A study was conducted to investigate the impact of several lipid sources that supplied mainly long-chain fatty acids (FA), for their potential to reduce methane emissions from growing cattle. Sixteen Angus heifers (initial weight, 325 ± 41 kg) were used in the experiment, which was designed as a crossover with two groups, four 21-d periods, and four dietary treatments: control (no added lipid source), tallow, sunflower oil, and whole sunflower seeds. Lipid sources were added to supply 34 g fat kg-1 of dietary dry matter (DM), bringing the total dietary fat content to about 59 g kg-1 of DM. Adding tallow increased the dietary proportion of saturated FA (47 g 100 g -1 of FA), whereas sunflower oil and seeds decreased the proportion (21 g 100 g-1 of FA). The basal diets consisted of mainly whole-crop barley silage (650 g kg-1 of DM). Compared with the control, ad libitum intake was reduced (P < 0.001) with sunflower seeds, but not with tallow (P = 0.13) or sunflower oil (P = 0.53). About 14% less methane was emitted per animal when diets contained tallow or sunflower oil and 33% less methane was emitted when diets contained sunflower seeds (P < 0.001), compared with the control diet (177.4 g d-1). Relative differences in methane emissions among lipid sources were maintained after correction for intake of DM or gross energy. The methane reduction caused by tallow and sunflower seeds was partly due to decreased diet digestibility. Digestibility of neutral detergent fiber in the total tract decreased (P < 0.05) by 15% with tallow and by 20% with sunflower seeds compared with the control, with only a numerical reduction from control for sunflower oil (12%; P = 0.11). Consequently, digestible energy intake was about 4% higher (P < 0.001) for sunflower oil, but 3% lower (P = 0.02) with tallow and 12% lower (P < 0.001) with sunflower seeds, compared with the control. All lipid sources reduced methane emissions by an average of 17% when corrected for digestible energy intake (from 11.22 to 9.34 g methane Mcal-1; P = 0.01). We concluded that adding about 3% lipid to high-forage diets in the form of saturated or unsaturated long-chain FA decreases methane emissions, and could have substantial effects on methane inventories if implemented commercially. All three lipid sources suppressed methane production, but sunflower oil has good potential for on-farm adoption because it had minimal effects of fiber digestibility, increased the intake of digestible energy and the rate of gain of cattle, and lowered methane production. Although tallow and sunflower seeds are usually cheaper sources of lipid than sunflower oil, their cost effectiveness as methane abatement strategies would also need to account for their potentially negative effects on digestible energy intake and performance of cattle fed high-forage diets. key words: Beef cattle, diet, fat, greenhouse gases, lipid; methane, oil
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