Nutritive characteristics of irrigated perennial pastures in northern Victoria and the selection of nutrients by grazing dairy cows
Authors: Stockdale C. R. , Cohen D. C. , Doyle P. T. 
Published: September 2002

The aims of the work reported here were, first, to summarise the available
information on nutritive characteristics of perennial pastures in northern
Victoria and to examine factors that affected these characteristics. Second,
in order for farmers to use such information, we needed to confirm that
estimates of nutritive characteristics and selection differentials (the ratio
of the concentration of a nutrient in herbage consumed relative to that in
herbage on offer) defined in research with irrigated pastures in northern
Victoria could be applied to commercial dairy farm situations. Finally,
regression models were used to relate selection differentials for
in vitro dry matter digestibility (DMD), crude protein
(CP), neutral detergent fibre (NDF) and acid detergent fibre (ADF) to pasture
conditions at different times of the year. It was hypothesised that pre- and
post-grazing pasture height (or mass), botanical composition and season are
the factors most likely to affect the nutritive characteristics of irrigated
perennial pastures and the prediction of selection differentials.
Research and on-farm data were generally consistent. On average, DMD of the
pasture on offer was highest in August at 80%, declining to 65%
in March before starting to rise again. The trends in NDF tended to be the
inverse of those for DMD although NDF peaked in late summer (at 63.6%),
rather than early autumn. Crude protein was highest in July (21.3%),
declining to 13.4% at the start of autumn. Except for winter, increases
in herbage mass generally had a significant
(P<0.05) negative effect on DMD and CP, and a
significant (P&lt;0.05) positive influence on NDF.
Of the species variables, weeds and paspalum influenced all nutritive
characteristics in summer and autumn, while the proportion of dead material
was generally important at most times of the year. Increases in weeds,
paspalum and dead material significantly (P<0.05)
decreased DMD and CP, and significantly (P<0.05)
increased NDF.
Selection differentials for summer were closer to unity for DMD and NDF than
at other times of the year (P<0.05). Selection
differentials for CP in summer were the same
(P<0.05) as the mean values recorded in all other
seasons. Significant (P<0.05) factors that
accounted for variation in the selection differential data included botanical
composition and seasonal variables, with the clover and dead components being
the most consistent. In addition, post-grazing height and/or the amount of
herbage removed at grazing significantly (P<0.05)
affected selection differentials of all the nutritive characteristics. For DMD
and CP, the greater the amount of herbage removed, the lower the selection
differential; the reverse was true for NDF and ADF. It is concluded that
regressions generated from the data collated here could be used in decision
support tools to help inform farmers about the nutrients their cows obtain
from the pasture they graze. This means that, when feeding supplements,
farmers can make better decisions about the provision of the most appropriate
feeds to meet cow requirements. In the absence of decision support tools that
have incorporated the information generated in this study, we suggest that the
same average selection differentials could be applied in all dairying areas of
Victoria to perennial ryegrass–white clover-based pastures during
spring. These are 1.1, 1.3 and 0.8 for DMD, CP and NDF, respectively. These
values might also be used in autumn, but with less certainty because of
differences in the timing of the autumn break and greater variations in
species composition.