SUMMARY

Three studies were conducted to identify strategies to avoid milk fat depression in dairy cows when diets are supplemented with oils rich in polyunsaturated fatty acids. Polyunsaturated fatty acids are used for the production of milk enriched in omega-3 fatty acids and conjugated linoleic acid (CLA). A literature review conducted in the first study showed that the daily CLA recommendations most commonly reported for humans are 0.8 g / d (0.6 to 3.0 g / d), although all recommendations have been extrapolated from animal models and the few studies in humans report conflicting results.

Were selected 69 published articles, where dairy cows with different fats were fed, the fat content of the milk and the fatty acid profile were recorded. Taking into account changes in CLA and milk fat content, supplementation with fish oils, together with vegetable oils, would be the best strategy (395 mg cis-9, trans-11 CLA / l vs. 188 mg cis -9, trans-11 CLA / l, 2.1 fold increase). Average estimated human consumption in Europe, the United States and Canada is 0.21 g / d. If we assume a 2.1-fold increase in CLA content in milk, the average human consumption would increase from 0.21 to 0.46 g / day. Although there is sufficient data in nutrition about strategies to increase CLA content in milk, human needs have not been well established and, based on current recommendations, are unattainable even if all milk and dairy products were consumed as products enriched in CLA. In the second study, two experiments (in vitro fermentation and double continuous flow fermenters) were carried out to determine the effects of two lipases, a lipase inhibitor and three essential oils on ruminal fermentation and the apparent bio hydrogenation of linoleic acid and linolenic. The treatments were control, lipase 1 and 2, a lipase inhibitor, Oxy-propyl-thiosulfate; Eugenol and Cinnamaldehyde (experiment 1), and control, lipase 1, Oxy-propyl-thiosulfate and Cinnamaldehyde (experiment 2) at two pH levels (6.4 and 5.6).

In Experiment 1, Lipase 1 increased the apparent as bio hydrogenation of linolenic acid and reduced the efficiency of the intermediate steps of the bio hydrogenation of linoleic and linolenic acids, but these results were not observed in experiment 2. Oxypropylthiosulphate inhibited the apparent bio hydrogenation of linoleic and linolenic acids, and decreased total concentrations of volatile fatty acids in the two experiments.

The third study was based on the fact to feeding dairy cows with diets supplemented with flaxseed resulted in milk fat depression, but there is a wide range of sensitivity among cows. The objectives of this study were to compare the expression of mRNA in somatic milk cells in cows resistant or sensitive to milk fat depression and to identify the metabolic pathways and transcription factors differentially expressed in cows resistant or susceptible to depression of the milk fat. Four cows were selected from a commercial farm after switching from a control diet to a diet rich in flaxseed. Among them, two cows were resistant to milk fat depression, having a high milk fat content both in the control (4.06%) and in the diet supplemented with flaxseed (3.90%); And two cows were sensitive to milk fat depression, decreasing the fat content of milk after switching to the diet with flaxseed (from 3.87 to 2.52%, respectively).

Differential expression analysis between cows resistant or susceptible to milk fat depression allowed the detection of a large number of differentially expressed genes in both diets Control (n = 1,316) and Flaxseed (n = 1888). Analysis of metabolic pathways and key regulatory genes also allowed the detection of a large number of pathways (between 13 and 117) and key regulatory genes (between 27 and 294) differentially expressed in the different comparisons made between resistant and susceptible cows of the milk fat and between both diets (Control and Flaxseed).

The analysis of single nucleotide polymorphisms (SNPs) showed 641 SNPs only in cows resistant to milk fat depression and 1024 only in cows sensitive to milk fat depression among genes differentially expressed in all comparisons. The results suggest that cows resistant to milk fat depression may be activating a compensation mechanism to increase fatty acid synthesis in the presence of flaxseed. Taken together, the results of this thesis suggest the possibility of avoiding milk fat depression by selecting animals resistant to milk fat depression using transcriptomic technologies.