Rumen Microbial Growth Rates and Yields: Effect of Amino Acids and Protein

Abstract

Effects of amino acids [aa] upon microbial growth, optimum ratio of nonprotein to aa N for microbial growth, and incorporation of aa into micorbial cells were determined with a washed cell suspension in vitro and rumen [cow] microbial cells. Rumen microbial dry matter, N, RNA, DNA and substrate disappearance was greatest when a mixture of 18 aa was substituted for urea. Substitutions of mixtures of 10 essential aa, 8 nonessential aa and S-containing aa and glutamate also stimulated microbial growth. Acid hydrolyzed casein markedly improved microbial growth. Branched aa addition did not affect growth. The optimum ratio of nonprotein to aa N for microbial growth was 75% urea N and 25% aa N. With this amount of aa, an average of 53% of added aa was incorporated into microbial cells, 14% was fermented to CO2 and volatile fatty acids, and 33% remained in the supernatant. Both 100% urea and 100% aa in growth media were unfavorable for maximal microbial growth. With all carbohydrate substrates, 100% urea N supported the growth of 9 mg bacterial dry matter/100 mg substrate. Substitutions of aa for urea increased yields to >20 mg/100 mg. Microbial growth yields in CO2 incubations were less than when flasks were flushed with N. Yield of bacterial dry matter per unit of substrate was less under N than under CO2. [This study has relevance to ruminant nutrition; maximum microbial growth rates yield maximum animal performance.].