The effect of the supply of rumen degradable protein and metabolisable protein on negative energy balance and fertility in dairy cows

Reproduction in dairy cattle is negatively affected by a negative energy balance (NEB), a combination of the deposition or mobilisation of fat and protein. The mode of action of NEB on fertility is not always clear, but the severity, length, and probably also the nature of the NEB may be involved. Extensive mobilisation of fat is expected to have detrimental effects on liver function due to the accumulation of non-esterified long chain fatty acids, impairing the detoxification of ammonia into urea. Protein evaluation systems nowadays use the concept of metabolisable protein (MP) and distinguish between rumen degradable protein (RDP) and rumen undegradable protein (RUP). Mobilisation of protein itself does not seem to have negative effects on reproduction. However, when protein is extensively degraded in the rumen or used as an energy source, metabolic residues like ammonia and urea will result. Such residues may exert metabolic effects that are often detrimental to reproduction and fertility. Ammonia is believed to play a role starting before ovulation, whereas urea mainly interferes negatively after fertilisation. But, urea is also believed to aggravate the severity of NEB and its effect on fertility by preventing or delaying the start of cyclicity. Besides, urea has been shown to lower the pH in the uterine fluid, giving rise to disturbances in follicular development and embryonic growth. It is recommended to limit the level of rumen degradable protein in the diet to 10% in the DM.