Use of an Experimental High-Magnesium Tall Fescue to Reduce Grass Tetany in Cattle

Abstract

Literature Summary Study Description Applied QuestionsFull scientific article from which this summary was written begins on page 491 of this issue.Grass tetany (hypomagnesemia) causes economic losses from death or reduced performance of livestock throughout the world. Attempts to reduce the incidence of grass tetany through application of fertilizer Mg have not generally been successful. Dietary Mg supplementation adds to the annual labor and cost of production and does not always ensure all animals receive adequate amounts of the mineral. Selective breeding of forages for higher levels of Mg and lower tetany ratio may offer a long-term solution to the grass tetany problem.Objectives of this study were to evaluate an experimental selection of tall fescue, HiMag, for its ability to reduce the risk of grass tetany through higher Mg concentration and lower tetany ratio of forage, and higher blood serum Mg concentrations of beef cattle under grazing conditions. Twenty, 1-acre pastures on a predominantly Creldon silt loam soil at the Southwest Research Center near Mt. Vernon, MO, were planted to four endophyte-free tall fescue cultivars, AU Triumph, Kentucky-31, Martin, and Mozark, and the experimental cultivar, HiMag. The study consisted of five individual grazing trials using cross-bred beef steers, dry cows, or cow/calf pairs during three autumn and two spring seasons between 1993 and 1995.Forages were sampled each season and analyzed for Mg, Ca, K, and P concentration, and the tetany ratio was calculated. Cattle were weighed at the beginning, middle, and end of each trial to determine average change in weight. Blood serum was collected at each weighing for determination of Mg, Ca, K, and P status of the cattle.Can cool-season forage grasses be selectively bred to reduce the risk of grass tetany?Forage from HiMag tall fescue consistently had higher Mg (22%), Ca (18%), and P (9%) concentration than other tall fescue cultivars. Potassium levels were not different among cultivars. This resulted in a lower tetany ratio for HiMag compared with the other cultivars. Higher Mg concentration (>0.20%) and lower tetany ratio (<2.2) suggest that HiMag tall fescue can reduce risk of occurrence of grass tetany.

Seasonal Trends in Chemical Composition of Different Artificial Pastures

“…Grass tetany is associated with Mg deficiency in the blood of animals. The risk of grass tetany is increased at a K:Ca + Mg ratio of 2.2 or higher (Crawford et al, 1998). Grass tetany values, which changed from 1.69 to 0.77, in this research were less than the critical level.…”

“…Grass tetany, an important disease of livestock, is caused by mineral imbalance in feeds. The risk of grass tetany is increased at a K:Ca+Mg ratio of 2.2 or higher (Elkins et al, 1977;Crawford et al, 1998).…”

Effects of fertilisation and harvesting stages on forage yield and quality of hairy vetch (Vicia villosaRoth.)

“…Grass tetany, an important disease of livestock, is caused by mineral imbalance in feeds. The risk of grass tetany is increased at a K: (Ca + Mg) ratio of 2.2 or higher (Elkins et al 1977;Crawford et al 1998). …”

Nutrition and Health of Dairy Animals

“…Forages containing less than 0,2% Mg and a "tetany ratio" [K/(Ca+Mg)] greater than 2,2 have increased risk of inducing grass tetany (Crawford et al, 1998). The fertilizers containing N and K are the most important factors increasing [K/(Ca+Mg)] ratio in forages.…”