Microbes in the rumen enable the cow to digest plant material, therefore cows are very efficient at extracting energy from forages such as pasture.
Many of the nutritional recommendations widely provided are based on data derived from cows fed a total mixed ration (TMR) but these are not always applicable to grazing dairy cows. This is because pasture and pasture-fed cows have unique characteristics:
Carbohydrates (sugars)
In theory, milk production is maximised when soluble sugars and starches are 35-40% of the diet. Although spring pasture contains less than this, the structural carbohydrates (cellulose) in good quality, leafy pastures are highly digestible (70-85%) and are degraded relatively quickly, thus supplying similar energy to soluble sugars and starches. This means there is enough readily available energy in pasture for the dairy cow.
Fibre
Recommendations from total mixed rations suggest that neutral detergent fibre (NDF) should make up 27-33% dry matter intake with effective fibre (the fibre most effective at stimulating rumination and salivation) making up 20% of dry matter intake.
The neutral detergent fibre (NDF) content of pastures is generally in excess of these requirements and although the ‘effective’ fibre in pasture is estimated to be lower (17-20% DM) than a TMR diet (and rumen pH are sometimes lower than recommended), this does not negatively affect digestion or microbial growth. Further there are no performance benefits of adding additional effective fibre (for example straw or hay) to a pasture-based diet.
Protein
Recommended protein levels for TMR- fed cows in early lactation, is a diet containing about 18% crude protein, of which 65% is degradable, while 35% is not digested in the rumen (by-pass protein).
At most times, good quality pasture contains more protein than cows require. Even though the protein in pasture is highly degradable (70-90%), fast rumen passage rate means there is still sufficient dietary protein that by-passes the rumen.