Through studying the effects of diets with different nutritional levels on the production performance, serum biochemical parameters, milk composition, rumen microbial community structure and function, and lamb growth performance of Shaanbei white cashmere goats during the twin-bearing lactation period, this research aimed to provide the scientific evidence for establishing a precision nutrition supply technical system for twin-bearing lactating Shaanbei white cashmere goats, which would help reduce breeding costs, enhance economic efficiency, and improve the sustainable development capacity of the Shaanbei white cashmere goat industry.

This investigation established a precision feeding paradigm for twin-bearing Shaanbei white cashmere ewes during peak lactation through a multifactorial experimental design. Thirty-two genetically uniform primiparous ewes (2-3 years old, (46.60±1.45) kg) were stratified by parity and body weight, then randomly allocated to four dietary treatments using a completely randomized design. The ewes were respectively fed the basal diet (CON group), low nutritional level diet with a nutritional level of 112% of the basal diet (LEL group), medium nutritional level diet with a nutritional level of 124% of the basal diet (MEL group), and high nutritional level diet with a nutritional level of 136% of the basal diet (HEL group). Trial sheep were weighed for initial body weight (IBM) and final body weight (FBW) under fasting conditions before morning feeding at 0 and 90 days of the trial. Daily feed provided and residual feed were recorded during the trial period to calculate body weight gain (BWG), average daily gain (ADG), and dry matter intake (DMI). For ewes, estrus detection was performed twice daily starting from 45 days postpartum to record estrus timing. Pregnancy diagnosis was conducted via rectal ultrasonography 30 days after mating to calculate pregnancy rates. Lambs were weighed every two weeks after birth, and weaning days were recorded to calculate lamb ADG. From each group, 5 ewes were randomly selected to collect fasting venous blood samples at 0, 30, and 60 days postpartum for serum separation, respectively, and 10 serum biochemical parameters including triglycerides (TG) were measured. For the same batch of 5 ewes, manual milking (100 mL) was performed at 0, 30, and 60 days postpartum to analyze milk composition parameters, such as milk fat and milk protein. On the 60th day of the trial, before morning feeding, rumen fluid was collected from 6 randomly selected ewes per group for metagenomic analysis.

(1)The medium-energy diet group (MEL) demonstrated significantly higher maternal body weight gain compared with the other three groups (P<0.01), and alongside a markedly shorter postpartum estrus interval than the control group (CON), while both MEL and high-energy diet groups (HEL) exhibited significantly greater average daily gain (ADG) and earlier weaning ages in their lambs compared with CON and low-energy diet groups (LEL) (P<0.01). (2) At 60 days postpartum, MEL ewes showed significantly elevated serum glucose (GLU) concentrations (P<0.05) and both MEL/HEL groups displayed significantly higher lipid profiles (total cholesterol, triglycerides, HDL, LDL) compared with CON/LEL groups (P<0.05). (3) Milk analysis revealed significantly increased fat and protein contents in MEL/HEL groups (P<0.05) with HEL milk showing exceptionally elevated urea nitrogen levels (P<0.05). (4) Metagenomic profiling showed no alpha diversity differences (Ace, Chao1, Shannon, and Simpson indices) among groups (P>0.05) but principal coordinate analysis revealed significant structural variations in rumen microbiota composition (P<0.05). Notable taxonomic shifts included higher Bacteroidota abundance in CON vs. HEL (P<0.05) and greater Bacillota abundance in HEL vs. CON (P<0.05), with HEL group exhibiting significantly elevated Selenomonas genera compared with CON/LEL (P<0.05). Functional analyses demonstrated nutrient-level dependent effects: KEGG pathway enrichment for Two-component system and Flagellar assembly in higher-nutrient diets (MEL/HEL) contrasted with Nucleotide metabolism and Carbon fixation pathway enrichment in lower-nutrient diets (CON/LEL) at level 3 (P<0.05). Carbohydrate-active enzyme profiling revealed progressive reductions in GH28, GH2, and CBM13 genes (P<0.05) with increasing dietary energy density, paralleled by significant enrichments in CBM43, GT9, and GT41 genes (P<0.05).

In conclusion, the optimal diet for lactating Shaanbei white cashmere goats was the medium nutritional level diet group (MEL) (metabolizable energy: 9.91 MJ·kg^-1, crude protein: 10.64%).