Influences of dietary crude protein and rumen protected conjugated linoleic acid on the efficiency of nitrogen utilization in lactating cows

In this trial the effects of a reduction of the dietary crude protein concentration, with or without addition of rumen protected conjugated linoleic acid (rpCLA), on DMI, milk yield, nutrients digestibility and N efficiency (milk N/intake N) were examined. Twenty Holstein-Friesian cows in middle lactation, homogeneous for parity, BW, body condition score, DIM, and milk yield, housed in pens of 5 were fed 4 diets containing 150 (CP15) or 120 g of CP/kg DM (CP12) with both CP15 and CP12 supplemented with 0 or 80 g/d of rpCLA, according to a 4×4 Latin Square design. Each period lasted 2 weeks for adaptation and one for experimental measures. The CP12 diet was formulated from CP15 by replacing soybean meal with barley meal to contain similar NDF content (37% DM). DMI and milk yield were recorded daily. Digestibility was evaluated using ADL as a marker, milk was analyzed for its chemical constituents. Individual data, averaged by group within period, were subjected to ANOVA. The reduction of dietary CP did not influence DMI, DM and NDF digestibility but reduced CP true digestibility from 0.82 to 0.77 (P<0.001), milk yield from 29.0 to 27.7 kg/d (P=0.072), milk protein yield from 1.00 to 0.91 kg/d (P=0.006), and increased N efficiency from 0.31 to 0.36 (P<0.001). Addition of rpCLA reduced DMI from 21.4 to 19.6 kg/d (P=0.018), milk fat production from 0.99 to 0.93 kg/d (P=0.001) and improved N efficiency (P=0.019). Differently from what observed in literature on beef, no CP × rpCLA interaction was observed. The significant effect of rpCLA on N efficiency suggests that these bioactive molecules would exert influences on N metabolism, irrespectively by the dietary CP density used. The strong reduction of dietary CP had influences on milk and milk protein yields much smaller than those expected, as NRC model predicts for the same CP15 and CP12 diets milk yields of 23.8 and 13.9 kg/d, respectively. This would reflect the existence of some compensating mechanisms, such as N recycling, operating when cows are kept under sub-optimal N supply