We propose that the minimum linoleic acid intake level should be targeted for 90-95% of the sows (rather than the average), which is especially important in an all or none scenario, where sows either maintain their pregnancy or not. Thus, sows that are compromised by having lower intake than the average of the population will consume adequate levels of linoleic acid to allow for a successful pregnancy.

This ultimately requires knowledge of seasonal lactation intake and the variation associated with feed intake, such that the minimum linoleic acid intake (g/d) can be achieved by most sows. For example, if the average daily feed intake of sows during the summer is predicted to be 5.5 kg per day and the bottom 10% of the sows are expected to consume 4.0 kg per day, then the dietary concentration of linoleic acid can be calculated to be 3.1% to achieve 125 g per day of linoleic acid intake for at least 90% of the sows in the population.

This also implies that diets should be formulated according to seasonal variation in intake, with dietary specifications for linoleic acid being greater in the summer than in the winter. Formulating diets by adding lipids according the EFA specifications rather than using lipids to formulate high-energy diets may present cost savings because of lower lipid inclusion rates. However, the cost of lipid sources relative to their content of EFA should be considered (Table 1). For example, when formulating diets based on their cost per unit of metabolizable energy (ME), animal-vegetable (A-V) blend, poultry fat, and choice white grease (CWG) are most cost effective because they are the lowest cost per unit of energy (0.107, 0.111, and 0114 $/Mcal of ME, respectively). On the other hand, when formulating on a linoleic acid basis, corn oil and soybean oil are most cost competitive ($2.04 and $2.59/unit of linoleic acid, respectively).

It should be noted that soybean oil also contains a substantial amount of α-linolenic acid compared to other commercially available lipid sources. Our research, not shown herein, suggested that a minimum dietary intake of α-linolenic of 10 g/day during lactation was beneficial for subsequent reproductive performance.

We conclude that a minimum dietary intake of 125 g/d of linoleic acid together with a minimum of 10 g/d of α-linolenic acid should be provided to at least 90-95% of the sows. This should be done to achieve maximum sow reproductive efficiency through multiple mechanisms, including rapid return to estrus, high maintenance of pregnancy, and large subsequent litter size in mature sows, which appear to be especially susceptible to EFA deficiency.

The findings described here represent a tremendous step forward towards resolving seasonal anestrous that has plagued the swine industry for decades.