The use of fats in ruminant nutrition is widely accepted. Fat supplementation is connected to improved milk, fat yield, better health status, and improved fertility. While there are several reasons fertility problems occur, (i.e. low pregnancy rates or ovarian cysts) a diet with higher energy will be usually suggested. To obtain this the easiest way is to increase the fat content.
Lipids are a complex universe of organic compounds. The ones linked to basal and clinical nutrition are the fatty acids, naturally contained in raw materials or added to the diet as feed additives. They can also be part of storage tissues (adipose tissue) in the form of triglycerides (fatty acid esters and glycerol) or the free form as NEFA circulating into the bloodstream. Fatty acids differ for many aspects: the number of carbons, double bonds (unsaturated fatty acids), and the ability of the organism to synthesize the molecule or not (non-essential or essential fatty, acid respectively). Due to these characteristics, each fatty acid can result being useful or dangerous for the animal. Therefore, arbitrarily increasing the fat content of the diet may end up with unwanted side effects.
During periods of negative energy balance (NEBAL), the cow mobilizes body fat as an energy source. Genetic selection for high milk and fat yield has been favored these animals have an elevated tendency to increase their BCS during pregnancy and lose a lot of body weight after calving. Excessive weight loss is negatively related to the adequate production of hormones (GnRH, FSH, LH) necessary for a new pregnancy. Diets for non-pregnant cows at milk peak are frequently supplemented with rumen-protected lipids, especially palmitic (C16:0) and stearic (C18:0) acid. This improves milk production and milk fat but it is also frequently related to even more depressed fertility. This is because the hypothalamus, unable to distinguish whether NEFAs are derived from the diet or adipose tissue, interprets the high concentration of fatty acids in the blood as an excessive mobilization of body fat consequent to a hard NEBAL.
Recent research has demonstrated the negative role of increased concentration, both in the follicular fluid and plasma, of fatty acids such as C14:0, C14:1, C16:0, C16:1, C17:1, C18:3 n-6 (γ-linolenic acid), C18:2 n-6 (linoleic acid), and C20:0. In particular, C16:0, C18:0 e C18:1 are toxic to the follicle (granulosa cells) and affect the quality of the oocyte as well as the maturation and growth of the blastocyst. However, it is important to underline that saturated fatty acids can improve hepatic cholesterol production, the precursor of ovarian synthesis of estrogens and progesterone.
Polyunsaturated long-chain fatty acids and omega-3 (PUFA ω-3) will affect positively dairy cows’ metabolism. In particular, α-linolenic acid (C18:3 n-3, ALA), an essential fatty acid highly concentrated in fresh grass and whole linseed. Starting from ALA, the animal can synthesize eicosatetraenoic acid (C20:5 n-2, EPA) and docosahexaenoic acid (C22:6 n-3, DHA). EPA and DHA are precursors of series-3 prostaglandins (i.e. PGE3 and PGF3α) that have anti-inflammatory activity by inhibiting the series-2 synthesis. Scientific research has linked the high content of ALA, EPA, and DHA in the follicular fluid and blood to positive effects on fertility.
Another essential PUFA is linoleic acid (C18:2 n-6, LN) part of the ω-6 group, highly present in soybean, cottonseed, sunflower seed, and corn. The LN has opposite physiological effects than PUFA ω-3. Starting from LN the animal synthetizes C20:3 n-6 (DGLA) and subsequently PG-1 (i.e. PGE1, PGF1α). Also arachidonic acid (C20:4 n-6, AA), a precursor of PG-2 (i.e. PGE2, PGF2α) derives from DGLA.
Figure 1: Metabolism of PUFA ingested with diet
Prostaglandins are used in the practice to “force” the lyses of the corpus luteum or to ameliorate uterine involution 20 days from calving. Naturally, they are produced by the endometrium after an estrous cycle not followed by pregnancy or better the lack of their inhibition by the tau-interferon (INF-τ) produced by the embryo before the placentation causes their release.
ω-3 PUFA inhibit prostaglandins secretion from the endometrium, increasing the probability of the embryo surviving before the engraftment. At the same time, ω-3 PUFA increase cellular sensitivity to insulin, facilitating glucose uptake and reducing the negative effects of the insulin-resistance typical of the transition phase and the first weeks after calving. Conversely, ω-6 PUFA promote insulin-resistance.
A high concentration of ω-6 PUFA is useful after calving to promote a correct involution of the uterus and its balanced bacterial environment, as well as at the beginning of the dry period, where ω-3 PUFA are useful in preparation to calving and after the puerperium until the new pregnancy is assessed. Summing up, ω-6 has pro-inflammatory while ω-3 are anti-inflammatory properties.
Fresh cow diets are rich in linoleic acid from whole soy, cotton, or corn. As reported in Figure 2, the content of linoleic acid in a typical fresh cow diet is about 440 g/day, while the α-linolenic acid is about 40 g/day.
Figure 2: Fatty acids composition of a standard diet for lactating dairy cows (DM 24 kg, CP 16.75%, starch 26%, CF 4.3%) composed by corn silage, alfalfa hay, ryegrass hay, integral cotton, corn meal, soybean meal, sunflower extraction meal, vitamins, and minerals
The presence of a high concentration of hydrogen ions in the rumen (a dairy cow at milk peak can produce about 700 g/day) causes the saturation (around 69 to 90%) of the unsaturated fatty acids ingested so that the concentration of ω-3 and ω-6 PUFA available for the animal is different and lower than their concentration in the diet. The saturation depends on multiple factors such as the ruminal pH, passage rate, or starch content of the diet. This should be considered during the dietary balance of ω-3 and ω-6 PUFA. An overdosing of free ω-3 fatty acids in the rumen increases the risk of isomers production, such as trans-10 or cis-12 C18:2 or the conjugated linoleic acid (CLA). CLA has a positive effect on human health but reduces milk fat concentration. 2.5 g are able to decrease the lipid content in milk by 25%.
To balance the diet with ω-3 PUFA in a correct way, rumen-protected sources are extremely useful. This kind of supplementation by-passes the rumen has a high bioavailability for the animal and protects the milk fat concentration.For more information: email@example.comOriginal article here.