Fat supplements differing in physical form improve performance but divergently influence methane emissions of grazing beef cattle

Abstract The objective of this experiment was to determine how physical form of fat supplements affect forage intake, ADG, and enteric CH 4 emissions from steers grazing tallgrass-prairie in late summer. Steers (n = 20; BW = 269 ± 35 kg) were acclimated to a portable, automated head-chamber system ( AHCS ; GreenFeed; C-Lock Inc., Rapid City, SD), and were randomly assigned to one of four treatments, either no supplement ( CON ), whole cottonseed ( WCS ), a supplement with bypass fat ( BYP ; Megalac, Arm and Hammer Animal Nutrition, Princeton, NJ), or a supplement containing soybean oil ( SBO ). Control steers gained less ( P P  =  0.13) performance to WCS and SBO, but WCS had significantly lower ( P  =  0.05) ADG than SBO. Supplemented steers had greater DMI than the CON ( P  =  0.04) due to the additional supplement, which did not reduce forage intake ( P  =  0.08). The WCS treatment reduced ADF digestibility ( P  =  0.02) compared to the other treatments, but aNDF digestibility was not affected by supplementation ( P  =  0.11). The supplements containing rumen available fat had similar ( P  =  0.88) CH 4 emissions and were lower than BYP and CON ( P P  =  0.75) CH 4 emissions. Supplements reduced emission intensity (g CH 4 /kg ADG) compared to the CON ( P  ≤  0.03). However, SBO had lower ( P  =  0.05) emission intensity than WCS. Across all treatments, increases in forage intake and ADF digestibility increased CH 4 emissions ( P P  =  0.03). Both rumen available fat supplements reduced daily CH 4 emissions ( P P = 0.05) ADG compared to SBO. Hence, SBO had a lower ( P  =  0.05) CH 4 emission intensity than WCS. WCS reduced ADF digestibility compared to the other treatments and greater ADF digestibility increased CH 4 emissions, therefore it is safe to say that this is the mechanism for CH 4 reductions by WCS. The mechanism for SBO is less apparent but may be related to shifts in microbial populations. All supplements improved the sustainability of the tall-grass prairie system, albeit by different mechanisms.