Fibrobacter succinogenes (along with Ruminococcus albus and Ruminococcus flavefaciens) is one of the predominant cellulolytic bacterial species in the rumen responsible for digesting fibrous plant cell walls.

Eructation (belching) is the physiological reflex by which ruminants expel large quantities of fermentation gases (chiefly $\text{CO}_2$ and $\text{CH}_4$) from the reticulorumen through the esophagus and out the mouth.

Rumen protein degradation and microbial protein synthesis:

• Assertion (A) is true: Ruminants require an adequate dietary supply of Rumen Degradable Protein (RDP) to support the metabolic growth and cellulolytic activity of the rumen microbiome.
• Reason (R) is true: RDP is hydrolyzed by rumen microorganisms into peptides, amino acids, and ultimately ammonia ($\text{NH}_3$); rumen bacteria utilize this ammonia as their principal nitrogen source to synthesize high-quality microbial crude protein.
• Reason (R) directly explains why ruminants need dietary RDP.
• Therefore, both (A) and (R) are true and (R) is the correct explanation of (A).

Rumen fermentation and Volatile Fatty Acid (VFA) proportions:

• When ruminants consume a roughage-rich fibrous diet (e.g. hay or pasture), cellulolytic and fibrolytic bacteria (Ruminococcus albus, Ruminococcus flavefaciens, Fibrobacter succinogenes) predominate in the rumen.
• Fibrolytic fermentation shifts the pathway primarily toward acetate production, yielding an acetate:propionate ratio of $3:1\text{ to }4:1$ (acetate comprises $65\%\text{ to }70\%$ of total VFA).
• High-concentrate/grain diets, in contrast, favor amylolytic bacteria producing higher proportions of propionate.
  
  Option analysis:
  Acetic acid is the predominant volatile fatty acid on roughage/hay diets, making Option 4 the correct answer (Key 4).