Dietary cation–anion difference and tetany index of timothy forage fertilized with liquid swine manure
Incidence of metabolic disorders increases when dairy cows (Bos taurus) are fed forages that have a high dietary cation-anion difference (DCAD) (>250 mmolc kg–1 dry matter, DM) or high grass tetany (GT) index (>2.2), both associated with high forage K concentration, often caused by applications of liquid swine manure (LSM). We determined how DCAD and GT index of timothy (Phleum pratense L.), grown on two soils with different K availability, were affected by mineral or LSM fertilization. Experimental treatments were: unfertilized control, mineral fertilizer, raw LSM, and liquid fractions of four treated LSM types (decanted, filtered, anaerobically digested, and flocculated), applied in spring and after the first of two harvests each year. Forage DCAD was lowest (–108 mmolc kg–1DM) with the flocculated LSM due to its higher Cl content. Forage DCAD with other LSM types was similar to that with mineral fertilizer. Forage GT indices with LSM and mineral fertilizer were higher than that of the unfertilized control but still within an acceptable range for cows. The DCAD and GT index were greater on soils with high K availability. From spring growth to summer regrowth, these values decreased for soils with low K availability and increased for soils with high K availability. Compared with mineral fertilizer, LSM applied to timothy did not increase the risk of metabolic disorders for dairy cows; a Cl-enriched LSM can substantially decrease DCAD and lower the risk of milk fever.