Foxtail millet (Setaria italica L.) silage compared peanut vine hay (Arachis hypogaea L.) exhibits greater feed efficiency via enhancing nutrient digestion and promoting rumen fermentation more efficiently in feedlotting lambs

Foxtail millet (Setaria italica L.) is an important alternative crop plant that is cultivated in Northern China. To explore its forage-use potential, a feeding trial of 60 healthy male feedlotting lambs weighing 23 ± 0.36 kg was conducted to determine the effects of replacing peanut vine hay (PVH) with different ratios (0%, 20%, 60%, and100%) of foxtail millet silage (FMS) as forage in rations on growth performance, nutrient digestibility, rumen fermentation characteristics, and bacterial community. The feeding trial was completed in two stages (Stage 1: days 1–42, forage: concentrate = 25:75; Stage 2: days 43–84, forage: concentrate = 20:80). The results show that dry matter intake (DMI) and average daily gain (ADG) increased linearly with the inclusion level of FMS in the diet (P < 0.05). Increasing the FMS replacement of PVH in rations quadratically increased the nutrient digestibility of dry matter, organic matter, crude protein and ether extract (P < 0.01) during the two stages, but the improvement of neutral detergent fiber (NDF) in response to FMS inclusion was observed during stage 2 (P < 0.01) instead of stage 1. Increasing the forage inclusion level of FMS linearly increased the digestibility of acid detergent fiber (ADF) at two stages (P < 0.05). Regarding rumen fermentation characteristics, increasing the FMS replacement of PVH in rations decreased rumen pH and linearly increased microbial crude protein (MCP) for the two stages (P < 0.05) and linearly increased NH₃-N and total volatile fatty acid (VFA) (P < 0.05) during stage 2 instead of stage 1. Regarding rumen VFA pattern, increasing the FMS replacement of PVH in rations increased molar propionate proportion and remarkably decreased the acetate: proportion ratio after lamb feeding, suggesting that rumen fermentation shifted to glucogenic propionate production. Furthermore, the rumen contents of each treatment were subjected to high-throughput sequencing analysis of 16 S rRNA genes in the V3-V4 hypervariable region; here, increasing the FMS replacement of PVH in the rations did not change the alpha diversity indexes, except for the Simpson index, including Chao1, ACE, and OTUs in rumen, while increasing the FMS level in the rations linearly increased the Simpson index (P < 0.01). Increasing the FMS replacement of PVH in the rations did not alter the relative abundance of the major bacteria phyla, nor most of the major genera of rumen in lambs, except for an increasing trend in Proteobacteria. Increasing the FMS replacement of PVH in rations numerically increased starch hydrolysis bacteria, including Prevotella (P = 0.06) and Selenomonas (P = 0.05), while it linearly decreased fiber hydrolysis bacteria involved Ruminococcus and unclassified_Ruminococcaceae (P < 0.05), with numerical improvement occurring in unclassified_Lachnospiraceae (P = 0.11) in the rumen. In brief, increasing the FMS replacement of PVH in rations exhibited greater feed efficiency with increased nutrient digestibility, promoting the total VFA production as available energy in the rumen of feedlotting lambs. Rumen fermentation shifted toward glucogenic propionate production in response to dietary FMS inclusion, and this shift was found to be associated with the increased growth abundance of Prevotella and Selenomonas and the inhibition of Ruminococcus in the rumen.