LEUCINE AND PROTEIN-METABOLISM IN THE LACTATING DAIRY-COW MAMMARY-GLAND - RESPONSES TO SUPPLEMENTAL DIETARY CRUDE PROTEIN-INTAKE

Mammary gland protein metabolism, determined by an arteriovenous diffe rence technique, was monitored in four Holstein-Friesian dairy cows in response to supplemental dietary protein (provided as rumen-protected soyabean meal) during late lactation (weeks 24-30). Each cow was offe red two isoenergetic diets composed of grass silage (170 g crude prote in/kg dry matter) plus either a low (108 g/kg) or medium (151 g/kg) cr ude protein concentrate in a single crossover design involving two 21 d periods. On day 21, arteriovenous measurements across the mammary gl and mere made during a 13 h continuous i.v. infusion of [1-C-13]leucin e and with frequent (2 hourly) milk sampling during the final 6 h. Alt hough total milk. yield was slightly increased (+ 1 kg/d) by protein s upplementation, milk protein yield was not significantly affected. Who le body protein flux (protein synthesis plus oxidation) was not signif icantly affected by supplementation. Total mammary gland protein synth esis (milk plus non-milk protein) was also not affected by supplementa tion but on both diets gland synthesis was always greater (by 20-59%) than milk protein output. The fractional oxidation rate of leucine by the mammary gland was significantly increased by protein supplementati on (0.047 v. 0.136). Although the enrichment of leucine in secreted mi lk protein continued to increase, the final value (at 13 h) was 0.94 o f the arterial plasma free leucine plateau value (not significantly di fferent), suggesting almost exclusive use of plasma free leucine for m ilk protein synthesis. Based on current feeding schemes for dairy catt le, a fixed proportion (0.65-0.75) of the additional protein intake ( 490 g/d) should have been partitioned into milk protein. Instead, leu cine oxidation by the mammary gland was increased. Whether oxidation o f other amino acids was also enhanced is unknown but if amino acid oxi dation and the 'additional' non-milk protein synthesis occurring in th e gland are not crucial to milk synthesis, then by reducing such activ ities improvements in the efficiency of converting absorbed amino acid into milk protein can be achieved.