Effect of different levels of fat addition on growth performance in grow-

The ability of fats to be combusted for energy is the most important reason for fat addition in commercial diets during pig production. Research on different levels of fat supplementation always involve both energy density and ratio of protein/lysine:energy (Gu and Li, 2003). Because of the differences in maturity of the GI tract and metabolic activities, pigs at different age or weight have a different response to fat supplementation and, as a result, have a different optimal ratio of protein/lysine:energy.

Beneficial effects have been generally reported in grow-finishing pigs from 20 to 100 kg, including improved growth rate, reduced feed intake, and improved F/G. However, the value of dietary fat supplementation in weanling pigs from 4 to 20 kg remains inconsistent (Leibbrandt et al., 1975; Coffey et al., 1982; Pettigrew et al., 1991; Gu and Li, 2003; Liu et al., 2018). No difference in growth performance was reported when weanling

linearly as dietary fat supplementation increased from 0 to 12% (Lawrence and Maxwell, 1983). Piglets have limited ability to digest high oil diets, but grow-finishing pigs can digest diets with fat supplementation as high as 10% very well (Leibbrandt et al., 1975). Due to the higher digestibility and gross energy content of fats compared to the other feed ingredients, adding fat to the diets increases digestible and metabolizable energy density and, as a result, reduces feed intake and improves feed efficiency (Li and Patience, 2016). The response in growth performance to fat supplementation also varies depending on the balance of different nutrients. When protein:energy ratio was not fixed with increasing fat supplementation, fat supplementation had different effects in pigs depending on the dietary energy density, via the direct effect in limiting the total energy intake of pigs. The critical lower limits of energy density were suggested to be 3,760, 3,290, and 2,350 kcal of DE/kg for pigs weighing less than 20 kg, between 20 to 50 kg, and greater than 50 kg, respectively (Black, 1995). Both ADFI and F/G linearly decreased, while ADG linearly increased as dietary ME density increased without adjusting other nutrients including amino acids and minerals, when pigs were fed with five diets containing a graded level of ME ranging from 3.05 to 3.61 MJ/kg by modulating the inclusion rate of canola oil, wheat, and barley in grow-finishing pigs (Beaulieu et al., 2009). However, in their second study, when lysine was adjusted to maintain the same lysine content per unit of digestible energy, ADG was not affected, but ADFI and F/G were both reduced by increasing fat supplementation.

When protein:energy ratio remains constant with the increasing fat supplementation, ADG was normally not affected by fat supplementation either from different sources or levels, while reduced ADFI and F/G were consistently reported (Pettigrew et al., 1991; Liu et al., 2018). When pigs were fed either soybean oil or choice white grease at levels of 3% or 6%, no differences were detected on overall ADG between fat added group and non-fat added group from 65 to 122 kg, while fat addition reduced overall ADFI and F/G (Liu et al., 2018). When pigs were fed diets with 6% fat from soybean oil, choice white grease, palm oil, animal-vegetable blend, or tallow, a similar response in overall ADG from 72 to 130 kg was observed, while differences in the effects of fat sources on ADFI were reported (Liu et al., 2018). When dietary fat supplementation increased from 0 to 2.5, and 5 % from animal-vegetable blend fat and tallow, both overall

ADFI and overall F/G were linearly reduced by increasing fat supplementation, while ADG was not affected (Averette Gatlin et al., 2002). In summary, fat supplementation with a good balance with other nutrients especially amino acids does not affect ADG, but does reduce ADFI and F/G. The difference in fat sources exists because of their different fatty acid profile, which makes them variable with regard to providing energy.

2.4.5 Effect of different levels of fat addition on the metabolism of fat in grow- finishing pigs

Increased fat deposition in terms of backfat thickness is consistently reported with fat supplementation (Coffey et al., 1982; Pettigrew et al., 1991; Averette Gatlin et al., 2002; Benz et al., 2011b; Kellner et al., 2015). The deposited fat in pigs originates from both dietary FA and de novo synthesized FA (Jump et al., 2005; Kloareg et al., 2007). The de novo lipogenesis primarily occurs in the adipose tissue in swine, it is different with humans and rodents, where lipogenesis either primarily or exclusively occurs in the liver (O'Hea and Leveille, 1969). Increasing dietary fat supplementation suppressed fatty acid synthase function, and reduced the rate of de novo lipogenesis in adipose tissue (Kellner et al., 2014; Kellner et al., 2017). Large amounts of dietary fatty acids (especially non-essential FA) are deposited without modifications of either chain length or degree of saturation in pigs, through which de novo lipogenesis is suppressed (Ellis and Isbell, 1926; Allee et al., 1971; Kellner et al., 2014; Beld et al., 2015; Kellner et al., 2017).

Although fat deposition increases with increasing dietary fat supplementation, dietary fatty acids inhibit lipogenesis in the adipose tissue. The increasing level of dietary fat (corn oil) from 1 to 13% significantly depressed (60 to 70% ) in vitro lipogenesis, although body fat content was increased due to the increasing direct deposition of dietary fat (Allee et al., 1971). When pigs were fed with diets of either 5% fat or starch, the fat supplementation decreased the mRNA abundance of FASN in adipose tissue, and mRNA abundance of ACACA, ATGL, PPAR-α, PRKAG-1, and SCD in the liver compared to the

displaying inhibitive effects on lipogenesis in adipose tissue. This effect is supported by the results that mRNA abundance of FASN and the transcription factor SRENP-1, which regulates the expression of key enzymes involved in the lipogenesis pathway, were reduced when dietary fat sources were high in SFA and omega-6 FA (Kellner et al., 2017). Pigs fed diets with 5% corn oil tended to have greater adipose tissue expression of FASN than pigs fed with 5% fish oil, tallow or coconut oil (Kellner et al., 2017). When pigs were fed diets with no added fat or 11% fat (tallow, sunflower oil, linseed oil, blend oil, and fish oil), the mRNA abundances were highest in pigs fed diets with no added fat, while lowest in pigs fed diets with 11% tallow (Duran-Montgé et al., 2009). The result indicates a reduced lipogenesis in pigs fed 11% tallow diet. In most recent studies, the addition of 5% dietary fat in the form of coconut oil, corn oil, fish oil, and tallow all reduced FASN abundance compared to pigs fed a control diet without fat addition (Kellner et al., 2017). And when pigs were fed 3% corn oil (56.84% omega-6 FA) compared to the pigs fed 3% tallow (2.81% omega-6 FA), greater abundance of FASN was observed (Kellner et al., 2016; Kellner et al., 2017).

High dietary fat supplementation suppresses lipogenesis in adipose tissue and affects lipid metabolism activity in liver of pigs, while intake of omega-3 fatty acids suppressed the mRNA abundance of genes involved in lipolysis in both adipose tissue and the liver (Allee et al., 1971; Kellner et al., 2017). Dietary SFA had a more potent inhibitive effect in de novo lipogenesis than omega-6 fatty acids. Correlation coefficients, established between dietary fatty acid composition and mRNA abundance of genes affected by dietary fat treatments in adipose and liver, indicated that dietary SFA (-0.898) and MUFA:SFA ratio (0.885) were more sensitive to PPAR-α than MUFA (0.828) or omega-6 FA (not significant) (Kellner et al., 2017). According to the research of Kloareg et al. (2007), de novo synthesized fatty acids are mostly SFA (such as palmitic and stearic acid) or MUFA (such as palmitoleic or oleic acid), while amount de novo synthesized long-chain FA is very small. When pigs consume diets with a high content of SFA, the deposition of these SFA may need less metabolic processing compared to the case when pigs consume and deposit more omega-6 FA, such as linoleic acid (Kellner et al., 2017).