Effect of pre-grazing herbage mass and daily herbage allowance on rumen, plasma and milk fatty acids

PALLADINO, R.A., O´DONOVAN, M., J.J. MURPHY, M. MCEVOY, AND KENNY, D.A.

Montreal

Congreso; 2009 Joint ADSA-CSAS-ASAS Annual Meeting.; 2009

ADSA-CSAS-ASAS

Conjugated linoleic acid (CLA) in milk is dictated by intake of dietary precursors and the extent of ruminal biohydrogenation. Grazing management may change both precursor intake and biohydrogenation through its influence on pasture composition. This study evaluated the effect of two levels of pre-grazing herbage mass (HM; 1700 and 2500 kg/ha) and two levels of daily herbage allowance (DHA; 16 and 20 kg/cow/d) on the fatty acid (FA) composition of rumen fluid, plasma and milk in grazing dairy cows. Four rumen fistulated Holstein dairy cows were randomly allocated to four treatments (high HM, high DHA; high HM, low DHA; low HM, high DHA; low HM, low DHA). Rumen (0700, 1100, 1500 and 2000 h), plasma (0700 and 1500 h) and milk (pm milking) samples were collected during each of four periods from June to August in a Latin square design. Samples were analyzed for FA by gas chromatography. Data were analyzed by mixed model ANOVA with cow as a random effect. The model included terms for HM, DHA, period and the interaction HM x DHA. For rumen and plasma samples, time of sampling was included as well as the interaction of HM x time, DHA x Time and HM x DHA x time. No CLA was detected in either rumen or plasma samples. The milk CLA concentration was in agreement with previous reports under grazing conditions (12.8 × 2.20 g/kg of total FA) but was not affected by treatment (P>0.05). Rumen concentration of vaccenic acid (VA) was highest in cows grazing the low HM, high DHA pasture (17.0 g/kg of total FA; P<0.05). Additionally, rumen VA concentration increased from 0700 to 2000 h (from 13.1 to 14.7 g/kg of total FA; P<0.01). Concentrations of VA in plasma were not affected by treatment or time of sampling (P>0.05). Milk VA was higher in cows grazing low HM (P<0.01). Linolenic acid (LNA) in plasma or rumen fluid was not affected by time (P>0.05). However, milk LNA was higher for cows grazing low HM (3.6 vs. 5.6 g/kg of total FA for high and low HM respectively). In conclusion, under the conditions of the present study, there was little effect of grazing management on the concentration of health promoting FA in milk.