Influence of N shortage and conjugated linoleic acid supplementation on some productive, digestive, and metabolic parameters of lactating cows

• A reduction of dietary CP from 15 to 12.2% reduced DMI by 7.7% and milk yield by 4.8%.
• Protein restriction was accomplished by replacing soybean meal with barley grain.
• Protein restriction decreased N intake by 122 g/d and N in milk by only 14 g/d.
• The apparent efficiency of N use increased 0.31–0.36 for a six-week period.

The effects of a shortage of dietary N and of a rumen protected CLA (rpCLA) supplement on DMI, rumination, rumen fluid characteristics, milk yield (MY), and milk N/N intake ratio (ENU) were studied in mid-late lactating cows. Twenty cows housed in 4 pens in groups of 5, homogeneous for parity, days in milk (DIM) and MY, were fed on 4 different diets: with 150 g (CP15) or 123 g (CP12) of CP/kg DM, with or without an rpCLA supplement (containing 6.34 g/d of C18:2c9,t11 and 6.14 g/d of C18:2t10,c12). A 4 × 4 Latin Square experimental design was used with periods of 3 wks, although the sequence of the 4 treatments (CP15 or CP12, with or without rpCLA) was such that each group received CP15 or CP12 for 6 consecutive wks. The CP12 diet was formulated from CP15 by replacing soybean meal with barley grain to maintain similar energy, fiber content and feed particle size. Rumination activity, DMI, and MY were recorded daily. Rumen fluid was analyzed for VFA and ammonia N content, and milk for quality traits. Nutrient digestibility was estimated using Lignin(sa) as a marker. Period, treatment and group (random) were included as sources of variation in the statistical analysis. Dietary CP restriction tended to reduce DMI (−7.7%; P = 0.09) and digestibility, but increased time spent in rumination (+10%; P = 0.009), decreased rumen fluid ammonia N (−36%; P < 0.001), and reduced MY (−4.8%; P = 0.047) and milk protein content (−4.7%, P = 0.026); it had no influence on the ratio between energy-corrected milk yield and DMI. CP restriction reduced N intake by 122 g/d and N in milk by 14 g/d, did not influence N in feces, but increased ENU from 0.31 to 0.36 (P < 0.01). A low marginal response of 115 g milk N/kg to the increased N intake from soybean meal was found. The addition of rpCLA tended to reduce DMI (−8.1%; P = 0.07) and decreased milk fat content (−15%; P = 0.002), but decreased N in milk only when added to CP12 (CP × rpCLA interaction, P = 0.016). A shortage of N supply increases ENU without apparent alteration of BW, BCS and blood metabolites. Long-term investigations to clarify the role of body N reserves and of a shortage of CP supplies on N partitioning are needed.