The objectives were to evaluate the effects of dietary fish oil on plasma metabolite, hepatic fatty acid composition, and total triacylglycerol concentrations. Multiparous Holstein cows (n = 42) were completely randomized to 1 of 3 treatments at 3 wk prepartum. Treatments were no supplemental lipid or supplemental lipid from either Energy Booster (Milk Specialties Co., Dundee, IL) or fish oil. Treatment diets were fed from -21 d relative to expected date of parturition until 10 d postpartum. Treatments were fed as a bolus before the a.m. feeding. The dose of lipid fed during the prepartum period was 250 g, whereas approximately 0.92% of the previous day's dry matter intake was supplemented postpartum. Blood was collected 3 times weekly for determination of plasma metabolites. Liver biopsies were performed at 21 and 10 d before expected date of parturition and 1 and 14 d after parturition to determine fatty acid compositions and total triacylglycerol concentrations. Dry matter intake, milk yield, and loss of body weight or body condition score were not affected by supplementing the diet with lipid or by the source of lipid. Supplemental lipid tended to increase plasma glucose and decrease nonesterified fatty acids during the postpartum period. Furthermore, plasma beta-hydroxybutyrate was reduced during the postpartum period in the lipid-supplemented treatments. However, source of supplemental lipid had no influence on any blood metabolite. Supplemental fish oil altered the fatty acid composition of liver phospholipids and triacylglycerols, decreasing total saturated fatty acids and increasing total n-3 and long-chain polyunsaturated fatty acids (>20 carbon fatty acids). Despite the altered fatty acid composition, hepatic total triacylglycerol concentrations were unaffected by supplemental fish oil. Furthermore, the improved metabolic profile following lipid supplementation did not decrease hepatic total triacylglycerol concentrations.

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BackgroundFeed intake plays an important economic role in beef cattle, and is related with feed efficiency, weight gain and carcass traits. However, the phenotypes collected for dry matter intake and feed efficiency are scarce when compared with other measures such as weight gain and carcass traits. The use of genomic information can improve the power of inference of studies on these measures, identifying genomic regions that affect these phenotypes. This work performed the genome-wide association study (GWAS) for dry matter intake (DMI) and residual feed intake (RFI) of 720 Nellore cattle (Bos taurus indicus).ResultsIn general, no genomic region extremely associated with both phenotypic traits was observed, as expected for the variables that have their regulation controlled by many genes. Three SNPs surpassed the threshold for the Bonferroni multiple test for DMI and two SNPs for RFI. These markers are located on chromosomes 4, 8, 14 and 21 in regions near genes regulating appetite and ion transport and close to important QTL as previously reported to RFI and DMI, thus corroborating the literature that points these two processes as important in the physiological regulation of intake and feed efficiency.ConclusionsThis study showed the first GWAS of DMI to identify genomic regions associated with feed intake and efficiency in Nellore cattle. Some genes and QTLs previously described for DMI and RFI, in other subspecies (Bos taurus taurus), that influences these phenotypes are confirmed in this study.

a b s t r a c tThe aim was to evaluate growth, carcass traits, feed efficiency reranking, body composition, calpain system activity and meat quality in Nellore steers that were phenotypically ranked for high and low residual feed intake (RFI). Seventy-two Nellore steers (16-21 month-old, 334719 kg BW) had free access to a feedlot diet for 70 d (feeding period 1, P1). Daily dry matter intake (DMI), body weight gain (ADG) and ultrasound carcass traits were measured individually. The 12 lowest and the 12 highest RFI steers were classed as low-and high-RFI groups and were fed for a second feeding period (feeding period 2, P2). Spearman's rank correlation was performed for RFI and gain-to-feed ratio (G:F) measured over P1 and P2. The carcass traits, meat quality and calpain system activity were evaluated at slaughter, and body composition was determined. In P1, low-RFI steers had greater G:F (0.159 vs. 0.134; Po0.001), lower DMI (9.30 vs. 11.1 kg/d; Po0.0001), lower RFI (À 0.80 vs. 0.85 kg/d; Po0.0001), and tended to have lower rates of rump fat gain (4.48 vs. 6.05 mm; P¼ 0.06), but no differences were observed for ADG, BW, and other traits measured by ultrasound (P40.05). In P2, no differences between RFI classes were observed for G:F and DMI as a percentage of BW (P40.05), and small differences were observed for DMI (8.25 vs. 8.99 kg/d, Po0.05) and RFI (À 0.28 vs. þ0.29 kg/d, P¼0.06). The ranking correlations for RFI and G:F measured consecutively in P1 and P2 were low to moderate (r¼ 0.11-0.40). Low-RFI steers had lower requirements of metabolizable energy (ME) for maintenance (131 vs. 160 Mcal/kg EBW 0.75 d À 1 , Po0.05), but no differences were observed for fat and protein gain, retained energy and efficiency of ME use for gain (P40.05). No differences between low-and high-RFI steers were observed for carcass traits at slaughter; however, low-RFI steers had 8.1 kg less gastrointestinal fat than high-RFI steers. There were no RFI effects on meat shear force and the activities of m-calpain, m-calpain and calpastatin (P40.05). High-RFI steers had greater MFI in 1 d aged LM (53.9 vs. 40.8, Po0.05). The variation in feed efficiency between high-and low-RFI Nellore steers is related to differences in energy requirements and deposition of fat on internal organs. The ranking for feed efficiency is altered as cattle become older. The selection for improved RFI in Nellore cattle may reduce feed requirements for beef production without affecting meat tenderness and enzymatic activity of the calpain system.