Animal Nutrition & Veterinary Physiology

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).