Fatty acid composition in adipose tissue of pasture- and feedlot-finished beef steers1

Twenty-seven Angus crossbred steers were used to evaluate the effects of finishing beef cattle on pasture without or with N fertilization of the pasture versus feedlot finishing beef steers on fatty acid composition in subcutaneous adipose tissue. A completely randomized design with repeated measures was used to arrange steers into 3 treatments: grazing on tall fescue (TF) without N fertilizer (TF−NF), grazing on TF with N fertilizer (TF+NF), and feeding TMR on feedlot (FLT). For the pasture treatments, it was hypothesized that N fertilization would affect fatty acid composition of adipose tissue in pasture-fed beef steers because of its potential effects on nutrient and energy utilization. Three replicated pastures or group pens with 3 steers per replicate were assigned into each treatment. A total of 168 kg of N fertilizer per hectare was applied in 3 split applications at 56 kg/ha to the TF+NF. From May through September 2010 (total of 16 wk), pasture-finished steers were grazed on replicated 0.47-ha paddocks, while steers on the FLT were fed a finishing diet containing 76% barley grain. Subcutaneous adipose tissue biopsies were obtained on wk 4, 12, and 16. Total fat percentage in TF pasture did not differ due to N fertilization, and similar total fat concentrations were also measured between TF pasture and the FLT on wk 12 and 16. Nitrogen fertilization increased PUFA proportion (mg/100 mg of total fatty acid) in TF grass on wk 4 and 12, including C18:3 n-3 (P < 0.01). Applying N fertilizer to TF increased C18:0 on wk 12 and 16 and cis-9, trans-11 CLA proportion on wk 16 in adipose tissue of steers (P < 0.02). Pasture-finished steers had greater proportions of C18:0, cis-9, trans-11 CLA, and C18:3 n-3 throughout grazing compared with those on the FLT treatment (P < 0.01). However, C18:1 cis-9 and C18:2 n-6 proportions in adipose tissue of pasture-finishing steers decreased compared with FLT steers throughout study (P < 0.02). Increased SFA and decreased MUFA and n-6:n-3 ratio were observed in pasture-finished steers on wk 4, 12, and 16 compared with FLT steers (P < 0.01). Grazing on TF pasture increased cis-9, trans-11 CLA, C18:3 n-3, and SFA proportions in adipose tissue; however, TF pasture-finished steers had less backfat, rib fat, and rib-eye area compared with feedlot-finished steers (P < 0.04).