فهرست مطالب سامان لشکری

Flaxseed is an oilseed that can be used as a rich source of high quality protein and fat for dairy cows (Neveu et al., 2014). Flaxseed contains high levels of linolenic acid, averaging 18% of the total seed weight and 53% of the total fatty acids. Recently, there has been a renewed interest in using flaxseed in animal rations as it can be used to alter the fatty acid composition of milk and meat products and; therefore, provides functional health benefits for the consumer (Petit, 2010). However, achieving an increase in the concentration of polyunsaturated fatty acids in dairy products is challenging because most unsaturated fatty acids are extensively biohydrogenated in the rumen.
Material and In the present study, the investigated flaxseed was unheated (raw) or in two different ways heat processed (rolled roasted and extruded). Flaxseeds were roasted in an industrial roaster with a flame and blower at 140 to 145 °C (turning speed of 2.5 circles per minute; tunnel diameter of 50 cm). The roasted flaxseed was then processed in a roller mill equipped with rollers. Extrusion of flaxseed was performed at 140 to 145 °C with a resident time of 43 s using a multipurpose twin-screw extrusion system. Twelve multiparous Holstein lactating cows at early lactation stage from the same group in the herd with an average body weight of approximately 640 ± 10 kg were housed individually in a tie-stall barn. The cows were assigned to one of the four experimental diets including 1) diet with no flaxseed (control), 2) unheated (5.5% DM), 3) rolled roasted (5.5% DM), and 4) extruded flaxseed (5.5% DM). All rations were formulated based on the NRC (2001) to contain approximately 18% CP and 1.6 Mcal net energy lactation (NEL) kg–1 on a DM basis. All diets were iso-energetic and iso-nitrogeneous. Cows were milked 3 times daily in a milking parlor at 05:00, 13:00 and 20:00. During the last 5 d of sampling periods, milk yield was recorded for all cows. Milk from individual cows was sampled at each milking in prelabeled 50 ml plastic vials. Milk samples were analyzed for fat and protein by Milk-O-Scan (Funke Gerber, LactoStar). Milk samples from individual cows were analyzed for FA composition. Milk fat was extracted and derivatized to methyl esters using Savage et al. (1997, 1999) method. For GC – MS analysis, an Agilent gas chromatography (2001, Palo Alto, CS, USA) with a 30 m to 0.25 mm HP-5MS capillary column coupled with an Agilent 5973 mass spectrometer (Agilent Technologies, Palo Alto, CA) was used. Analysis of variance was conducted using the MIXED procedure of SAS (2001) for all the statistical analyses in this study. Least square means were estimated using Restricted Maximum Likelihood method, and the model consisted of the fixed effect of treatment and random effects of cow.
The similar dry matter intake (DMI) between cows fed different experimental diets suggested that fatty acid content and profile of flaxseed had no impact on shortterm feed intake regulation. Similarly, Gonthier et al. (2004) reported that ground or extruded flaxseed can be fed up to 120 g/kg DM of diet without any adverse effects on DMI. In agreement with our results, it has been shown that low to moderate levels of ground or extruded flaxseed in the diets of dairy cows (up to 10% of the diet DM) did not affect the DMI (Gonthier et al., 2005; Petit, 2010). However, Neveu et al. (2014) reported that inclusion of flaxseed in dairy cows increased DMI. In the present study, inclusion of flaxseed in the diets did not affect the milk production. This is in agreement with Neveu et al. (2014) who reported that inclusion of flaxseed in the lactating cow diets did not affect the milk production. Maintained milk production was consistent with unaltered DMI. Reduced milk fat in cows fed diets containing extruded flaxseed might be due to an increase in rate of oil release from flaxseed into rumen fluid (Lashkari et al., 2015). Extrusion may increase the availability of polyunsaturated fatty acids in rumen (Lashkari et al., 2015) and, therefore, increase biohydrogenation of poly unsaturated fatty acids (Lashkari et al., 2017). Reduced milk fat in cows fed diet containing the extruded flaxseed could be due to an increase in dietary supply of poly unsaturated fatty acids which results in formation of trans-10 isomers (C18:1) in rumen and depresses fatty acids biosynthesis in the mammary gland (Piperova et al., 2000). Altered milk fat in our study suggests that ruminal biohydrogenation pathways and the mammary de novo fat biosynthesis or related genomics significantly affected in cows fed diets containing extruded flaxseed. In agreement with our findings, feeding extruded flaxseed at 14.8% of the diet decreased fat concentration from 4.1 to 3.5% in milk of dairy cows compared to a control diet (Martin et al., 2008). Concentrations of linolenic acid and conjugated linoleic acid increased as a result of flaxseed supplementation compared to control diet. However, linolenic acid concentration in cows fed extruded flaxseed was lower than cows fed raw and rolled roasted flaxseed. These results suggest extensive ruminal biohydrogenation of dietary linolenic acid in diets containing extruded flaxseed. In agreement with our findings, Gonthier et al. (2005) reported low transfer efficiency (i.e., 2%) of dietary linolenic acid to milk as a result of extruded flaxseed supplementation.
Results of the present study showed that rolled roasted flaxseed can be used as practical method to improve milk fatty acid profile without any adverse effect on performance of lactating cows.