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

Abstract

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.

Long-term liming changes pasture mineral profile

“…Many factors can impact both the supply and uptake of minerals, such as the availability of mineral nutrients in the rhizosphere, improvement in root growth allowing more soil to be explored 1 , altered uptake of nutrients per unit length of root 36 , and the moisture status of the soil 37 for some minerals, such as P that moves by diffusion or S that moves via mass flow. The growth stage of the pasture also alters the relative content of minerals due to growth induced dilutions 21 .…”

“…The action of lime in altering concentrations of macro elements in shoots could occur through change to the supply such as large quantities of Ca in lime competing with other base cations and changes in mineral availability in soils due to change in soil pH after liming 1 , 38 . This is in addition to dilution effects due to extra plant growth 20 , 36 . Consideration of all results indicates long-term liming would have beneficial role in reducing the incidence of hypocalcaemia and hypomagnesemia by improving Ca, Mg and Na concentration.…”