Nutritive characteristics of irrigated perennial pastures in northern Victoria and the selection of nutrients by grazing dairy cows

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.