Dietary cation - anion differences in some pasture species, changes during the season and effects of soil acidity and lime amendment

Abstract

The difference between cation and anion concentrations is an important property when assessing feed for dry dairy cows in order to avoid hypocalcaemia following calving. Dietary cation–anion difference (DCAD) is used to assess suitability of feed and predict the risk of milk fever; a value of –5 cmol(+)/kg dry matter (DM) or less is desirable. This work has examined the DCAD of 16 field-grown species found in pasture in southern Australia. The DCAD [cmol(+)/kg DM] at the flowering stage varied from 7 to 32 for grasses, 21 to 72 for legumes and 72 to 99 for dicot weeds. The average DCAD for legumes was 50 cmol(+)/kg DM, over 2-fold higher than the 20 cmol(+)/kg DM average for grasses. There was a substantial decline in DCAD of herbage as the season progressed. In a glasshouse experiment with five grass species in an acid soil, lime application increased yield and tended to lower the DCAD. Lime decreased uptake per unit root length of potassium and chlorine and increased uptake of calcium by phalaris and timothy. While DCAD is an important attribute of herbage for assessing its suitability for prepartum diets of dairy cows, the present data indicate that it would be prudent to also consider concentrations of calcium and other mineral nutrients in herbage, particularly when examining less familiar plant species or the effects of different cultural practices on the composition of herbage for such diets.