Previous studies have confirmed that dietary lipid types do not affect egg production and egg weight (Abbasi et al. 2019, Ceylan et al. 2011). It was reported that egg yield, egg weight, and feed intake did not change with the inclusion of different lipid sources (cotton, corn, flax, soybean, olive, sunflower, fish, tallow, and rendering) in the diet (Balevi and Coskun 2000). Omidi et al. (2015) used five different lipids rich in UFAs (fish, olive, grapeseed, canola, and soybean oils) in the diet and found that performance parameters during the laying period did not differ significantly between treatments. It has long been known that vegetable oils in the diet of chickens can increase egg weight, while fats from animal sources have less effect on it (Jensen et al. 1958, Shutze et al. 1958, Treat et al. 1960). To test this hypothesis, Whitehead et al. (1993) reported that corn oil in the diet of hens increases egg weight compared to fish oil, coconut oil, and tallow. Thus, egg size may increase when hens are fed diets high in oil and rich in unsaturated fatty acids (Galea 2011) which affects shell quality. It has been reported that feeding canola oil, which is high in linoleic acid, results in higher egg weight than feeding poultry fat (Bohnsack, Harms, Merkel and Russell 2002). The results of some studies comparing the effects of different vegetable oils in the diet of chickens showed that egg quality parameters were not influenced by the oil sources. For example, Batkowska et al. (2020) showed that the inclusion of 2.5% linseed or soybean oil in the diet for laying hens had no negative effect on egg quality. Similarly, Küçükersan and Küçükersan (2010) found no differences in egg quality when high UFA oils such as sunflower, fish, soybean, and hazelnut oil were included in the feed at a concentration of 3%. A recent study proved that linseed and soybean oil have no effect on quality traits, but increase the weight of chicken eggs (Batkowska et al. 2021). However, some studies reported that olive and sunflower oil in the diet increased the weight of eggs compared to sesame, cottonseed, hazelnut, maize, soybean, and fish oil, but the thickness of the eggshell was not changed by the lipid sources in the diet (Güçlü et al. 2008). Depending on the amount of added oil in the diet, the amounts of other feeds may change so that the fatty acid profile and nutrient composition of the diet may vary, and these changes may affect egg quality parameters. In our study, the experimental diets were formulated with the same amount of feed ingredients (Table 1.) and with the same content of seed-based oils (3.7% of the diet), so that the FA changes in the experimental diets were caused by the added oils and the FAs of the experimental diets in this study clearly had no effect on the egg quality parameters.
The colour of the egg yolk is another effective factor that can influence consumer choice as much as the size of the eggs and can be easily influenced by feed ingredients. For this reason, feed premixes containing carotenoids, which provide the preferred dark yellow yolk colour, are often used in the diets of laying hens. It is reported that unrefined seed oils can contain a detectable amount of carotenoids (Franke et al. 2010), but this amount depends on the carotenoid content of the seed. In addition, the literature mentions that different vegetable oils, depending on their content or mixed oil sources in the diet, can alter the pigmentation of the egg yolk (Faitarone et al. 2016). In this study, a* value, indicating redness, decreased significantly with the intake of HSO and decreased slightly with the intake of CAO, compared to CO, SBO, and SFO. The L* value, indicating the brightness of the yolk, increased in parallel with the decrease in a* value in the CAO and HSO groups. The results of the current study have shown that HSO and CAO can cause a pale-yellow yolk colour, which is not desired by customers, and this fact should be considered when selecting seed-based oils for laying hen diets.
The importance of the FA profile of egg yolk has increased with evidence linking the FA profile of the diet to cardiovascular disease in humans. Since it has been understood that the issue of dietary oil is important in designing a PUFA and Ʃn-3 enriched egg, studies have been presented that used lipids high in UFA or PUFA such as vegetable, nut, and fish oils in the diet of chickens. (Alagawany et al. 2019). The results of our study showed that dietary seed oils altered the DHA, ƩPUFA, and Ʃn-3 content of egg yolk (P < 0.05), and this change was directly related to the FA composition of the oil in the diet. Previous studies have also shown that SO, CO and SFO improved the n-6 content of egg yolk (Güçlü, Uyanık and İşcan 2008, Kang et al. 2006, Oliveira et al. 2010). CAO, which has the highest oleic acid content, increased the ƩMUFA content but decreased the ƩPUFA content of egg yolk. Similarly, CAO was reported to enrich the oleic acid content of the yolk instead of SO in the hens' feed (Gül, Yörük, Aksu, Kaya and Kaynar 2012). In addition, feeding laying hens with diets high in MUFA has been found to increase the oleic acid content in egg yolks (Milinsk et al. 2003). Oleic acid is not essential for humans and to produce ƩPUFA and Ʃn-3 design eggs CAO may not be the best choice. On the other hand, Ʃn-9 has been defined as a beneficial fatty acid for human health (Galán-Arriero et al. 2017) and CAO can be used in the diet of laying hens to improve the MUFA content in the egg yolk. The enrichment effect of dietary HSO on α-linolenic acid, DHA, and PUFA content in egg yolk was impressive. HSO caused the highest content of linolenic acid in the egg yolk. In humans, linolenic acid can be converted to a limited extent to EPA and DHA (Fraeye et al. 2012) and many studies have shown that higher intakes of omega-3 PUFAs, especially EPA and DHA, are linked to lower chronic diseases incidence (Djuricic and Calder 2021). Our previous study also confirmed that HSO in the diet significantly increases ƩPUFA and Ʃn-3 content in quail egg yolk compared to SBO and SFO (Göçmen et al. 2021). In another study, HSO in the diet was reported to significantly increase n-3 PUFA content in egg yolk (Goldberg et al. 2012). A recent meta-analysis confirmed that table eggs from chickens fed whole hemp seeds or their by-products containing HSO in varying amounts had higher levels of long-chain fatty acids ALA (α-linolenic acid), DHA and n-3 (Fabro et al. 2021). We concluded that it is possible to use HSO at 3.7% in the diet of laying hens without differences or negative effects on performance and egg quality parameters. SBO, SFO, and CO helped to increase the ƩPUFA and Ʃn-6 content of the yolk fat and improve the pigmentation of the yolk. Dietary CAO may contribute to the ƩMUFA, and Ʃn-9 content of the egg but may reduce the redness of the yolk. HSO is promising for the design of Ʃn-3-rich eggs, but changes in yolk colour should be considered when using HSO in the diet. On the other hand, HSO is a newer lipid source for animal nutrition, and further research on its use in laying poultry nutrition is needed.