Farmers are often told they need to balance pasture in the diet but that implies there’s either something missing or there’s something in excess that’s detrimental to the animal and milk production.
“And that’s simply not the case,” he said.
Well documented research in NZ and overseas shows 60% of the difference in production between a TMR and pasture-fed cow can be explained by the difference in dry matter (DM) intake.
Almost all of the rest of the difference in production between the two systems can be explained by factors other than nutrition in that they relate to the extra energy used for walking and the fact grazing cows produce milk with a higher milksolids (MS) percentage.
Some nutritionists argue that grass is inherently low in four of the essential amino acids needed by the mammary gland for milk production and while Roche agreed they do appear to be lower than the ideal, “bucket loads” of research carried out all over the world showed that adding the amino acids to the diet made not one iota of difference to production.
The excess of protein in grass overall meant any deficiencies in the constituent amino acids was well overcome.
Instead Roche said based on knowledge of the digestive and metabolic processes going on in the rumen, the first limiting factor to milk production in a pasture diet is almost always going to be energy.
Only once the energy requirements of the cow have been met will production be limited by factors such as minerals and vitamins, bypass protein, rumen degradable protein then fibre.
The yield of microbial protein from rumen bacteria, which is then used then by the cow in milk production, is determined by the amount of effective rumen degradable protein (ERDP) and metabolisable energy (ME).
The cow needs approximately 8g of ERDP/megajoule (MJ) ME eaten. But at an ME of 11 and intake of 14kg DM, as in a typical dry summer the cow needs 1232g ERDP.
“That means nine percent of the drymatter in grass must be EDRP – just nine percent, almost all of the crude protein in grass is EDRP. Effectively you’d have to get down to 12% or less crude protein in summer before cows are short of protein.
“They may be short of feed and if they’re short of feed they’re short of energy but they won’t be short of protein.”
Supplementing them with urea, as advocated by some nutritionists through summer just never works in these simple supplementary feeding systems, he said.
Roche is concerned farmers in the North Island in particular are feeding expensive protein supplements like soybean meal when they aren’t deficient in protein and, even when they are deficient in protein, because they aren’t set up to feed it with low physical wastage.
Because of the high proportion of maize silage being fed on some farms during summer dry periods some nutritionists advise feeding soybean meal.
Although, this will increase milk production when crude protein is less than 12-14%, NZ research suggests it is not profitable to feed.
Even at a milk price of $8.50/kg MS, economically he couldn’t make it stack up and that’s with low levels of physical wastage of the feed.
