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:
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.
This is because the building blocks of all carbohydrates (soluble sugars and starches, and structural carbohydrates) are essentially the same (a simple sugar e.g. glucose) with the only chemical difference being the type of bonds that joins the sugar molecules. Bugs in the rumen can break all these bonds supplying the pasture-fed cows with energy.
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.
This is because the lower rumen pH in pasture-fed cows is generally caused by an increase in acetic acid (such as vinegar) and does not result in rumen upset. In comparison, a drop in rumen pH in a TMR-fed cow is usually associated with increased lactic acid which can have detrimental effects (rumen acidosis, lameness).
Additionally, it is sometimes suggested that if the NDF content of the diet is too high, dry matter intake will be limited and occasionally the below equation is used to predict intake:
Dry matter intake = (120 ÷ NDF%) ÷ 100 x liveweight
However, this would suggest a 500 kg cow eating pasture at 40% NDF can only eat 15 kg DM or produce 1.5 kg MS which is not true. In reality, a 500 kg cow eating good quality pasture with an NDF of 40 – 45% will still eat 17 -20 kg DM and produce 2 kg MS. In fact, when cows are grazing good quality pastures NDF content has only a very small impact on intake.
This is because the NDF in good quality pastures is highly digestible and rapidly degraded. However, as pasture quality declines, and digestibility and degradation rate decrease, NDF will play a bigger role in regulating intake.
Thus the fibre in poor quality hay, silages or ryegrass that has not been managed properly, or in some tropical grasses (kikuyu) can play a role in limiting intake.
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.
When protein is degraded in the rumen, ammonia is produced and is used by the rumen microorganisms for their own growth. Any excess ammonia is transported in the blood to the liver, where it is converted to urea and either excreted: primarily via urine, although a small amount ends up in milk, or recycled back to the rumen.
The process of converting ammonia to urea is not energetically expensive to the dairy cow and in pasture-based systems, high dietary intakes of crude protein are not detrimental to health or reproduction.
Wait a sec, Milk urea?
Milk urea is a by-product of the breakdown of dietary protein in the rumen, and, it is an approximate indicator of dietary protein. Briefly, in a pasture based system, high MU levels are not detrimental to performance or reproduction, and generally it is not economical to bring in protein supplements if MU levels are low. For more information, see the Milk urea page.
Can milk urea concentrations help to improve environmental footprint?
Although milk urea concentrations are positively associated with urinary nitrogen concentrations, the implications of small changes in the urinary nitrogen concentration on environmental nitrogen loading needs to be considered with other system factors.
Numerous management and resource factors determine the environmental outcome on farm and the impact of any change must be determined by considering the whole farm system (e.g. stocking rate and dry matter intake). The addition of low protein supplement to reduce milk urea and subsequent intensification in the absence of changes in other inputs, management practices or infrastructure, could lead to increased nitrogen leaching per hectare.