Effect of Two Selected Levels of Padina gymnospora Biowaste and Enteric Methane Emission, Nutrient Digestibility, and Rumen Metagenome in Growing Sheep

A study was conducted on growing sheep to investigate the effect of two selected levels of biowaste of <i>Padina gymnospora</i> on feed intake, digestibility, daily enteric methane (CH₄) emission, growth performance, and rumen metagenome. We randomly divided the 18 growing male sheep into three groups of six animals each. The animals were fed on a basal diet comprising finger millet straw (<i>Eleusine coracana</i>) and a concentrate mixture in a 35:65 ratio. The sheep in the control group (C) were offered a concentrate mixture without waste, whereas the wheat bran in the concentrate mixture in test group I (A₂) and test group II (A₅) was replaced (<i>w</i>/<i>w</i>) with the biowaste of <i>Padina gymnospora</i> at a level of 3.07 and 7.69%, respectively. The biowaste of <i>Padina gymnospora</i> at the above levels in concentrate constituted 2 and 5% of the diet. A significant decrease of 28.4% in daily enteric CH₄ emission (g/d) was reported in the A₅ group, whereas the difference in daily enteric CH₄ emission between the C and A₂ & A₂ and A₅ groups did not prove significant. The inclusion of <i>Padina gymnospora</i> biowaste did not affect the nutrient intake and digestibility among the groups. The inclusion of <i>Padina gymnospora</i> biowaste in the A₅ group resulted in a significant reduction (<i>p</i> = 0.0012) in daily CH₄ emissions compared with group C; however, no significant differences were observed in daily CH₄ emissions between groups C-A₂ (<i>p</i> = 0.0793) and A₂-A₅ (<i>p</i> = 0.3269). Likewise, the adjustment of data to CH₄ emissions per 100 g of organic matter intake indicated a substantial decrease in the A₅ group relative to C. The energy loss in CH₄ as a percentage of GE relative to group C decreased significantly (-23.4%) in the A₅ group; however, this reduction was not associated with an increase in productivity, as almost similar average daily gain (<i>p</i> = 0.827) was observed in the groups. The replacement of wheat bran with the biowaste of <i>Padina gymnospora</i> significantly decreased the numbers of total protozoa and <i>holotrichs</i> in the A₅ group. Irrespective of the group, the Bacteroidota was the single largest phylum in the rumen metagenome, representing >60% of the microbiota. However, the abundance of Bacteroidota was similar among the groups. The methanogenic phyla Euryarchaeota was the 5th most abundant; however, it constituted only 3.14% of the metagenome. The abundance of <i>Desulfovibrio</i> was significantly higher in the A₅ group as compared with the control. In conclusion, the significant increase in the abundance of sulfate-reducing bacteria and reduction in protozoal numbers led to a significant reduction in CH₄ emissions with the incorporation of biowaste of <i>Padina gymnospora</i> at a 5% level of the diet.