Objective: The effect of wheat percentage in diets offered to lactating cows on variations of milk fat content and profile of milk fatty acids (FA) was studied, focusing on odd-chain FA and trans intermediates of ruminal biohydrogenation.
Materials and methods: Two cows equipped with a ruminal canula successively received diets based on maize silage, and comprising 0, 20, 34 and again 0% wheat on a dry matter basis. Each diet was used during 12 days. The diet was distributed in two equal meals at 08:00 and 17:00, and wheat was top-dressed on silage. Milk samples were taken at the evening milking, and samples of ruminal contents were taken hourly from 08:00 to 16:00 on days 10 and 11 in the first period, and on days 5, 10 and 11 in the 3 subsequent periods.
Results and discussion: Compared with the initial control, after 10 days adaptation, diet with 20% wheat significantly lowered mean ruminal pH (6.03 vs 6.77) and milk fat content (33.0 vs 44.1), and significantly increased the percentage of odd-chain FA in milk fat (2.38 vs 1.48). The trans-10C18:1 / trans-11C18:1 ratio increased from 0.34 to 0.82, but the difference was not significant.
After 10 days adaptation, the diet with 34% wheat resulted in low ruminal pH (5.8), low milk fat content (22.4), and high percentage of odd-chain FA (3.03) and high trans-10C18:1 / trans-11C18:1 ratio (12.2). All these values were significantly different from both initial values and values observed with 20% wheat.
After 10 days with the control diet following acidogenic diets, mean ruminal pH and milk fat content returned near initial value (6.98 and 41.1, respectively), but odd-chain FA and the trans-10C18:1 / trans-11C18:1 ratio (1.80 and 2.14, respectively) remained significantly higher than initial values. This suggests that the effects of a ruminal acidosis can remain a long time after returning to a non-acidogenic diet.
Values observed after 5 days adaptation to the three diets were intermediate between values at the end of the previous period and values after 10 days adaptation, and significantly different from values after 10 days adaptation for all presented parameters except the trans-10C18:1 / trans-11C18:1 ratio (P = 0.25).
The correlation coefficients between mean ruminal pH and milk fat content, proportion of odd-chain FA and the trans-10C18:1 / trans-11C18:1 ratio were significant (0.84, 志0.87 and 志0.62, respectively). However, this low latter value was due to a weak relationship when pH was over 6.2, and a large increase of the trans-10C18:1 / trans-11C18:1 ratio when mean ruminal pH was under 6.2.
These results are consistent with present knowledge on trans-10 FA as a result of low ruminal pH and a cause of low milk fat content. They show that the trans-10C18:1 / trans-11C18:1 ratio can exhibit very large variations when the mean ruminal pH is under 6.2.
Conclusion and perspective: Induced acidosis resulted in lowered milk fat content, and higher proportion of odd-chain FA and a higher trans-10C18:1 / trans-11C18:1 ratio. Milk fat content and proportion of odd-chain FA were linearly related to mean ruminal pH. On the contrary, trans-10C18:1 / trans-11C18:1 ratio only exhibited variations when mean ruminal pH was low, and these variations were in a large range, making this ratio a possible candidate for biochemical characterisation of acidosis
