Effects of season and time since defoliation on the nutritive characteristics of three irrigated perennial pasture species in northern Victoria 2. Macro-minerals

Abstract

During 1993–94, the nutritive characteristics of white clover (Trifolium repens cv. Haifa), ryegrass (Lolium perenne cv. Ellet) and paspalum (Paspalum dilatatum) were examined at weekly intervals on 4 occasions. On each occasion, they were initially defoliated with a drum mower and allowed to regrow for up to 9 weeks; dates of the 4 initial harvests were 24 September, 26 November, 28 January and 25 March. The objective of the part of a larger study that is reported here was to examine changes in macro-minerals of the 3 species as they regrew after defoliation, in order to start providing information for informed decisions on the need for supplemental minerals in northern Victorian dairy systems. The ranges in mineral elements recorded in this study were 0.16–0.43% for phosphorus, 1.18–3.41% for potassium, 0.26–1.49% for calcium, 0.20–0.54% for magnesium, 0.05–0.44% for sodium, 0.79–2.02% for chlorine and 0.15–0.37% for sulfur. A major reason for the large ranges was due to variations between the 3 species. White clover was richer in a number of minerals than were the grasses, particularly calcium, potassium and magnesium, although phosphorus was also generally higher. Perennial ryegrass had high sodium concentrations, which were probably related to high watertables, with the associated propensity for salinity problems, that are endemic in irrigated northern Victoria. It is suggested that, on the basis of the pasture on offer, phosphorus is the major mineral that can potentially limit milk production by grazing dairy cows in this environment, although possible deficiencies were also demonstrated for calcium, sodium and sulfur. Where a mineral has its highest concentration in the leaves, lax grazing management or the use of short pastures should result in higher concentrations in the pasture eaten than that on offer. Leaves were not always the richest source of a mineral although this was almost always the case for all minerals in ryegrass leaves. In white clover, phosphorus, calcium, magnesium and sulfur were more concentrated in the leaves than in the stem. In paspalum, calcium, magnesium and sulfur were more concentrated in the leaves. White clover stems had higher concentrations of potassium and chlorine than leaves, while with paspalum this applied to sodium and chlorine. It was concluded that the species composition of a pasture will have a large impact on the supply of minerals for grazing dairy cows, although the leaf : stem ratio, as determined by time since the last defoliation, will also have a role in determining whole plant composition.