Effect of drying method on accuracy of n-alkane estimation of forage intake

Abstract

N-alkane marker techniques are commonly used in the measurement for forage intake in grazing animals. Traditionally sample preparation of feed for subsequent n-alkane analysis has been conducted on freeze-dried samples, however oven-drying is both less time consuming and less expensive. Ten mixed age rumen fistulated wethers were housed in individual metabolism cr ates in the Animal Physiology Unit of Massey University for 30 days. The wethers were randomly allocated to one of four treatments fed either 0.5, 1.0, 1.5 or 2.0x maintenance requirement based on individual liveweight, with 2, 3, 3 and 2 animals in each respectively. Fresh ryegrass/white clover pasture was offered twice-daily. Following a 10 day adaptation period, each animal had an n-alkane capsule inserted via rumen fistulae from d0-d20. Feed and faeces was sub-sampled for subsequent analysis of C31 , C32 , C33 , C35 and C36 alkane content on alternate days from d5-20. Capsules were removed temporarily on alternate days from d0, for measurement of the plunger travel, as an indirect measure of capsule release rate. All faecal samples were oven-dried at 60°C until a constant dry weight was achieved. Herbage sub-samples collected on d5, 9, 13 and 17 were stored at -5°C before freeze-drying, while subsamples collected on d7, 11, 15 and 19 were oven dried at 60°C for 48 hours. The capsules achieved a constant release rate within four days post-insertion and remained at a steady rate up to day 20 postinsertion. No difference was found between estimated intakes using either C31:C32 or C32:C33 alkane ratios. Day had no significant effect on either actual or estimated intakes. Oven-drying the feed was found to produce a weak linear relationship whereas freezedrying the feed samples produced a much stronger relationship when compared to in vivo intake. A significant difference (P