Effect of incremental amounts of Asparagopsis taxiformis on ruminal fermentation and methane production in continuous culture with orchardgrass herbage

Abstract

Enteric methane (CH4) emissions from ruminants are a significant contributor to total greenhouse gas emissions from the livestock sector. Previous research has demonstrated that the red seaweed Asparagopsis taxiformis strongly inhibits ruminal methanogenesis, but its effects on CH4 production, nutrient digestibility, and ruminal fermentation when added to a cool-season herbage diet have not been assessed. A 4-unit, single-flow continuous culture fermentor system fed orchardgrass (Dactylis glomerata L.) herbage was used to assess the effects of incremental addition of dietary A. taxiformis (0, 5, 10, or 15 g/kg DM) on nutrient digestibility, volatile fatty acid (VFA) concentration, pH, CH4 production, and N metabolism. Treatments were randomly assigned to fermentors in a randomized block design with 7 d of treatment adaptation and 3 d of sample collection. Fermentors were fed a total of 76 g of DM per day, split equally into 4 feedings (07:30, 10:30, 14:00, and 19:00 h). In the last 3 d of each experimental period, daily samples of total effluent were taken for analyses of ammonia N and VFA. One liter of effluent was collected daily (3 L total) for a composite sample that was blended and lyophilized for analysis of DM, ash, neutral and acid detergent fiber, crude protein, and total purines. Headspace CH4 concentration in each fermentor was measured every 15 min (192 readings/d) using a Fourier transform infrared gas analyzer, and pH was recorded every 2 min. Data were analyzed using the GLM procedure of SAS with orthogonal polynomial contrasts. Methane production in fermentors receiving all 3 levels of A. taxiformis was reduced by 99.9% within the first 24 h of each experimental period and remained predominantly below the limit of detection for the remaining 9 d. Apparent digestibility of neutral and acid detergent fiber, as well as apparent and true DM digestibilities, linearly decreased with A. taxiformis inclusion. Total VFA concentration decreased quadratically, and molar proportion of acetate decreased in a cubic manner. Overall, A. taxiformis acutely inhibited CH4 production in continuous cultures fed orchardgrass herbage, but also negatively affected fiber digestibility and total VFA concentration, especially at higher feeding rates. In vitro results indicated that A. taxiformis could be effective in reducing enteric CH4 production in grazing ruminants, but in vivo research is needed to determine how A. taxiformis may impact ruminal fermentation and milk production or average daily gain in grazing systems.