Effect of dietary CP and rumen protected CLA on performance of lactating cows

The effects of dietary protein level (CP) and of rumen protected conjugated linoleic acid (rpCLA) on dry matter intake (DMI), milk yield, rumen activity, and nutrients digestibility were evaluated. Twenty Holstein-Friesian cows, homogeneous for parity, body weight, body condition score, DIM, and milk yield, were housed in 4 pens and fed 4 diets containing 15% (CP15) or 12% of CP (CP12), supplemented or not with 80 g/d of rpCLA, following a 4×4 Latin Square design. Each period lasted 3 weeks. The CP12 diet was formulated from CP15 by replacing soybean meals with barley, to contain similar NDF content (37% DM). DMI, rumen activity, milk yield and quality were daily monitored. Diet digestibility was evaluated using ADL as marker. Data were analyzed using GLM procedures in SAS. The reduction of dietary CP content did not influenced DMI, DM and NDF digestibility but slightly reduced true CP digestibility and increased rumination activity from 16.8 to 19.8 min/kg DMI (P=0.009). Reduction of dietary CP decreased milk yield only 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), but N efficiency was improved (+14%, P<0.001). Supplementation 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), slightly increased rumination activity (P=0.06), without effect on diet digestibility and improved efficiency of N utilization (+7%, P=0.019). Results shown that a strong reduction of dietary CP influences milk yield less than it was expected from simulation using the NRC model which predicts for CP15 and CP12 milk yields of 23.8 and 13.9 kg/d, respectively. The effects of a reduction of diet CP on rumen degradation of feeds would be mitigated by a compensative increase of rumination activity. The significant effect of rpCLA on the efficiency of N utilization suggest that these bioactive molecules can exert influences on N metabolism.