Fatty acid composition of intra-muscular triacylglycerols of steers fed autumn grass and concentrates

The objective of this study was to determine the impact of increasing the rate of carcass growth of steers grazing autumn grass by either supplementing with concentrates or by increasing grass supply on fatty acid composition of triacylglycerols within the M. longissmus thoracis. One hundred and ten steers (567 kg) were assigned to ten dietary treatments in a 3 (daily grass allowances: 6, 12 and 18 kg dry matter (DM)/animal) by 3 (daily concentrate allowances: 0, 2.5 and 5 kg/animal) factorial experiment with a control group offered concentrates ad libitum. Animals were slaughtered after 100 days and lipid extracted from samples of the M. longissmus thoracis for fatty acid analysis. There was an interaction (P < 0.001) between grass and concentrate allowance for the proportions of major fatty acid classes which reflected the patterns observed for individual fatty acids. Thus, supplementary concentrates had little effect on total fatty acids when offered with the high grass allowance, but high concentrate supplementation of the low grass allowance decreased the saturated fatty acid (SFA) and increased the monounsaturated fatty acid (MUFA) proportions. The high increment of concentrates at the medium grass allowance decreased the SFA and increased the MUFA proportions compared to the low increment of concentrates. In the absence of concentrates the high grass allowance decreased the proportion of SFA (P < 0.001) and increased MUFA proportion (P < 0.001). Intra-muscular fat from the positive control group had lower (P < 0.05) SFA and higher (P < 0.05) MUFA proportions than that of animals offered the low grass allowance alone or with 2.5 kg concentrates or the medium grass allowance alone or with 5 kg concentrates. There was an interaction between grass and concentrate allowance for polyunsaturated fatty acid (PUFA): SFA ratio whereby this ratio was higher (P < 0.05) for the first concentrate increment on the medium grass allowance than this grass allowance either unsupplemented or supplemented with the high concentrate allowance. There was no effect of grass or concentrate allowance on the n-3: n-6 PUFA ratio but this ratio tended (P < 0.1) to be higher for all grass-based treatments compared to the control group. It is concluded that similar carcass growth rate can be achieved from an allowance of 18 kg autumn grass DM/day and 5 kg concentrates or from ad libitum concentrates and that muscle from the former ration has a lower fat concentration and a more favorable n-3 and n-6 PUFA profile, from a human health perspective. (C) 2002 Elsevier Science B.V. All rights reserved.