Correspondence between in vitro and in vivo rumen methane production obtained with different starch sources and starch levels

Abstract Text:

To investigate the relationship between in vitro and in vivo methane (CH₄) production measured simultaneously, 16 rumen cannulated lactating dairy cows were assigned to 4 blocks with a 2 x 2 factorial arrangement of treatments. The treatments were based on concentrates formulated to contain starch varying in source (slowly fermentable (S) vs. rapidly fermentable (R); native vs. gelatinized corn grain) and level of starch (low (L) vs. high (H); 270 vs. 530 g/kg of concentrate DM). The grass silage to concentrate ratio of the total diet was 60:40 (DM basis). After 12 days of adaptation, the cows were housed in respiration chambers for 5 days to measure CH₄ production, replicated in 4 periods. In each period rumen fluid was obtained from each of 4 donor cows adapted to one of the 4 different diets for about 16 d. Gas production (GP) and CH₄ was measured (in duplicate per period) for each substrate from the same diet as fed to individual donor cow using automated GP system with CH₄ measured at distinct time points. Rumen fermentable organic matter (OM) in concentrates was determined by in situ technique and in grass silage estimated by NIRS analysis.

Table   1.   Chemical composition (g/kg DM) of total mixed diet
Diet	CP	NDF
SL-diet	156	441
SH-diet	157	385
RL-diet	156	440
RH-diet	163	378

In vitro CH₄ production (24 h) was lower with R than with S starch (42.9 vs. 49.5 mL/g of incubated OM; P=0.004), and higher with L than with H (49.8 vs. 42.6 mL/g of incubated OM; P=0.002). In vivo, an increased rate of fermentation, but not increased level of starch, resulted in a lower CH₄ production per unit rumen fermentable OM (55.6 vs. 61.1 L/kg of fermentable OM; P=0.007). Across the diets tested, in vitro CH₄ correlated well with in vivo CH₄ production expressed per unit fermented OM (R²=0.54; P=0.040), but not with in vivo CH₄ production expressed per unit ingested OM (R²=0.04; P=0.878). These results indicate the complexity of rumen fermentation conditions needs to be considered to predict in vivo CH₄ production from in vitro measurements. In conclusion, in vitro CH₄ production was only indicative of the trend of in vivo rumen CH₄ production from different combinations of source and level of starch when in vivo CH₄production was expressed per unit rumen fermented OM, but not when expressed per unit ingested OM.

Keywords: methane, in vitro, in vivo