Tannins are a complex group of polyphenolic compounds found in a wide range of plant species commonly consumed by ruminants. They are conventionally classified into two major groups: the hydrolysable and the condensed tannins. Although for a long time tannins were thought to be detrimental to ruminants, their effect may be either beneficial or harmful depending on the type of tannin consumed, its chemical structure and molecular weight, the amount ingested, and the animal species involved. High concentrations of tannins reduce voluntary feed intake and nutrient digestibility, whereas low to moderate concentrations may improve the digestive utilisation of feed mainly due to a reduction in protein degradation in the rumen and a subsequent increase in amino acid flow to the small intestine. These effects on nutrition are reflected in animal performance.

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In an attempt to develop strategies for enhancing the nutritional value of sheep milk fat, dairy ewe diet was supplemented with 3 incremental levels of marine algae (MA), in combination with sunflower oil, to evaluate the effects of these marine lipids on milk fatty acid (FA) profile and animal performance. Fifty Assaf ewes in mid lactation were distributed in 10 lots of 5 animals each and allocated to 5 treatments (2 lots per treatment): no lipid supplementation (control) or supplementation with 25 g of sunflower oil/kg of DM plus 0 (SO), 8 (SOMA(1)), 16 (SOMA(2)), or 24 (SOMA(3)) g of MA (56.7% ether extract)/kg of DM. Milk production and composition, including FA profile, were analyzed on d 0, 3, 7, 14, 21, and 28 of treatment. Neither intake nor milk yield were significantly affected by lipid addition, but all MA supplements decreased milk fat content from d 14 onward, reaching a 30% reduction after 28 d on SOMA(3). This milk fat depression might be related not only to the joint action of some putative fat synthesis inhibitors, such as trans-9,cis-11 C18:2 and probably trans-10 C18:1, but also to the limited ability of the mammary gland to maintain a desirable milk fat fluidity, that would have been caused by the noticeable increase in trans-C18:1 together with the lowered availability of stearic acid for oleic acid synthesis through Delta(9)-desaturase. Furthermore, all lipid supplements, and mainly MA, reduced the secretion of de novo FA (C6:0-C14:0) without increasing the yield of preformed FA (>C16). Supplementation with sunflower oil plus MA resulted in larger increases in cis-9,trans-11 C18:2 than those observed with sunflower oil alone, achieving a mean content as high as 3.22% of total FA and representing a more than 7-fold increase compared with the control. Vaccenic acid (trans-11 C18:1) was also significantly enhanced (on average +794% in SOMA treatments), as was C22:6 n-3 (DHA) content, although the transfer efficiency of the latter, from the diets to the milk, was very low (5%). However, the highest levels of MA inclusion (SOMA(2) and SOMA(3)) reduced the milk n-6:n-3 ratio, but MA supplements caused an important increase in trans-10 C18:1, which would rule out the possibility that this milk has a healthier fat profile before determining the specific role of each individual FA and ensuring that this trans-FA is at least innocuous in relation to cardiovascular disease risk.

Addition of marine algae (MA) to the diet of dairy ruminants has proven to be an effective strategy to enhance the milk content of some bioactive lipids, but it has also been associated with the syndrome of milk fat depression. Little is known, however, about the persistency of the response to dietary MA in sheep. Based on previous experiments with dairy ewes fed sunflower oil plus MA, it was hypothesized that the response might be mediated by time-dependent adaptations of the rumen microbiota, which could be evaluated indirectly through milk fatty acid (FA) profiles. Animal performance and milk FA composition in response to MA in the diet were studied using 36 Assaf ewes distributed in 6 lots and allocated to 2 treatments (3 lots/treatment) consisting of a total mixed ration (40:60 forage:concentrate ratio) supplemented with 25 g of sunflower oil (SO)/kg of dry matter plus 0 (SO; control diet) or 8 g of MA/kg of dry matter (SOMA diet). Milk production and composition, including FA profile, were analyzed on d 0, 6, 12, 18, 24, 34, 44, and 54 of treatment. Diet supplementation with MA did not affect milk yield but did decrease milk fat content. Differences in the latter were detected from d 18 onward and reached -17% at the end of the experiment (i.e., on d 54). Compared with the control diet, the SOMA diet caused a reduction in milk 18:0 and its desaturation product (cis-9 18:1) that lasted for the whole experimental period. This decrease, together with the progressive increase in some putative fat synthesis inhibitors, especially trans-10 18:1, was related to the persistency of milk fat depression in lactating ewes fed MA. Additionally, inclusion of MA in the diet enhanced the milk content of trans-11 18:1, cis-9,trans-11 18:2, and C20-22 n-3 polyunsaturated FA, mainly 22:6 n-3. Overall, the persistency of the responses observed suggests that the ruminal microbiota did not adapt to the dietary supply of very long chain n-3 polyunsaturated fatty acids.