Supplementation of palmitic acid (PA) to some or all lactating cows in a herd is a practice known to improve fat content, although questioned from a consumer health, milk processing and environmental point of view. The objective of the present study was to evaluate whether one could rely on bulk tank fatty acid (FA) profile to easily monitor the use of this feeding strategy. The database generated by a survey on the use of PA in dairy cow rations was used and matched with corresponding bulk tank FA profiles. A total of 989 herds were part of the original database. Of these, 52 herds had been feeding a source of fat rich in PA (≥85% as PA) to all lactating cows in the herd for more than 6 mo and 467 herds had never fed PA over the past 2 years. Herds feeding PA to some of the cows or feeding products containing mixed FA were excluded, leaving 519 herds for the present study. Bulk tank FA composition was analyzed by FTIR every other day from September to December 2017 and was averaged for each herd. Of the herds feeding PA, most herds (65.4%) reported feeding between 200 and 300 g/cow/d, while some herds were feeding less than 200 g/cow/d (23.1% of herds) or more than 300 g/cow/day (11.5% of herds). Supplementation of PA influenced milk and FA composition (Table 1): fat, C16:0, and C18:0 FA concentrations in milk increased, while protein, C14:0 and C18:1 FA concentration in milk remained unchanged with PA. Relative proportions (% of total FA) of de novo and mixed FA were respectively decreased and increased by PA addition to the diet. Although indicators presented in the table are influenced by PA supplementation, ranking of herds by any of these variables does not allow to confirm whether herds are supplementing PA or not. From this data set, bulk tank FA profile alone cannot serve as a tool to diagnose PA supplementation in Quebec dairy herds. Table 1.

Composition (g/100 g milk)	PA	No PA	SE	P-value
Fat	4.08	4.04	0.02	0.04
Protein	3.29	3.32	0.01	0.008
C14:0	0.445	0.445	0.004	0.96
C16:0	1.155	1.124	0.010	0.003
C18:0	0.362	0.348	0.003	≤0.001
C18:1	0.875	0.864	0.007	0.12
Composition (g/100g FA)
De novo FA	31.22	31.60	0.15	0.02
Mixed FA	33.41	33.08	0.14	0.02
Preformed FA	35.37	35.32	0.26	0.87