Feed energy (UFL) intake is the first limiting nutrient for the milking cow, however adequate protein (PDI) intake must also be provided for high milk solids production according to Teagasc.
It says grazed grass is an excellent source of both but generally contains a surplus of crude protein (nitrogen) relative to energy. It is sometimes suggested that this negatively affects feed efficiency and fertility. However, it notes that surplus crude protein in grass is a by-product of the most cost-effective feed available.
Teagasc research has shown that there is little economic cost to having surplus crude protein in grazing diets, unlike for indoor diets where purchased feed protein is expensive. The feed energy cost associated with excreting surplus dietary N is quite small (2% of intake) unless grass crude protein levels reach 25% or higher.
Teagasc says there is some evidence that individual cows with elevated milk urea have lower conception rates.
However, research in Australia/New Zealand shows little difference in milk urea levels between grazing herds with good or poor fertility. Energy intake and cow body condition are much more important feed factors affecting fertility outcomes.
In summary, Teagasc says there is usually a surplus of protein N in grazing diets but this should not have any major negative effect under good management conditions. Some simple steps to managing excess crude protein N in a grazing diet are:
- supplement with high energy (0.94 UFL+) and low protein rations (e.g. 12%) during grass deficits; and,
- avoid blanket spreading of N fertiliser during the breeding season and in very dry weather.