A lick block (LB) is a solidified mixture of molasses, urea, minerals, filler, coagulant and binder that is supplemented to livestock mainly in relatively extensive rearing systems. It provides nutrients, such as soluble sugars, proteins, minerals and vitamins to balance dietary intake and can improve rumen fermentation and facilitate digestion and absorption of nutrients. These supplements improve livestock production, reproduction and carcass quality. In addition, LB can partially replace concentrate, serve as a delivery vehicle for additives such as enzymes and drugs and mediate the distribution of grazing livestock. This paper classifies and analyzes representative research; discusses the types, ingredients and current status of the utilization of LB; and systematically reviews the processing technology, quality assessment, influencing factors of intake, action mechanism and application. This review can provide a basis for the development, popularization and application of novel LB products.

Copper (Cu) is essential for the health of livestock, however, the optimal source and level of dietary Cu for yaks are uncertain. To fill this important gap, we designed an in vitro study to examine the effects of three Cu sources, namely Cu methionine (Met-Cu), Cu chloride (CuCl₂) and tribasic Cu chloride (TBCC), at five levels, namely 5, 10, 15, 20 and 25 mg/kg DM (includes Cu in substrate), on rumen fermentation in yaks. In vitro dry matter degradability (IVDMD) and amylase activity were greater (p < 0.05) with added Met-Cu than the other two Cu sources, and ammonia nitrogen (NH₃-N), microbial protein (MCP) and propionate contents were greater with Met-Cu and CuCl₂ than with TBCC. Total gas production and lipase activity were greater with Met-Cu and TBCC than CuCl₂ (p < 0.05), which meant that the metabolizable energy yield was greater in the two former Cu sources than the latter, but CH₄ production did not differ (p = 0.92) among Cu sources. IVDMD and lipase activity were greatest (p < 0.05) at 15 mg Cu/kg DM in the substrate and MCP, isobutyrate, butyrate and isovalerate contents, and amylase and trypsin activities were greatest or second greatest at 10 and 15 mg Cu/kg DM. It was concluded that Met-Cu was the best source of Cu and 10 to 15 mg Cu/kg DM was the optimal level for yaks, at least under in vitro conditions.

Copper, manganese, and iodine are part of a yak’s required trace elements. However, knowledge about their dietary requirements is scarce. Therefore, an experiment was conducted to evaluate rumen fermentation, blood parameters, and growth performance and screen out the optimum levels of trace elements in yaks’ diet. Here, 18 three-year-old castrated yaks were randomly divided into four groups, which fed with diets containing basal (CON: 4.40, 33.82, and 0 mg/kg) and low-level (LL: 10.00, 40.00, and 0.30 mg/kg), middle-level (ML: 15.00, 50.00, and 0.50 mg/kg), and high-level (HL: 20.00, 60.00, and 0.70 mg/kg) copper, manganese, and iodine for 30 days. With the increase in trace elements, yaks’ daily weight gain (DWG), rumen pH, ammonia nitrogen, microbial protein (MCP), and volatile fatty acids levels and serum triglycerides and urea nitrogen levels showed firstly increasing and then decreasing trends and reached the highest values in ML, and serum ceruloplasmin and total superoxide dismutase (T-SOD) activities showed continuously increasing trends. Yaks’ DWG, rumen MCP, butyrate, and valerate levels and serum triglycerides, urea nitrogen, ceruloplasmin, and T-SOD levels in ML were significantly higher than CON. Therefore, the recommended levels of copper, manganese, and iodine in growing yaks’ diet are 15.00, 50.00, and 0.50 mg/kg (ML), respectively.

Objective: This study aimed to elucidate the mechanisms underlying milk composition divergence between naks (female yaks) and Simmental cows (S-cows) by integrating longitudinal multi-omics analyses of gut microbiota and metabolomes.Methods: We determined the gut microbiota and metabolites of both species over a 54-day period (day 26 to 80 of lactation) of ten naks and ten S-cows. Gut microbiota dynamics were assessed via 16S rRNA sequencing, while serum and fecal metabolomes were profiled using ultra-high performance liquid chromatography–tandem mass spectrometry. Statistical analyses included Wilcoxon rank-sum tests, linear discriminant analysis effect size (linear discriminant analysis>2, p0.70).Results: Milk yield was lesser (0.53–0.91 vs. 2.07–3.88 kg/d) but concentrations of fat (5.63%–6.30% vs. 3.30%–3.74%), protein (5.66%–6.30% vs. 3.39%–3.74%), and conjugated linoleic acid (CLA) (1.74%–2.35% vs. 1.40%–1.75%) were greater (p<0.001) in nak than Scow milk. Species-specific microbial signatures emerged. In naks, the g-Family-XIIIAD3011-group and g-norank-Ruminococcaceae were correlated with bile acid metabolism and CLA synthesis via 13-hydroxyoctadecadienoic acid transport. Additionally, the naks gut had a greater concentration of 13-hydroxyoctadecadienoic acid, a precursor of CLA, which may be transported to mammary cells via phosphatidylcholine and converted to CLA under the catalysis of fatty acid desaturase2. S-cows harbored g-Succinivibrio and g-Eubacterium-ruminantium-group, which are linked to galactose utilization and mTORmediated amino acid allocation. Metabolomics revealed naks-enriched steroid biosynthesis and taurine pathways (false discovery rate<0.05), while S-cows exhibited a lactating network associated with greater milk yield.Conclusion: Host-specific gut microbiota mediated nutrient allocation trade-offs. Naks optimized lipid-rich milk through bile acid and CLA metabolic networks, whereas S-cows enhanced yield via microbial-galactose synergies. This research underscores the pivotal role of the gut microbiome in mediating milk composition and suggests that microbiome manipulation could be a promising strategy to enhance milk quality in ruminants.

Feedlots rely on corn-based total mixed rations (TMR) to finish yaks. However, corn is markedly deficient in lysine and, therefore, we hypothesized that feedlot yaks supplemented with rumen-protected lysine (RPLys) would improve performance. To test this hypothesis, twelve 2.5-year-old male yaks (122 ± 5.3 kg) were selected, and divided into a control (CON) and RPLys-supplemented (RPL) group. All yaks were provided with a pelleted diet that consisted of 25.0% corn stalk, 31.6% corn grain, and 24.0% corn by-products; while RPL yaks were supplemented with 37.0 g/d RPLys. Dry matter intake was not affected (p = 0.671) by RPLys supplementation, but the average daily gain was greater (p < 0.05; 1.46 vs. 1.25 kg/d) and the feed-to-gain ratio was lesser (p < 0.01; 3.39 vs. 3.90) in RPL than CON yaks. Serum urea nitrogen concentration and aspartate aminotransferase were greater (p < 0.05) in the CON than the RPL group. However, plasma lysine concentration was greater (p < 0.05), while threonine tended to be greater (p = 0.065) in RPL than CON yaks. Rumen ammonia-N concentration was lesser (p < 0.05) in RPL than CON yaks, but pH and volatile fatty acids concentration did not differ (p > 0.10) between groups. The relative abundances of the ruminal bacterial phyla of Firmicutes and Elusimicrobiota were greater (p < 0.05), whereas of the phylum Bacteroidota and genus Butyrivibrio were lesser (p < 0.05) in RPL than CON yaks. In general, the rumen microbiota was altered toward more abundant N utilization taxa in RPLys-supplemented yaks. RPLys-supplemented yaks had elevated plasma lysine and improved feed conversion ratio, providing the first evidence that bypass lysine improves the growth performance of yaks on corn-based diets in feedlots.