**3. Rumen-inert fat and the productive and reproductive responses in ruminants**

90 Milk Production – An Up-to-Date Overview of Animal Nutrition, Management and Health

acids are commonly found in oil and fishmeal.

ruminants.

tissues (Table 2).

(Adapted from Staples *et al*., 1998)

source of fat.

fatty acids or hydrogenated fats.

the stearic FA (C18:0). Eicosapentaenoic acid (C20:5, ω-3) and docosahexaenoic acid (C22:6, ω-3) undergo very little hydrogenation in the rumen (Mattos *et al.,* 2000). These two fatty

When we use a source of fat, not protected or not a rumen-inert fat, with high levels of PUFA, most is lost due to the biohydrogenation, which is particularly important for PUFA ω-6 and ω-3, which are considered as essential in the diet and which have an important role on reproductive, immunological, metabolic and hormonal functions. Therefore, this type of susceptible fat to interact in the rumen, are known as active fat and its use is limited in

New technologies have generated chemically modified fats that allow its use at higher levels and with a lower level ruminal interaction, which reduces the deleterious effects of lipids on the ruminal bacteria activity. This kind of fats is known as inert fat, by-pass fat, or protected fat. In this regard, Jenkins (2004) defines the inert greases as those fats that have been specifically designed to have very little, or no negative effect on the digestibility of foods in ruminants. Rumen-inert fats are often carboxylate calcium salts (calcium soaps), saturated

The use of calcium soaps allows the incorporation of a higher level of unsaturated fatty acids in the diet of ruminants. This is particularly important in the case of essential fatty acids (ω-6 and ω-3) which not only provide an energy effect *per se*, but may have specific effects on the metabolism of tissues and organs (Staples *et al.,* 1998). The melting point of rumen-inert fats, is usually above 100°C and solubility occurs at pH levels below 5.5. These values of temperature and pH are present in the rumen, which allows the fat to by-pass the rumen. However, at the level of the abomasum and first portion of the duodenum pH levels are much lower, allowings the dissociation of the carboxylate salt, leaving the fatty acids available for absorption. Therefore, it can be concluded that supplementation of rumen-inert fats to ruminants generates an increase in the availability of unsaturated fatty acids at the intestinal level, and therefore may increase the absorption and their incorporation into

> Linoleic acid appearing in small intestine (g/day)

Source of fat Fat source fed Linoleic acid fed

Whole cottonseeds 2.8 300 30 to 120 Whole soybeans 2.8 300 30 to 120 Yellow grease 0.45 77 8 to 31 Tallow 0.45 23 2 to 9 Calcium soaps (8,5% ω-6) 0.45 38 25 to 34

**Table 2.** Estimated amount of linoleic acid that reaches the small intestine in cows, depending on the

(Kg/day) (g/day)

When the rumen-inert fat is included into the diet it incorporates more PUFA, which is not only an energy source, but also generates non-energetic benefits related to the impact of these FA on the metabolism, hormonal and immune responses. The energetic effect is related to the greater amount of energy that lipids have, which helps to reduce the NEB during the postpartum period, which translates into increased production of luteinizing (LH) and follicle-stimulating hormone (FSH) by the pituitary gland, generating more growth and follicular development and promoting the ovulation (Diaz *et al.,* 2009).

Non-energetic effects are associated with the type of FA presents in the supplied fat, these effects are related with the increase in cholesterol (particularly the HDL fraction or "good cholesterol") levels, direct effects on ovaries and uterus, increase the levels of progesterone (P4) and the modulation of the prostaglandins production (especially PGF2α). In addition, rumen-inert fat has direct effects on hormones and growth factors involved with the reproductive and productive activity (insulin, IGF-I, among others). Most of these non-energy effects are favored when PUFA (ω-6 and ω-3) are included in the diet. Figure 1 shows various of the non-energetic proposed mechanisms, that can generate this type of FA.

**Figure 1.** Proposed mechanisms of action by which PUFA supplementation can affect reproductive function. (Adapted from Staples *et al*., 1998).

The impact of this type of fat is associated with the energy balance of the animal, particularly in the cow during early lactation, decreasing the deleterious effects of the NEB and improving the reproductive activity during the early postpartum. On this regard, Staples *et al*. (1998) mentioned that 11 out of 18 studies reviewing the effect of fat in cow's reproduction, reported an increase in reproductive performance, either because it improves the conception rate at first service or by increasing in the overall conception or pregnancy rates.

In addition, the supplementation with high levels of PUFA can generate a favorable response in milk production and composition, and an improvement in the fatty acid profile of the milk, particularly the levels of conjugated linoleic acid (CLA). Angulo *et al*. (2005) have described that some anticancer and antidiabetogenic properties of the CLA as well as the prevention of atheroma formation, the potentiation of the immune response and the improvement of bone mineralization. It may also increase levels of essential fatty acids (ω-6 and ω-3) in milk, which can cause the milk produced by cows or goats fed with this type of fat, would be a nutraceutical food.
