The study reported here compared the nutritive characteristics of the 3 most
common irrigated perennial pasture species grown in northern Victoria as they
regrew after defoliation at various times during the year. In addition, the
relative influence of changes to the proportions of morphological components
and the nutritive characteristics of the individual components on the quality
of whole plants was examined. 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 during spring–autumn, 1993–94. On each occasion,
pastures were defoliated with a drum mower and allowed to regrow for up to 9
weeks; defoliation dates were 24 September, 26 November, 28 January and 25
March.
The variation in estimated metabolisable energy [obtained from
in vitro dry matter (DM) digestibility], crude
protein and detergent fibre concentrations within species was significantly
(P<0.01) less than between clover and the grasses.
White clover was consistently high in metabolisable energy (9.3–11.2
MJ/kg DM) and crude protein (17.7–27.7% DM), and low in
neutral detergent fibre (27.8–39.8% DM) in all periods. At the
other extreme, paspalum had a metabolisable energy content that peaked at 9.3
MJ/kg DM, and fell as low as 7.4 MJ/kg DM. Paspalum also had low
protein (7.5–14.7% DM) and very high neutral detergent fibre
(61.9–69.9% DM) concentrations. Ryegrass varied greatly in
metabolisable energy concentration between the 4 periods, being high in autumn
(average of 10.2 MJ/kg DM) and low in summer–autumn (average of 8.4
MJ/kg DM). Metabolisable energy apart, there were few differences in the
crude protein and detergent fibre contents of ryegrass and paspalum. Perennial
ryegrass is generally considered a superior feed to paspalum, but the data
indicate this is not always the case under irrigation in northern Victoria.
The nutritive characteristics of the plant fractions (leaf, stem, dead,
inflorescence) were analysed separately to give an indication of the limits to
selection by grazing cows. Differences in metabolisable energy between leaf
and stem in both white clover and perennial ryegrass suggested that grazing
dairy cows could consume a pasture diet which is likely to be slightly higher
in energy than that in the herbage on offer. With paspalum, this is unlikely
to be the case because differences in energy content between plant parts were
small. However, with all species, cows should be able to consume herbage that
is significantly higher in protein, and lower in detergent fibre, than that on
offer because of differences in their concentrations in leaf and stem. The
nutritive characteristics of morphological components of each species remained
relatively constant throughout the study. Therefore, it would seem that it is
the proportions of these fractions in the plant, together with severity of
grazing, that will largely determine the degree of selection that can occur.
High lipid perennial ryegrass growth under variable nitrogen, water and carbon dioxide supply
