous weeds found in pastures and hay fields. Phytoestrogens produced by clovers and other plants mimic estrogen in disrupting estrus cycling in cows, ewes and mares.
Our attention has always been on the adverse effects of plant-secondary metabolites on animal health, but are there any secondary metabolites that benefit health and performance of cattle? The present concern with synthetic growth promoters has led to research that evaluates various plant-secondary metabolites for antimicrobial activity in the rumen, anthelmintic properties or positive physiological effects on cattle performance. Our research has focused on beta acid metabolites produced by hops plants and isoflavones produced by legumes.
The role in ammonia reduction
Alpha and beta acids are produced in the flowers, commonly referred to as cones, of hops plants that have been used as a preservative by the brewing industry for centuries.
We recently conducted laboratory studies that showed beta acids to exhibit antimicrobial activity in the rumen. Beta acids in the laboratory using rumen fluid demonstrated that they can inhibit the hyper-ammonia-producing bacteria (HAB) that break down protein to form ammonia.
Ruminants can recapture some of the ammonia, but a portion of the ammonia is converted to urea and excreted. Therefore, inhibiting HAB could improve the quality and quantity of protein available to the ruminant and reduce ammonia production (see Figure 1).
Soy hulls and fescue toxicosis
In a second laboratory experiment, beta acids functioned similarly to ionophores in reducing ruminal acid production and increasing bacterial production of propionate relative to acetate, which can increase the available energy to the animal.
Although laboratory studies have shown benefits of beta acids that can lead to growth promotion, field experiments will be required before it can be concluded that the benefits observed for rumen fluid in the laboratory will carry over to the whole animal in benefitting health and performance. This will be exciting research if beta acids can be concluded to be available as naturally occurring ionophores.
Isoflavones are secondary metabolites produced by most legumes, such as soybeans, clovers and alfalfa. Our research on isoflavones actually began with a grazing experiment with steers grazing toxic endophyte-infected tall fescue pasture to determine if combining the daily feeding of soy hulls and ear implantation with estradiol can mitigate the adverse effects of fescue toxicosis.
Cattle exhibiting fescue toxicosis have poor weight gain and reproductive performance, maintain rough hair coats during the summer, are vulnerable to severe heat stress in moderate air temperatures and consistently have low blood concentrations of the prolactin hormone. Feeding soy hulls that provided a daily consumption of approximately 0.8 percent of body weight and without implantation increased average daily weight gain by 32 percent over the daily gain on the pasture-only treatment.
Compared to the pasture-only treatment, the implant-only treatment increased average daily weight gain by 13 percent. However, combining the two treatments increased average daily gain by 71 percent.
Feeding soy hulls with or without implants also resulted in fewer steers having rough hair coats, and these steers had 2.5-fold greater serum prolactin concentrations than those not fed soy hulls.
The substantial increase in average daily weight gain by combining soy hull feeding and ear implantation with estradiol was not understood, but follow-up laboratory analyses determined that soy hulls contain isoflavones, which have estrogenic activity.
Compounds exhibiting estrogenic activity are those that can mimic estradiol in promoting growth. It was further shown that the estrogenic activity of isoflavones from the soy hulls and the estradiol from the implants were additives in substantially increasing the estrogenic activity in the blood of steers fed soy hulls and implanted with estradiol.
Red clover extract and ergot alkaloids
We have conducted studies to determine the benefits of the isoflavone biochanin A on ruminant health and performance. Biochanin A is produced in high amounts by red clover and was demonstrated in a laboratory experiment to inhibit the HAB.
Grazing experiments are presently being conducted to verify the benefit of inhibiting these ruminal bacteria in improving steer weight gain and determine the optimum percentage of red clover needed to deliver amounts of biochanin A that can improve growth rates of beef calves.
Biochanin A also was shown with wether goats exposed to ergot alkaloids in a pen experiment to relieve the persistent constriction of blood vessels that occurs when ruminants consume ergot alkaloids. Ruminants undergoing ergot alkaloid-induced constriction of blood vessels cannot regulate their body temperatures, which causes them to be vulnerable to severe heat stress.
In one experiment, the goats were dosed with ergot alkaloids and their carotid arteries were monitored using color Doppler ultrasonography. Once their carotid arteries were constricted, the goats received doses of both biochanin A rich red clover extracts plus ergot alkaloids, and the arteries were found to relax.
Another experiment initially dosed the goats with ergot alkaloids and biochanin A, and after a week, the goats were dosed with only ergot alkaloids. The carotid arteries constricted as soon as the dosage of biochanin A was ceased. These experiments provided evidence that biochanin A can function as a natural-occurring beta-agonist in reversing the constriction of blood vessels caused by ergot alkaloids.
A look at the research horizon
It should be emphasized that our research is evaluating the use of plant-secondary metabolites as antimicrobial growth promoters with other beneficial effects, such as improved blood circulation.
However, those secondary metabolites, such as isoflavones, that have estrogenic activity could have negative impacts on the fertility of breeding cows and heifers. Although certain plant metabolites could eventually be identified to have benefits on reproductive performance, these compounds have not yet been identified.
Our present research focusing on natural growth promoters is showing promise in developing the use of beta acids and isoflavones for growth promotion. Further evaluations are needed to determine the effectiveness of these compounds in promoting growth and to develop feasible and cost-effective approaches to delivering compounds to cattle.
