Something new is afoot underfoot, something we can grow to combat something we can’t see that is becoming resistant to all the medicines we can drench and inject.
A little wordplay, perhaps, but the topic itself is not playful at all. I’m talking, of course, about internal parasites – specifically the nematodes that populate the intestinal tract. They suck blood, damage intestinal cells, cause anemia and scours, reduce performance and sometimes kill animals. For more than 40 years we’ve had the luxury of controlling these nematodes by drenching or injecting any number of handy-dandy commercial drugs (anthelmintics). But those halcyon days are coming to an end.
More and more, we’re seeing veterinary reports that our favorite nematodes have developed resistance to these once-formidable drugs. Most troubling are the reports of nematodes that are resistant to all commercial drugs. On many farms around the country, none of the anthelmintics work. For farmers and ranchers, this is a nightmare scenario worse than the most fearsome horror movie, because it is real.
But something new is appearing on the horizon, something we can grow. Some forages contain compounds that reduce nematode populations in livestock or help the animals cope better with these parasites without the use of commercial drugs. We can grow these forages in our pastures, and the compounds they contain are called tannins.
You’ve probably heard of tannins, as in “tea” and “tanning leather.” The chemistry of tannins is filled with mind-numbing details, of course, but here I want to discuss their potential for controlling parasites. In brief, tannins are complex phenolic compounds of high molecular weight that, under certain conditions, will bind to proteins and other large molecules. They are astringent – they make your mouth pucker. In general, high tannin levels reduce a plant’s palatability. And although tannins are secondary compounds, some plants produce them in rather large quantities.
But tannins are not all alike. They are divided into two basic categories: hydrolyzable tannins and condensed tannins. The hydrolyzable tannins can be toxic to livestock, especially ruminants. They cause gastroenteritis and extensive damage to the liver and kidneys. Oak toxicity, for example, is due to hydrolyzable tannins, and it occurs mainly when animals consume large amounts of young oak leaves or green acorns. But hydrolyzable tannins are not what interests us now.
Condensed tannins
Our interest in this article is the condensed tannins, particularly those found in some legumes and other forages. Chemists have characterized a large assortment of these condensed tannins, all with long, unpronounceable names. Condensed tannins can affect forages in many ways. Because they bind with proteins, tannins can alter the digestibility of proteins and thus change a forage’s nutritional value. Their strong, astringent taste can reduce palatability and intake. They can prevent legume bloat by binding to certain plant proteins. And, in some situations, they can reduce the effects of nematode parasites. Let’s focus on this last attribute.
Over the past few years, researchers have studied one legume species in particular: sericea lespedeza (Lespedeza cuneata). Sericea is a warm-season perennial found throughout the southern and southeastern U.S. It grows particularly well in low-fertility, low-pH soils where other legumes refuse to thrive, and it’s used for grazing and hay as well as erosion control on hillsides and reclaimed soils. The scientific reports have been fairly consistent: Sheep and goats that grazed sericea lespedeza or were fed sericea hay showed lower fecal egg counts (FECs) and had fewer nematodes in their gastrointestinal tracts than the control animals. Sericea seems to suppress many species of nematodes, from the barber pole worm (Haemonchus contortus) in the true stomach to a variety of nematode species that live in the lower tract.
Sericea lespedeza is not the only forage legume with high levels of tannins, although it is the most studied. Other legumes with significant amounts of condensed tannins include birdsfoot trefoil (Lotus corniculatus), big trefoil (Lotus pedunculatus), sainfoin (Onobrychis viciifolia) and sulla (Hedysarum coronarium). Low levels of condensed tannins are also found in nonlegume forages like chicory (Chicorium intybus) and plantain (Plantago lanceolata), although these latter species may also contain other types of compounds with anti-nematode properties.
Finding the right balance
Tannins require a balance. The amounts in a plant make a difference. Too much will make the forage so unpalatable that animals will refuse to eat it. Too little will be ineffective against the nematodes. Researchers have estimated that the sweet spot may be 30 to 50 grams of condensed tannins per kilogram of forage dry matter, although this may vary because each forage species contains its own array of tannins. Sericea lespedeza and birdsfoot trefoil contain around 50 grams per kilogram. I’ve seen tannin levels in big trefoil listed from 80 to 200 grams per kilogram (and big trefoil is notoriously unpalatable to most livestock, at least at certain times of the year). Chicory seems to contain less than 5 grams per kilogram. In contrast, alfalfa contains only 0.5 gram per kilogram, and perennial ryegrass contains less than 2 grams per kilogram. Not surprisingly, neither alfalfa nor perennial ryegrass has shown any effectiveness against nematodes.
These tannin levels may become more usable if we convert them to percentages. Recall that 1 kilogram equals 1,000 grams. Therefore, 100 grams per kilogram equals 10%, and 50 grams per kilogram equals 5%. Thus, the recommendation listed in the previous paragraph of 30 to 50 grams per kilogram means that forages should contain 3% to 5% condensed tannins for effective control of nematodes. Let’s think about this: 5% of the dry matter is really a lot of material. For example, 1 ton of sericea hay with tannins at 5% dry matter would contain 90 pounds of condensed tannins – the weight of some newborn calves. Compare these levels with other feed components: Forages generally contain only 1% to 2% fat; and corn grain, the commercial source of corn oil, contains only 4% fat.
(I know what you’re thinking: 5% of 1 ton equals 5% of 2,000 pounds, which equals 100 pounds, not 90 pounds. But . . . our hay contains 10% moisture, which equates to hay at 90% dry matter. Which means that 1 ton of this hay contains 1,800 pounds of dry matter. Therefore, 5% of 1,800 pounds equals 90 pounds.)
On a mechanistic level, do we know exactly how tannins reduce nematodes? In a word, no.
But there are two feasible explanations. One is direct, one is indirect. (And both may be happening at the same time.)
Why it could work
The direct explanation is that the condensed tannins are somehow directly toxic to the parasitic nematodes. Researchers have observed that tannins result in lower nematode populations in the gut and smaller numbers of viable eggs released in the manure. Although the exact mechanisms are unknown, condensed tannins may somehow alter or interfere with nematode metabolism so that the parasites cannot thrive in the gut or reproduce properly. Lower egg production would also reduce nematode contamination of the pasture, something researchers have measured. And that, in turn, would reduce parasite ingestion during subsequent grazing rotations.
The indirect explanation is that condensed tannins improve the protein nutrition of the host animal, so the animal is generally healthier and its immune system can better cope with a parasite infection. In other words, better protein nutrition enhances the animal’s resistance (preventing the infection) and resilience (coping with the effects of the infection if it does come). Livestock nutritionists have known for years that condensed tannins can increase the amount of bypass protein in a feed, which would increase the amount of amino acids absorbed from the small intestine. It’s reasonable to speculate that these extra amino acids could be critical for boosting the immune system and repairing the tissue damage caused by the parasites.
This is all new research and very exciting indeed. I’ll admit that most of it has been focused on sheep and goats. What about cattle, especially yearlings or weaned calves on pasture? It probably works –probably– but there’s still a lot we don’t know.
