Abstract
The aim of this study was to analyze the effect of fat and protein supplementation to dairy cattle rations on milk fat triacylglycerol (TAG) composition, fatty acid (FA) positional distribution in the TAG structure, and milk solid fat content (SFC). Fifty-six lactating Holstein-Friesian cows were blocked into 14 groups of 4 cows and randomly assigned 1 of 4 dietary treatments fed for 28 d: (1) low protein, low fat, (2) high protein, low fat, (3) low protein, high fat, and (4) high protein, high fat. The high protein and high fat diets were obtained by isoenergetically supplementing the basal ration (low protein, low fat) with rumen-protected soybean meal and rumen-protected rapeseed meal, and hydrogenated palm FA (mainly C16:0 and C18:0), respectively. Fat supplementation modified milk TAG composition more extensively compared with protein supplementation. Fat supplementation resulted in decreased concentrations of the low molecular weight TAG carbon number (CN) 26 to CN34 and medium molecular weight TAG CN40, CN44, and CN46, and increased concentrations of CN38 and the high molecular weight TAG CN50 and CN52. Increased contents of C16:0, C18:0, and C18:1cis-9 in TAG in response to fat supplementation were related to increases in the relative concentrations of C16:0 and C18:0 at the sn-2 position and C18:0 and C18:1cis-9 at the sn-1(3) positions of the TAG structure. Increased concentrations of high molecular weight TAG species CN50 and CN52 in response to fat supplementation was associated with increased milk SFC at 20, 25, and 30°C. Our study shows that important alterations in milk TAG composition and structure occur when feeding hydrogenated palm FA to lactating dairy cattle, and that these alterations result in an increased SFC of milk fat. These changes in milk SFC and TAG composition and structure may improve absorption of both fat and minerals in milk-based products for infants and may affect processing of milk fat.
Highlights
Bovine milk fat is mainly composed of triacylglycerols (TAG; 98%), which are formed by the esterification of 3 fatty acids (FA) to a glycerol backbone
Our results show that milk TAG composition as analyzed by GC-FID was affected independently by supplementation with protein and fat, with a protein × fat interaction affecting only TAG CN42 (P = 0.049; Table 1), which increased in response to protein supplementation but only in the absence of fat supplementation
The relative concentrations of 11 TAG groups were affected by fat supplementation, whereas only 4 TAG groups were affected by protein supplementation
Summary
Bovine milk fat is mainly composed of triacylglycerols (TAG; 98%), which are formed by the esterification of 3 fatty acids (FA) to a glycerol backbone. The supplementation of saturated long-chain FA (SLCFA) into dairy cattle rations is a nutritional strategy commonly used to increase the energy density of the diet in support of milk production (Ashes et al, 1997; Nichols et al, 2018). In contrast to UFA that are susceptible to biohydrogenation in the rumen (Baumgard et al, 2000), SLCFA are largely rumen-inert, and commonly do not result in negative effects on fiber digestibility or milk fat depression at supplementation levels up to 3% of dietary DM (Ashes et al, 1997; NRC, 2001; Nichols et al, 2018). Previous studies have reported that oil supplementation to dairy cattle rations (e.g., palm oil, canola oil, olive oil, soy oil, and linseed oil) decreased the milk concentration
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