This paper reviews how tannins, through their effects on rumen lipid metabolism, can affect the composition of ruminants' meat and milk fat. Tannins are a heterogeneous group of plant secondary compounds known for both beneficial and detrimental effects on animals' digestive physiology. Tannins supplementation of ruminants' diets alters both in vivo and in vitro unsaturated fatty acids biohydrogenation and hence the profile of fatty acids outflowing the rumen, which can influence milk and meat content of beneficial fatty acids such as linolenic acid (c9,c12,c15-18:3), vaccenic acid (ti 1-18:1) and rumenic acid (c9,t11-18:2), among others. Published information indicates that tannins could inhibit biohydrogenation though affecting ruminal microorganisms. Some studies found increments in linolenic, rumenic and/or vaccenic acids in meat and milk fat using different sources of tannins; however, the effects of tannins supplementation on milk and meat fatty acid profile are not consistent, and there are contradictory results published in the literature. Effects of tannin supplementation on fatty acids biohydrogenation are affected by the chemical type of tannins, the complexity of their interactions with dietary components, and the potential microbial adaptation to tannins. In addition, the duration of the tannins-feeding period may also affect milk and meat fatty acid profile. Characterizing the effects of each specific tannic compound on different biohydrogenation steps and on the microbial species conducting them, as well as the interaction between specific tannin compounds and other dietary components can help to take greater advantage of tannins potential to contribute to improve human health through promoting beneficial fatty acids in ruminants products

In recent years, beef industry has improved production processes to ensure quality and certification for overseas meat markets. However, there is limited scientific information about the nutritional quality of beef from Chile. The aim of the present study was to evaluate the nutritional quality of beef produced in Chile from different production systems classified according to the type of finishing diet. Two-hundred and five animals from 13 livestock farms were used, 80 steers were finished on grazing pasture, 79 on pasture supplemented with grain or concentrate and 46 steers were finished on a typical feedlot system in the central zone of Chile. Longissimus thoracis intramuscular fat, fatty acid composition and cholesterol content were determined. Beef from pasture-fed animals showed higher content of vaccenic acid (C18:1 trans-11) rumenic acid (CLA cis-9 trans-11) and n-3 fatty acids, and lower n-6:n-3 ratio than beef from feedlot animals. However, Chilean beef from the three production systems showed similar intramuscular fat percentages and a n-6:n-3 ratio lower than 4.0. Results indicate that the Longissimus muscle from Chilean beef could be recommended as a source of red meat for a healthy diet.

Research is ongoing to find nutritional methane (CH₄) mitigation strategies with persistent effects that can be applied to grazing ruminants. Lipid addition to dairy cow diets has shown potential as means to decrease CH₄ emissions. This study evaluated the effects of oilseeds on CH₄ emission and production performance of grazing lactating dairy cows. Sixty Holstein Friesian cows grazing pasture were randomly allocated to 1 of 4 treatments (n = 15): supplemented with concentrate without oilseeds (CON), with whole cottonseed (CTS), rapeseed (RPS) or linseed (LNS). Oilseeds were supplemented during weeks 1–16 (spring period) and 17–22 (summer period), and the autumn period (wk 23–27) was used to evaluate treatment carryover effects. Cows fed CTS decreased CH₄ yield by 14% compared to CON in spring, but these effects did not persist after 19 weeks of supplementation (summer). Compared to CON, RPS decreased milk yield and CTS increased milk fat concentration in both spring and summer. In summer, CTS also increased milk protein concentration but decreased milk yield, compared to CON. In spring, compared to CON, CTS decreased most milk medium-chain fatty acids (FA; 8:0, 12:0, 14:0 and 15:0) and increased stearic, linoleic and rumenic FA, and LNS increased CLA FA. There were no carry-over effects into the autumn period. In conclusion, supplementation of grazing dairy cows with whole oilseeds resulted in mild effects on methane emissions and animal performance. In particular, supplementing with CTS can decrease CH₄ yield without affecting milk production, albeit with a mild and transient CH₄ decrease effect. Long term studies conducted under grazing conditions are important to provide a comprehensive overview of how proposed nutritional CH₄ mitigation strategies affect productivity, sustainability and consumer health aspects.