Each year, we pray for the rainfall needed to produce the green grass we hope for every spring. If you’ve owned livestock for more than about 15 minutes, however, you know that with nearly every good thing that comes along, there’s something to watch out for.
This time of year, several factors come together to produce a perfect storm: grass tetany. Grass tetany is characterized by a low level of magnesium in the blood, which results in cows who become rigid or unable to rise, and often become uncharacteristically belligerent and aggressive. Some are simply found dead. Grass tetany happens this time of year due to the collision of several factors associated with the cow and with the grass.
Milk production consumes a significant amount of the cow’s magnesium, with peak milk volume produced in the several weeks following calving. We also see cows in their 3rd – 5th lactation (about 5-7 years of age) being the highest producers, increasing their risk. Further, there is some evidence that cool weather may directly and negatively impact her management of magnesium. Finally, magnesium is not stored in the body to any extent – it is a simple in-and-out system. So, when a cow runs low in magnesium, unless she has consumed enough recently to make it immediately available, she can’t pull it out of storage to get her by. In spring calving herds, this peak milk production coincides with the presence of early spring grasses.
Early spring grass, to make things worse, is not helpful to the cow who is under this lactational stress with regards to magnesium. First, new spring grass is inherently deficient in magnesium. Second, this new grass is high in carbohydrate, potassium and protein, which actually reduce the amount of magnesium the cow can absorb. Low sodium and high water content further contribute to the problem. As all of these factors come together, there comes a point at which insufficient magnesium is available for normal function of many body systems.
Treatment of grass tetany involves administration of oral or intravenous magnesium solutions, which should be done with caution, as these cattle can be quite aggressive. Additionally, rapid administration of magnesium-containing solutions can be fatal. Cattle found dead should undergo necropsy and evaluation of their magnesium level. Postmortem testing of magnesium level is typically done on the fluid of the eye; blood can be used to check levels in live cattle. If a veterinarian is not available or there will be a delay in performing a necropsy at the time a cow is found, an eye should be immediately removed – you have to beat the buzzards to it – and placed in a plastic bag and refrigerated until it can be submitted to the lab.
So how do we ensure that the cow has enough magnesium to support the milk production she needs to raise her calf? Well, we certainly don’t want to wish away spring pasture! Prevention of spring grass tetany can begin the fall before with a soil test, which can guide fertilizer application – fertilizers high in nitrogen and potassium should be used with care and dividing nitrogen application may be advisable. In addition to pasture management, cows should have access to a direct magnesium source. The most common sources for cattle on pasture are “high-mag” mineral blocks, tubs or loose mineral. There are a variety to choose from in these forms, and it is important to read the label to determine how many mineral stations are indicated for a given pasture size and stocking density. These mineral sources should be put out with cattle 2 weeks prior to grazing. Although the cattle do not store or save up magnesium during these two weeks, this time allows them to get used to the mineral feeder and increase their consumption of the mineral to a level that is protective. Then, the mineral needs to be ever-present in the pasture during the grazing season. Recall that there is no significant storage of magnesium in the body, it’s a daily in-and-out system. A delay in getting mineral refilled can result in cows developing a deficiency.
High-quality mineral supplementation is important in all classes of animals throughout the year. Consumption of the mineral is key. It is commonly presumed that if the cattle need it, they will eat it. We have found that some cows simply won’t eat certain mineral preparations. If that occurs, it will be important to find a mineral that the cattle consume at the label rate. Providing a high-magnesium mineral to lactating beef cows in the spring is of particular importance and cannot be skipped.