Drought Stress and Pasture Quality

Drought causes pastures to dry up, weeds to flourish, and horses to get thin without supplemental feeding. But there are other things going on in your fields that you need to know about in order to protect your horses from problems such as laminitis, colic, diarrhea, and toxic weeds. The problem isn't over when the rains come; conversely, the much-needed, drought-breaking rains can cause additional problems to grazing horses.

One might think that green grass is highest in nutrients and brown grass is lowest. This is true for some nutrients, but not all. When dead grass is rained on, water-soluble nutrients are leached out. But under drought conditions, they remain; your pasture might actually be higher in non-structural carbohydrates, which includes sugars, starch, and fructan.

Most cool-season grasses accumulate sugar and fructan (a type of sugar) under drought stress. When plant growth slows due to lack of water, the balance between photosynthesis that creates sugar, and growth that uses sugar, is lost. Sugar levels rise in intense sunlight, and high sugar levels trigger formation of fructan, which is the storage form of carbohydrate in cool-season grasses. Warm season grasses do not make fructan, preferring starch for carbohydrate storage.

Fructan reserves are like the fat on a hibernating bear: The more fructan plants can accumulate before they go dormant, the more energy reserves they have to sustain them during the drought and allow them to get a competitive advantage when conditions are again favorable for growth. Unfortunately for horses eating these plants, fructans are a known trigger for laminitis in susceptible horses.

Orchard grass varieties that best survived a long-term drought under Mediterranean conditions were found to have sugar and fructan levels up to 63% of dry matter in the green, living leaves enclosed in the leaf buds. In a study of wheatgrass in the high desert of the United States, sugar and fructan levels increased seven-fold in the grass crowns and roots under drought stress compared to irrigated control plots.

Other species prefer to turn their fructan reserves to sugar under drought stress. In tall fescue subjected to drought stress under greenhouse conditions, sucrose content increased 258% in the leaf base, and fructan decreased to 69% of that in control plants that got water.

When selecting plants to survive drought, plant breeders either purposely or inadvertently select those with the highest levels of fructan and sugar, because they tend to be the hardiest.

Then the Rains Came
So what happens to all these hoarded non structural carbohydrates when it rains and growth starts again?

Fructan and starch are the storage forms of carbohydrate. Neither is transportable in a plant because they are too big. To be used by the new growing point, they have to be broken down to sugars that are small enough to travel from the lower storage portions of the plant up to the new growth. Re-watering stimulates production of enzymes that break long-chain fructan into shorter chains, and ultimately to the sugar components. Those first new green shoots can be very high in sugar. The type of sugar produced after the drought breaks can vary depending on the species of grass. Ryegrass is an exception in that it makes even mores fructan upon re-watering.

So, the consequence of a drought-breaking rain that stimulates new growth in your pasture is a rapid change in carbohydrate form that might affect your horse's digestive system or cause a metabolic crisis in a horse with impaired glucose metabolism. The effect of the change on grass carbohydrate concentration and form in pasture-fed horses has not yet been researched; although digestive upsets and laminitis frequently occur with seasonal change and periods of rapid pasture growth after a drought-breaking rain.

Studies on the fermentation of fructan and sugar in the production of silage show that different species of bacteria tend to prefer specific types of plant carbohydrate. Because bacteria eat mostly the terminal ends of fructan chains, shorter chain fructans are fermented faster than longer chain fructans. These are the same kinds of bacteria that ferment the carbohydrates in your horse's cecum.

If the type and chain length of the carbohydrates change quickly, the bacterial populations can also change rapidly. Those preferring the new form of carbohydrate might proliferate, producing lactic acid and changing the population dynamics within the hindgut. This is the scenario where endo- and exo-toxins might be produced.

We are all aware that changing the carbohydrate levels in our horses' diets too quickly can lead to diarrhea, gas, colic, and even laminitis in susceptible individuals. Long-term drought--and a drought-breaking rain--make for sudden changes in the carbohydrate form and concentration in your pasture, and thus in your horse's diet if he is on pasture.

Weeds in Drought-Stressed Pasture
When shallow-rooted grasses dry up, sometimes the only succulent forage left is weeds with deep tap roots. Weeds can also accumulate sugar, starch, or fructan under drought stress. This makes weeds a lot more appealing to your horse, and he might start eating things usually avoided when grass is green and well-hydrated.

Dandelion, thistles, and chicory are common weeds often relished by horses even under normal conditions. All three contain inulin, the same form of fructan used to induce laminitis in clinical studies.

Many plant toxins concentrate under drought conditions. Weeds can accumulate nitrates, oxalates, alkaloids, prussic acid, and cyanogenic glycosides, which can cause a wide variety of equine health problems including digestive upset, mineral imbalances, photosensitivity, and liver, kidney, and neurological damage. Fescue infected with endophytes might have dramatically increased levels of ergovaline under drought stress. This alkaloid causes constriction of blood vessels, which can cause abortion in late-term mares and has been associated with laminitis.

Preventive Strategies
Pregnant or growing animals grazing drought-stressed pastures might need supplemental protein and vitamins, but energy needs might be adequate. Diarrhea or excess gas is a signal that pasture access might need to be limited to allow for adaptation to new carbohydrate levels. If your horse is susceptible to problems associated with excess carbohydrates, test your pasture for non-structural carbohydrates (NSC, the ones we've been discussing) when your pasture is under adverse conditions such as drought. Samples with any green plants must be frozen immediately and shipped cold to retain sugar. Compare these levels with the NSC content of hay your horse seems to do well on.

If your pastures are too high in NSC for your horse, limit pasture access and feed more hay. Make sure he's not so hungry he starts nibbling on weeds with possible plant toxins. When the rains come again, exert caution and consider controlling access by dry-lotting part of the day, using a grazing muzzle, or forcing strip grazing with portable fencing until grass is two to three inches tall. This allows grass growth to use excess carbohydrates accumulated during the drought.

About the Author

Kathryn Watts, BS

Kathryn Watts, BS, is the director of research for Rocky Mountain Research and Consulting and a passionate forage researcher. Her web site is www.safergrass.org.

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