Project introduction - Pensarnau

A review carried out by Cornell University of studies relating to high protein rations and fertility in dairy cows noted that although high dietary protein stimulates milk production, high protein has often been associated with decreased reproductive performance.

The biological value of dietary protein for the lactating dairy cow is directly related to the energy status of the cow and the balance of absorbed amino acids relative to their requirements. In lactating cows, dietary Crude Protein comprises RDP (Rumen Degradable Protein) and RUP (Rumen Undegradable Protein) fractions. Through normal ruminal fermentation, RDP provides a source of ammonia for microbial protein synthesis. Some of the ammonia escapes incorporation by the microorganisms, diffuses out of the rumen into portal blood, and is detoxified in the liver by conversion to urea. The quantity of rumen ammonia produced and the amount that escapes for conversion to urea directly reflects both dietary RDP and the availability of fermentable carbohydrates in the rumen to support microbial growth and protein synthesis.

A second source of urea produced by the liver is from deamination and metabolism of amino acids. Circulating amino acids originate from RUP, microbial protein, and body protein stores. Amino acids not taken up for utilization in milk protein synthesis are deaminated by the liver to yield energy substrates and urea. Although the production of ammonia and urea can be minimized by balancing RDP and RUP, high dietary intake to support milk production and variation in rumen microbial protein yield make accurate prediction of the availability of amino acids very difficult.

Consequently, most high producing cows consume protein in excess of requirements, and blood urea concentrations are increased.

Urea circulating in the bloodstream is measured as urea nitrogen in either plasma (PUN) or serum fractions and is often referred to generically as blood urea nitrogen (BUN). Typically, BUN peaks about 4 to 6 h after meals because of RDP catabolism, while metabolism of RUP contributes to BUN continuously throughout the day. Urea passes easily from blood into the milk within the mammary gland. Milk urea nitrogen (MUN) provides a rapid, non-invasive, and less expensive means of assessing BUN and of monitoring overall protein metabolism in lactating cows.

Measurements of PUN or MUN have provided a useful index for studying the association between metabolism of dietary protein and reproductive efficiency.

Across many studies increased PUN or MUN concentrations were correlated with decreased fertility in dairy cows both in confinement and grazing herds.