The Last 20 Cents; is often the best 20 cents
Most of you will be familiar with the image to the right. It shows that pasture grasses contain the most energy prior to seeding. As seeds develop and the grass dries out, the nutrient value continues to fall. Protein should be considered a type of energy. Without protein the cow cannot utilise other types of energy efficiently to produce milk. The protein percentage of pasture declines at a faster rate than metabolizable energy. Pasture tests show that on the first of October protein percentage is about 30.0%, falling to under 20.0% by the first of November, most likely 10.0% by the first of December, and 5.0% by the first of January depending on where you live and the types of pasture that you grow.
This means that a cow eating 15 dry kg of pasture in October ingests 4.5 kg of protein, 3.0 kg protein in November, 2.25 kg in December and only 1.5 kg of protein in January. To produce any amount of standard 4.0% fat and 3.2% milk a cow must first eat 1.0 kilogram of protein to stay alive and a further 80 grams of protein for every litre that she produces. That 1.0 kg of protein she must eat to stay alive remains the same regardless of how much milk that she is eating (we call this maintenance protein). Let’s see how the cow would go over spring eating pasture and 5 kilograms of low protein grain. A cow eating this diet should have enough energy to make 30 litres in every instance assuming that grass didn’t run out.
As the table shows unless you have irrigation then only in October does the amount of protein provided by pasture exceed that provided by energy. The sharp falls in milk production in November, December and January are almost entirely a result of declining pasture quality as the grasses go to head. Increasing fibre levels, declining sugar levels and possible availability don’t help either.