Effect of supplementing high-fiber or high-starch concentrates or a 50:50 mix of both to late-lactation dairy cows fed cut herbage on methane production, milk yield, and ruminal fermentation

High-fiber and high-starch concentrates are used as supplements to meet the energy demand of lactating cows or to fill herbage shortage in pastoral systems. Additionally, concentrates can be a tool to mitigate enteric methane (CH₄) per unit of DMI (CH₄ yield) and per unit of milk production. This study aimed to compare the effects of supplementing herbage-fed late-lactation dairy cows with high-fiber (FIB), high-starch (STA), or a 50:50 mix of both concentrates (MIX) compared with no supplementation (CON) on CH₄ emission parameters and ruminal fermentation. Multiparous cows (Friesian × Jersey, average ± SD; 3.8 ± 0.8 yr old; 519 ± 50 kg BW, 211 ± 10 DIM; n = 32) were blocked by lactation number and then randomly allocated to receive (on a DM basis) the following: 0 kg/d of concentrate (CON), 5 kg/d of STA, 5 kg/d of FIB, or 5 kg/d of MIX (2.5 kg each of STA and FIB). After at least 14 d of diet adaptation, the CH₄ emissions, DMI, and milk production were measured in respiration chambers for 48 h, ruminal fluid was collected after chamber measurements, and short-chain fatty acids (SCFA) were analyzed. Daily milk production was recorded, and milk samples were analyzed for milk composition. The CH₄ production (g/d) trended to be greater in concentrate-fed cows than CON cows. The CH₄ yield was similar across the dietary treatments. The CH₄ per kilogram of TS (calculated as fat plus protein production) was 22% less and trended to be up to 24% less per kilogram of fat- and protein-corrected milk (FPCM) in concentrate-fed cows than CON cows mainly due to an increase in FPCM production. After morning and afternoon feeding, the time to reach peak CH₄ emissions was less in concentrate-fed cows than CON cows. The variation in the CH₄ emission rate after morning feeding was greater in concentrate-supplemented cows than CON cows. The ruminal SCFA profile was similar among treatments. Regardless of the dietary treatment, the acetate plus butyrate to propionate plus valerate ratio was positively associated with CH₄ yield, FPCM, and peak CH₄ emission rate and negatively associated with the CH₄ decline rate after peak emissions. High-starch, high-fiber, or mixed (50:50) concentrates trended to reduce the CH₄ per unit of FPCM by up to 24% in lactating cows eating fresh herbage as a basal diet.