• Steve McGill

Stop the drop in milk volume: maintaining dry matter intake

Talking with many farmers lately, cow’s milk production is starting to decline. Spring calving cows should not drop in milk production month on month by more than 6-7%. Though quite often we see drops in milk production far greater than this, so what is driving this?

Digestibility of our pasture is a key driver in this. As our grasses turns reproductive and produces seed head then the lignin content in the pasture increases to allow this

As plant lignin density increases, total NDF increases and NDF digestibility decreases. As lignin content increases, it bonds to the cellulose and hemicellulose that are normally available to the rumen bugs for metabolism as a carbohydrate source. This means that less of the cell wall or structural carbohydrates are now available for the rumen bugs for metabolism as a carbohydrate source. This results in lower metabolizable energy contribution from the plant. As Neutral Detergent Fibre (NDF) increases, Metabolisable energy (ME), Crude Protein and pasture sugar levels decrease. Milk production is usually one of the first things to suffer. However, it is not only the drop in metabolizable energy that causes this drop in milk production.

As a result cow’s dry matter intake decreases due to high NDF feeds taking longer in the rumen to be digested by the rumen bugs. As a result of this drop in dry matter intake the cow is also consuming less carbohydrates, rumen degradable protein (RDP) and undegraded dietary protein (UDP) as well as minerals. Especially in dryland farming situations as we enter the summer months then pasture growth rates and pasture availability start declining. Silage and Hay are feeds commonly fed to try and make up for a lack of available pasture but only further increase the total amount of NDF consumed by the cow. We can use Merten’s equation to predict what the dry matter intake will be as a % of the cows live weight.

Mertens equation: 120/NDF= % of liveweight as dry matter intake.

We then take that percentage and multiply by the live weight of the animal to give a predicted dry matter intake.

120/35 =




500kg x3.43% =

17.1kg DM/cow/day

500kg x 2.67% =

13.3kg DM/cow/day

Difference =

-3.8kg DM/cow/day

For a 500 kg cow the difference in dry matter intake between 35% NDF pasture and 45% NDF pasture is 3.8kg DM intake. Assuming the ME of the pasture is 11MJ ME per kilogram of dry matter then this increase in NDF will result in 41.8 MJ ME being lost from the cow’s diet. This will result in the cows starting to drop milk volume to react to this lack of energy.

This is where low NDF concentrates consisting of starch, protein, macro/microminerals and rumen buffers help maintain dry matter intake and milk production during this period. These concentrates can also be customised with minerals for your specific farming situation. Starch and protein can also be used as a glucose source for the cows and are the ultimate source of energy for maintaining milk production. Molasses is also a feed we are highly interested in, due to its high sugar levels and an NDF of 0. This means it doesn’t take up much space in the rumen. Molasses is quickly digested by the rumen microbes and helps maintain dry matter intake when NDF is limiting.

If you would like help with reducing post-peak decline of milk production on your farm, or animal nutrition advice on what concentrates you can feed to keep your cows milking well please contact us.

Steve McGill

BAppsc(ag), AARNe,


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