Sorting of diets by cows can go undetected, but can have a number of serious consequences.
Ruth Lawson, ruminant nutritionist for W.E. Jameson and Sons, says sorting of diets can lead to low butter fat levels, or butter fats which vary greatly between cows, reduced feed efficiency, poor fibre digestion, increased susceptibility to disease and reduced absorption of nutrients across the gut wall.
Dr Lawson says: “Sorting means the diet formulated is not the diet each cow eats. Cows which sort the total mixed ration [TMR] will eat a diet which is too high in concentrates and potentially suffer subacute ruminal acidosis [SARA].”
SARA accounts for substantial economic losses in the dairy industry due to its association with reduced feed intake, liver abscesses, milk fat depression, scour, laminitis and increased bacterial endotoxins and inflammation.
“Ruminal pH is determined by the balance between acid production in the rumen and acid removal from the rumen by absorption through rumen epithelial cells, neutralisation with buffers, and passage to lower digestive tracts,” she says.
She adds research conducted by Trevor DeVries, at the University of Guelph, in 2008, showed increased sorting for particle fractions which were higher in starch and lower in NDF was associated with reduced ruminal pH variables.
“This pattern of sorting has been associated with lower milk fat percentage in a range of studies, likely as a result of imbalances in intake of highly fermentable carbohydrates and effective fibre, leading to SARA,” she says.
“In these studies, milk fat decreased by 0.15 percentage points for every 10 per cent refusal of long forage particles in the ration.”
Dr Lawson adds that sorting of a TMR can also reduce the nutritive value of what remains in the feed bunk, particularly in the later hours after feed delivery.
“This may have a negative effect on cows that do not have access to feed when it is delivered,” she says.
“For example, high competition at the feed bunk may restrict subordinate heifers, or compromised animals, to feeding later in the day, and these cows may end up consuming a ration which is different from that formulated for their production and growth requirements.
“This, in turn, can affect production targets. For example, diets which are higher in NDF and lower in energy can reduce milk yield and increase ketosis.”
Dr Lawson also says it is important to prevent sorting in robotic milking systems to ensure the robot is working within its capability.
“If sorting does occur, there are large peaks in feeding activity after fresh feed is delivered,” she says.
“This is not good, as after that peak in feeding activity comes a spike in milking frequency. This can lead to queues at the robot at these peak periods.”
■ Adding water to a dry TMR has been suggested as a technique to reduce the extent of feed sorting
■ Moist feeds can also be helpful in reducing sorting behaviour as they can help to make the concentrate component stick to the forage
■ The concentrate portion of the ration can be soaked in water (and molasses) and left overnight before mixing with the forage component
■ However, there can often be an intake response to a more moist diet, which in turn can improve milk yield
■ Adding water can lead to reduced palatability caused by spoilage of the TMR in the trough, because high moisture TMR are less stable during periods of elevated environmental temperatures
■ This can be avoided by feeding more than once a day during hot periods
■ Straw should be well chopped at about 2-3cm in length or ground to avoid sorting. Alternatively, soya hulls can provide a good source of fibre and bring more energy than feeding straw