Improving Dry-matter Intake of Perennial-ryegrass Pasture by Dairy Cows

Dairy cows fed on perennial-ryegrass pasture produce higher-quality milk in terms of fatty-acid composition; however, they produce much less milk compared to dairy cows under other systems. The low dry-matter intake (DMI) has been identified as the main factor responsible for the lower milk production in pasture-based systems. To maximize milk production from those systems it is important to maximize DMI from pasture. Many interventions have been studied, such as: manipulating herbage allowance, feeding frequency, time of allocating the new pasture, pasture maturity, fertilization rate and the choice of perennial-ryegrass variety. The results have shown that pasture DMI increases in a curvilinear asymptotic manner as herbage allowance increases. However, increasing herbage allowance decreases milk production per hectare and increases production cost due to the increased need for pasture and land. More frequent allocation of pasture has shown no effect on pasture DMI. Allocating the new pasture after the evening milking has sometimes shown to increase pasture DMI without any negative effect in terms of productivity per hectare. Allocating a young and leafy grass has shown to increase DMI, but at the same time pasture holding capacity was reduced and milk production per hectare decreased. Using high-sugar grass was advocated as a way to improve DMI of dairy cows on pasture. Research findings with regard to the effect of high-sugar grass on DMI are very inconsistent. Some experiments reported a positive effect of high-sugar grass on DMI, but that was due to the confounded effect of high sugar and digestibility. Whereas, other experiments reported no effect, as in those experiments the highand low-sugar grasses had a similar digestibility. This indicated that high sugar is not always associated with higher digestibility especially in young and leafy perennial ryegrass. It also indicated that grass digestibility exerts a stronger effect on DMI than its sugar content does. It was hypothesized that feeding grass that has a fast clearing or degrading fibre would improve DMI. The results of several experiments that screened different perennial-ryegrass varieties for their fibre clearance and degradation rates under grazing indicated that variation in these traits between perennial-ryegrass varieties is very limited under the chosen conditions. This suggested that choosing varieties with fast degrading fibre, as a means to improve pasture DMI, is very limited.