The relationship betweenin vitrogas production,in vitromicrobial biomass yield and¹⁵N incorporation and its implications for the prediction of voluntary feed intake of roughages

Abstract

The relationship between in vilro gas prodaction, concomitant in v h apparent and true DM degradability has been examined in forty-two roughages. The partitioning of truly-degraded substrate between gas volume and microbial biomass yield and "N incorporation into cells was also investigated. The relevance of this partitioning for the regulation of DM intake (DMI) was examined for my-four roughages. The results can be summarized as follows. I n vitro gas production and in vifro apparent and true degradability are highly correlated (P<O.OOOl), r being 0.96 and 0.95 respectively. There is an inverse relationship between in viho gas production and microbial biomass yield (r -0.67, (P < 0.oOOl) and also ' % enrichment (P < 0.001) when the variables were related to a given unit of substrate truly degraded. Selecting roughages by in viho gas production may well be a selection against maximum microbial yield and a combination of in vitro gas volume measurements with a complementary determination of the substrate truly degraded is proposed, to calculate a partitioning factor (PF) reflecting the variation of short-chain fatty acid production per unit substrate degraded. PF is calculated as the ratio, substrate truly degraded : gas produced by it. PF was highly significant (P-zO.OOO1) in DMI prediction when included in stepwise multiple correlations together with in vitro gas volume variables reflecting the extent and rate of gas production; 11 % of the variation in DMI was accounted for by the PF. The total model, including extent and rate of gas production and the PF, accounted for 84% of the variation in DMI. Roughages producing proportionally less gas per unit substrate truly degraded had higher feed intakes.Gas production: Microbial activity: 15N incorporation: In v h degradability: Voluntary feed intakeThe association between rumen fermentation and gas production has long been known (Tappeiner, 1884 as cited by Marston, 1948). Quin (1943) examined the fermentation of glucose, lucerne (Medicago sutiva) hay and various types of straw by connecting a manometer device for the recording of gas production directly to the rumen cannulas of Merino sheep. Finding this technique too difficult to execute and of poor reproducibility, he switched to the measurement of in vitro gas production.

The influence of casein and urea as nitrogen sources on in vitro equine caecal fermentation

“…This can impose an inverse relationship upon the production of short-chain fatty acids and microbial biomass yield (Preston and Leng, 1987). It has been shown that this relationship also applies to in vitro gas production and microbial biomass yield when both variables are related to a unit of the substrate fermented (Blü mmel et al, 1997).…”

Relevance and Potential Use ofIn vitroGas Production Measurements to Evaluate Varying Ratios of Roughages and Protein Sources for Ruminants

“…1991; Blummel e t aZ., 1997). The difference between true degradability and apparent degradability was taken to represent microbial matter (Van Soest, 1994, Blummel et al, 1997, Ranilla et aZ., 2001. The PF and degradation efficiency factor (DEF) for the rations were calculated as follows:…”

“…Makkar (2004) remarked that in most in uitro studies, the rate and extent of gas production (GP) has been wrongly considered t o be equivalent to rate and extent of substrate (feed) degradation. Blummel et al( 1997) showed that GP and microbial yields per unit of substrate degraded are inversely related. The implication is that selecting diets on the basis that higher GP is synonymous with higher quality might result in selecting against maximal microbial yield, which would be inappropriate (Makkar, 2004).…”

“…Consequently, the importance of including other concomitant measurements alongside the GP has been advocated (Blummel et al, 1997). The concomitant measurements commonly considered include degradability (Deg) and microbial yield (MIC).…”

“…The concomitant measurements commonly considered include degradability (Deg) and microbial yield (MIC). Blummel et al (1997) calculated Deg:GP ratio, which they termed the 'partitioning factor' (PF) and found that feeds with high PF stimulated higher MIC.…”