Animal Health Matters: Fast growing green grass – how could that be bad for animals?

Russ Daly
Special to the Farm Forum

It’s a sight for sore eyes: sunny days, warm air, and green, green grass growing in the pastures – almost growing taller by the hour. Cattle producers and horse owners live for this time of year, when animals can be turned out to graze by themselves and feeding chores drop off the to-do list. How could this be anything but great?

While animals benefit from the nutrients in these forages, sometimes other factors in these plants can affect their health. Interestingly, these problems occur when grasses are growing their fastest.

One of those is grass tetany. When grasses enter their rapid growth phase, they outgrow their ability to absorb magnesium from the soil. Can’t cattle get along without magnesium for even a little while? Unfortunately that’s not the case.

When cattle and sheep diets are too low in magnesium, levels fall in the bloodstream and eventually the fluid bathing the brain and spinal cord. That’s when the nervous system’s functions get out of whack.

Grass tetany can come on suddenly. Affected cows toss up their heads and run around blindly for a short time before throwing themselves on the ground convulsing. Death can result fairly quickly: oftentimes the animals are simply found dead on pasture. For those found alive, IV magnesium and calcium solutions can be given by a veterinarian, with fairly good results.

Grass tetany most often affects lactating cows, since low calcium levels in the bloodstream are an exacerbating factor. Some well-fertilized pastures, particularly those with soils high in potassium, can be more prone than others to set off grass tetany. Giving cows access to high-magnesium mineral supplements is the main preventive measure; this should be part of the springtime turnout ritual regardless of the pasture.

A stranger phenomenon can also afflict cows grazing rapidly growing or already lush pasture. Atypical (or acute) interstitial pneumonia, or “AIP”, occurs when cattle graze grasses high in certain metabolites, particularly L-tryptophan. I’ll spare you the biochemical details, but this nutrient is converted in the rumen to other chemicals, some of which are directly toxic to lung cells.

Breathing difficulty is the result of this lung damage, as fluid builds up and air leaks out into parts of the lung it doesn’t belong. Cows stretch their neck out, breathe with their mouth open, and have a generally hard time getting air. The animals can die if this damage is extensive enough. Unlike grass tetany, there is no specific treatment for AIP. In fact, handling these animals can be fatal by itself if their exertion exceeds their respiratory capacity. Feeding cows supplements that contain ionophores can help ward off AIP, but can’t help animals already affected.

And for horses, there’s laminitis, better known as founder. There are a lot of opinions why grazing lush pastures causes horses to founder, but it probably is similar to grain overload. Metabolic byproducts break down the connection between the horse’s coffin bone (the foot bone within the hoof) and the hoof wall itself. The result is pain and a reluctance to move. If the process is allowed to continue, that bone can separate and rotate towards the sole of the hoof, a process that can’t be reversed. Acutely foundered horses should be promptly treated with anti-inflammatory drugs. Chronic founder results in overgrown toes and a regular need for corrective hoof trimming.

Besides their connection with lush, rapidly growing pastures, these diseases are also similar in that they develop soon after turnout to pasture. This means that keeping a close watch on your animals in those first few days on pasture is particularly important, especially when the grass is growing rapidly. At the first sign of any problem, remove the animals as safely as possible and wait until growth conditions slow down. Gradually exposing animals to new pastures (an increasing number of hours per day, for example) can help decrease these problems.

So while it’s fun to watch those animals enjoy their new pasture, don’t just turn back for home and forget about them after dropping them off. Watching them closely for the next few days can help you head off these fast growing pasture problems.