Performance, rumen fermentation, and enteric emissions measurement methodologies in commercial feedlot cattle fed ionophore-containing diets

Abstract

Ionophores are widely used in beef cattle diets to improve feed efficiency through alterations in ruminal fermentation. Most evaluations of ionophore responses and enteric methane (CH₄) emissions have been conducted under controlled research conditions rather than commercial feedlot environments, creating a need for additional evidence from large-pen production settings as well as methodological approaches capable of accurately characterizing ruminal fermentation and CH₄ production under commercial conditions. The objectives of this thesis were to evaluate (1) the effects of rotational feeding of monensin and laidlomycin propionate on cattle performance and ruminal fermentation and (2) methodological approaches to estimate enteric CH₄ production under commercial feedlot conditions. Experiment 1 evaluated growth performance of 584 crossbred beef heifers (initial BW = 290 ± 26 kg) assigned to receive either consistent dietary inclusion of monensin (MON; 200 mg/heifer/d) or rotational ionophore exposure (ROT), alternating every 28 d between dietary inclusion of monensin (200 mg/heifer/d) and laidlomycin propionate (75 mg/heifer/d) during a 112-d growing period. Heifers were limit-fed a common diet containing 1.26 Mcal/kg NEg and 14.3% crude protein (DM basis). Across the feeding period, ROT had increased average daily gain (1.54 vs. 1.41 kg/d; P ≤ 0.01) and improved gain efficiency (G:F; 0.171 vs. 0.162; P ≤ 0.01) compared to MON, while dry matter intake did not differ (P > 0.10) by design. Experiment 2 evaluated ruminal fermentation responses using ex vivo batch culture fermentation with rumen inoculum collected from ruminally cannulated steers exposed to either consistent dietary inclusion of monensin (200 mg/steer/d) or a rotational program of dietary ionophore inclusion of 56 d monensin, 28 d laidlomycin propionate (75 mg/steer/d), and a final 28 d monensin. Following the 28-d laidlomycin propionate feeding period (d 84), ROT tended to produce greater CH₄ during ex vivo fermentation (19.47 vs. 15.30 mL; P = 0.06) and exhibited a lower carbon dioxide (CO₂) to CH4 ratio (6.14 vs. 8.75; P = 0.03) compared to MON. To understand the reliability of the ex vivo batch culture techniques, an additional experiment was conducted where CH₄ production estimated from ex vivo batch culture fermentation was compared with in vivo measurements obtained using automated head chamber systems (AHCS; GreenFeed; C-Lock Inc., Rapid City, SD) in beef x dairy steers, including ruminally cannulated steers co-housed with commercial steers under commercial feedlot conditions. Methane estimates from ex vivo fermentation scaled using substrate intake (145.8 g CH₄/steer/d) or rumen fill (149.5 g CH₄/steer/d) were similar to pen-level AHCS estimates (139.2–143.9 g CH₄/steer/d; P > 0.10), whereas CH₄ values measured from ruminally cannulated steers using AHCS were substantially lower (39.3 g CH₄/steer/d; P < 0.05). Collectively, these findings demonstrate that rotational feeding of monensin and laidlomycin propionate can improve growth performance in growing cattle under commercial feedlot conditions and that batch culture fermentation-based approaches can generate CH₄ estimates comparable to pen-level in vivo measurements when appropriate biological scaling assumptions are applied.