Abstract By 2050, the US beef industry must produce an extra 40 million tons of beef to satisfy the global demand, while reducing methane (CH4) emissions. This surplus of cattle will be provided mostly by cow-calf operations, which rely primarily on grazing. Those pastures are often infested with weeds such as pigweed (Amaranthus spinosus), one of the most invasive in the US, known for its increased polyphenols concentration. Despite the known effect of polyphenols on CH4 reduction, few studies have explored pigweed as a feed additive. Our objective was to evaluate the effect of in vitro addition of pigweed on ruminal fermentation, digestibility, and CH4 production. We collected ruminal fluid from three American Aberdeen steers (374 ± 21 kg) receiving a backgrounding diet (80:20% corn silage:cotton gin trash, as fed). We incubated (24 h, 39°C) diet (0.7 g) plus the inclusion of 0, 2.5, 5, or 10% (of diet DM) of roots, stems, leaves, seeds, or the whole pigweed plant (WHO). Volatile fatty acids (VFA), CH4 (mmol g-1 organic matter), ammonia, pH, gas, in vitro dry matter, and organic matter digestibility (IVDMD and IVOMD) were determined. Using the MIXED procedure of SAS, data were analyzed as a randomized complete block design with a 5 × 4 factorial arrangement of treatments with plant part (n = 5) and inclusion level (n = 4) as fixed main factors. Incubation and bottle within incubation were random effects. Polynomial orthogonal contrasts were used to determine linear, quadratic, and cubic effects of inclusion level. Plant × level interactions were detected in acetate, propionate, and butyric molar proportion, total VFA, CH4 production, and IVDMD (P < 0.01). Increasing levels of stems increased acetate concentration (P = 0.03), while those from WHO led to a linear reduction of acetate (P < 0.01). The inclusion of 2.5% of leaves, seeds, and WHO increased propionate concentration (P = 0.05), but greater levels decreased it (P < 0.01). Increasing levels of all parts decreased acetate:propionate proportion (P < 0.001). Increasing levels of WHO resulted in a linear reduction of CH4 production (P < 0.05), while increasing inclusion of roots and stems increased CH4 production (P < 0.01). Quadratic reductions in CH4 production were observed after increasing the addition of leaves, seeds, and WHO (P < 0.001). Increasing levels of leaves and roots decreased ammonia concentration (P < 0.01) while stems increased it (P = 0.01). Except for stems and roots, there was a linear decrease in IVDMD with increasing levels of all parts (P < 0.001). The inclusion of low levels of seeds, leaves, and WHO promoted ruminal changes leading to a reduction of CH4 production without significantly compromising the digestibility.
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