Interpretive Summary: Effect of humic substances on rumen fermentation, nutrient digestibility, methane emissions, and rumen microbiota in beef heifers.
By: Jackie Walling
With consumer push-back regarding the use of antibiotics and hormones for increasing the growth of cattle, a study released in the September 2018 Issue of the Journal of Animal Science explores the use of humic substances (HS) as an alternative feed additive. Possible ways to reduce methane production in cows were also considered. Differing concentrations of HS were administered in cattle diets to identify how rumen fermentation, nutrient digestibility, methane production, and rumen microbial population were affected.
HS is an organic compound or a “mixture of acids which arise from the natural decomposition of plant and animal material by soil microorganisms” with a high affinity for nitrogen (N). They possess antimicrobial, absorptive, and detoxifying properties stimulating microbial growth in soil. It has been hypothesized HS may serve as a “natural” antibiotic stimulating microbial growth in the rumen of cattle.
Eight Angus/Hereford crossbred heifers were cannulated in the rumen and divided by weight into a 4x4 Latin Square design consisting of four 28-day periods and four dietary treatments. They underwent 14 days of adaptation being fed a carefully constructed diet once a day. HS, consisting of 50.7% humic acid and 4.4% fulvic acid, was added to the diet in treatments of either 0, 100, 200, or 300 mgHS/kg live BW. At 15 days, rumen fermentation was evaluated by collecting samples from four sites inside the rumen at hours 0, 3, 6, and 12 post feeding. Samples were compounded to determine microbial profiling, pH, volatile fatty acids, ammonia, and enumeration of protozoa. In four 24-hour periods between days 18 and 22, total urinary and fecal collection established apparent total tract digestibility. Methane measurements were taken in open circuit respiratory chambers on day 26. Details regarding chemical and statistical measurements can be found in the article.
Results showed dry matter intake remained similar across all treatments with the addition of HS. Volatile fatty acids, ruminal pH, and apparent total tract digestibility of DM were not affected. Also unaffected were purine derivatives, energy balance, BW, and methane production. As HS increased, ammonia-N concentration, total protozoa count, and starch digestibility showed a quadratic relationship. Digestibility of crude protein linearly increased. Total N retention improved and a linear decrease was seen in total N excretion. Illumina sequencing evaluated HS on the rumen microbiome revealing no differences among treatment groups. Rumen samples displayed 18 bacterial phyla and 64 bacterial families with a decrease of Proteobacteria and an increase of Synergistetes, Euryarchaeota, and methanobacteriaceae.
Past in vitro studies indicate HS decreasing ammonia-N concentration and protozoal counts. The results in vivo from this study differed showing a quadratic relationship of HS to those two parameters. Thought to inhibit protozoal effects, HS first increased protozoal numbers before showing an overall decrease when hitting the highest amount of HS. Even with this decrease, other rumen metabolism factors influenced by protozoa remained unaffected suggesting HS had little effects. Firmicutes (Gram-positive bacteria), however, were found in abundance leading to increased crude protein digestibility. Methane production remained unchanged as the average result of HS decreasing methane utilizing bacteria and increasing methane producing bacteria. Thanks to the high affinity HS carries for N, N retention increased which decreased N found in fecal excretions. HS may be a “natural” antibiotic prospect, but additional in vivo studies need further investigation.
For the full article, visit the Journal of Animal Science.