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Variation exists in milk protein concentration of dairy cows of the same breed that are fed and managed in the same environment, and little information was available on this variation which might be attributed to differences in rumen microbial composition as well as their fermentation metabolites. This study is aimed at investigating the difference in the composition and functions of rumen microbiota as well as fermentation metabolites in Holstein cows with high and low milk protein concentrations. In this study, 20 lactating Holstein cows on the same diet were divided into two groups (10 cows each), high degree of milk protein group (HD), and low degree of milk protein (LD) concentrations based on previous milk composition history. Rumen content samples were obtained to explore the rumen fermentation parameters and rumen microbial composition. Shotgun metagenomics sequencing was employed to investigate the rumen microbial composition and sequences were assembled via the metagenomics binning technique. Metagenomics revealed that 6 Archaea genera, 5 Bacteria genera, 7 Eukaryota genera, and 7 virus genera differed significantly between the HD and LD group. The analysis of metagenome-assembled genomes (MAGs) showed that 2 genera (g__Eubacterium_H and g__Dialister) were significantly enriched (P < 0.05, linear discriminant analysis (LDA) > 2) in the HD group. However, the LD group recorded an increased abundance (P < 0.05, LDA > 2) of 8 genera (g__CAG-603, g__UBA2922, g__Ga6A1, g__RUG13091, g__Bradyrhizobium, g__Sediminibacterium, g__UBA6382, and g__Succinivibrio) when compared to the HD group. Furthermore, investigation of the KEGG genes revealed an upregulation in a higher number of genes associated with nitrogen metabolism and lysine biosynthesis pathways in the HD group as compared to the LD group. Therefore, the high milk protein concentration in the HD group could be explained by an increased ammonia synthesis by ruminal microbes which were converted to microbial amino acids and microbial protein (MCP) in presence of an increased energy source made possible by higher activities of carbohydrate-active enzymes (CAZymes). This MCP gets absorbed in the small intestine as amino acids and might be utilized for the synthesis of milk protein.Key points• Rumen microbiota and their functions differed between cows with high milk protein % and those with low milk protein %.• The rumen microbiome of cows with high milk protein recorded a higher number of enriched genes linked to the nitrogen metabolism pathway and lysine biosynthesis pathway.• The activities of carbohydrate-active enzymes were found to be higher in the rumen of cows with high milk protein %.

Data from 1,010 lactating lactating, predominately component-fed Holstein cattle from 25 predominately tie-stall dairy farms in southwest Ontario were used to identify objective thresholds for defining hyperketonemia in lactating dairy cattle based on negative impacts on cow health, milk production, or both. Serum samples obtained during wk 1 and 2 postpartum and analyzed for β-hydroxybutyrate (BHBA) concentrations that were used in analysis. Data were time-ordered so that the serum samples were obtained at least 1 d before the disease or milk recording events. Serum BHBA cutpoints were constructed at 200μmol/L intervals between 600 and 2,000μmol/L. Critical cutpoints for the health analysis were determined based on the threshold having the greatest sum of sensitivity and specificity for predicting the disease occurrence. For the production outcomes, models for first test day milk yield, milk fat, and milk protein percentage were constructed including covariates of parity, precalving body condition score, season of calving, test day linear score, and the random effect of herd. Each cutpoint was tested in these models to determine the threshold with the greatest impact and least risk of a type 1 error. Serum BHBA concentrations at or above 1,200μmol/L in the first week following calving were associated with increased risks of subsequent displaced abomasum [odds ratio (OR)=2.60] and metritis (OR=3.35), whereas the critical threshold of BHBA in wk 2 postpartum on the risk of abomasal displacement was ≥1,800μmol/L (OR=6.22). The best threshold for predicting subsequent risk of clinical ketosis from serum obtained during wk 1 and wk 2 postpartum was 1,400μmol/L of BHBA (OR=4.25 and 5.98, respectively). There was no association between clinical mastitis and elevated serum BHBA in wk 1 or 2 postpartum, and there was no association between wk 2 BHBA and risk of metritis. Greater serum BHBA measured during the first and second week postcalving were associated with less milk yield, greater milk fat percentage, and less milk protein percentage on the first Dairy Herd Improvement test day of lactation. Impacts on first Dairy Herd Improvement test milk yield began at BHBA ≥1,200μmol/L for wk 1 samples and ≥1,400μmol/L for wk 2 samples. The greatest impact on yield occurred at 1,400μmol/L (−1.88kg/d) and 2,000μmol/L (−3.3kg/d) for sera from the first and second week postcalving, respectively. Hyperketonemia can be defined at 1,400μmol/L of BHBA and in the first 2 wk postpartum increases disease risk and results in substantial loss of milk yield in early lactation.

Although the effect of lactation stage is similar, the responses of milk yield and composition (fat and protein contents) to different types of lipid supplements differ greatly between goats and cows. Milk fat content increases with almost all studied fat supplements in goats but not in cows. However, the response of milk fatty acid (FA) composition is similar, at least for major FA, including conjugated linoleic acid (CLA) in goats and cows supplemented with either protected or unprotected lipid supplements. Goat milk CLA content increases sharply after either vegetable oil supplementation or fresh grass feeding, but does not change markedly when goats receive whole untreated oilseeds. Important interactions are observed between the nature of forages and of oil supplements on trans-10 and trans-11 C18:1 and CLA. Peculiarities of goat milk FA composition and lipolytic system play an important role in the development of either goat flavor (release of branched, medium-chain FA) or rancidity (excessive release of butyric acid). The lipoprotein lipase (LPL) activity, although lower in goat than in cow milk, is more bound to the fat globules and better correlated to spontaneous lipolysis in goat milk. The regulation of spontaneous lipolysis differs widely between goats and cows. Goat milk lipolysis and LPL activity vary considerably and in parallel across goat breeds or genotypes, and are low during early and late lactation, as well as when animals are underfed or receive a diet supplemented with protected or unprotected vegetable oils. This could contribute to decreases in the specific flavor of goat dairy products with diets rich in fat.

The objective of this study was to evaluate the effect of variations in milk protein composition on milk clotting properties and cheese yield. Milk was collected from 134 dairy cows of Swedish Red and White, Swedish Holstein, and Danish Holstein-Friesian breed at 3 sampling occasions. Concentrations of αS1-, β-, and κ-casein (CN), α-lactalbumin, and β-lactoglobulin (LG) A and B were determined by reversed phase liquid chromatography. Cows of Swedish breeds were genotyped for genetic variants of β- and κ-CN. Model cheeses were produced from individual skimmed milk samples and the milk clotting properties were evaluated. More than 30% of the samples were poorly coagulating or noncoagulating, resulting in weak or no coagulum, respectively. Poorly and noncoagulating samples were associated with a low concentration of κ-CN and a low proportion of κ-CN in relation to total CN analyzed. Furthermore, the κ-CN concentration was higher in milk from cows with the AB genotype than the AA genotype of κ-CN. The concentrations of αS1-, β-, and κ-CN and of β-LG B were found to be significant for the cheese yield, expressed as grams of cheese per one hundred grams of milk. The ratio of CN to total protein analyzed and the β-LG B concentration positively affected cheese yield, expressed as grams of dry cheese solids per one hundred grams of milk protein, whereas β-LG A had a negative effect. Cheese-making properties could be improved by selecting milk with high concentrations of αS1-, β-, and κ-CN, with high κ-CN in relation to total CN and milk that contains β-LG B.

Forty-five multiparous and 18 primiparous Holstein cows were fed three levels of crude protein (CP), each at three levels of neutral detergent fiber (NDF), to identify optimal dietary CP and energy. Cows were blocked by parity and days in milk into seven groups of nine and randomly assigned to an incomplete 9 x 9 Latin square trial with four, 4-wk periods. Diets were formulated from alfalfa and corn silages, high-moisture corn, soybean meal, minerals, and vitamins. Forage was 60% alfalfa and 40% corn silage on all diets; NDF contents of 36, 32, and 28% were obtained by feeding 75, 63, and 50% forage, respectively. Dietary CP contents of 15.1, 16.7, and 18.4% were obtained by replacing high-moisture corn with soybean meal. Production data were from the last 2 wk of each period. Spot fecal and urine samples were collected from 36 cows to estimate N excretion using fecal indigestible acid detergent fiber (ADF) and urinary creatinine as markers. There were no interactions (P>or= 0.08) between dietary CP and NDF for any trait; thus, effects of CP were not confounded by NDF or vice versa. Intake of DM and fat yield were lower on 15.1% CP than at higher CP. There were linear increases in milk urea and urinary N excretion and linear decreases in N efficiency with increasing CP. Increasing CP from 15.1 to 18.4% reduced milk N from 31 to 25% of dietary N, increased urinary N from 23 to 35% of dietary N, and reduced fecal N from 45 to 41% of dietary N. Decreasing NDF gave linear increases in BW gain, yield of milk, protein, true protein, lactose, and SNF, and milk/DM intake and milk N/N intake, and linear decreases in milk urea. However, fat yield was lower on 28% than 32% NDF. Reducing NDF from 36 to 28% increased purine derivative excretion by 19%, suggesting increased microbial protein. Increasing CP by adding soybean meal to diets fed cows averaging 34 kg/d of milk increased intake and fat yield but depressed N efficiency. Increasing dietary energy by reducing forage improved milk yield and efficiency and decreased excretion of environmentally labile urinary N.

A number of major scientific advances have been realized in the last 25 yr in determining the opportunities and limitations of altering milk composition through nutritional manipulation. Because of the greater sensitivity of milk fat to dietary manipulation than either protein or lactose, nutritional control of milk fat content and fatty acid composition received a great deal of attention. New information emerged linking ruminal production of trans fatty acid isomers with milk fat depression. As a result, research on fatty acid biohydrogenation intensified yielding new insight on the origin of specific trans fatty acid isomers originating from ruminal biohydrogenation and how these isomers were modified by the action of mammary enzymes. The discovery of conjugated linoleic acid (CLA) as a potent anticarcinogen also led to extensive work on enhancing its concentration in milk through nutritional manipulation and discovering the physiological effects of specific CLA isomers. New protected fats were developed in recent years that were designed to resist biohydrogenation and enhance the concentration of unsaturated fatty acids in milk. The nutritional factors receiving the most attention during the last 25 yr for their influence on milk protein content were forage-to-concentrate ratio, the amount and source of dietary protein, and the amount and source of dietary fat. New insights were tested on modes of action whereby fat supplements caused a decline in protein concentration. Changes in milk lactose concentration occur only in extreme and unusual feeding situations, but the basic biology of lactose synthesis and regulation are still being explored using modern molecular techniques. This paper highlights the major advances in controlling milk composition by dietary manipulation and how it influences the entire animal system from practical feeding studies to basic cellular work on mammary tissue metabolism.

The objective of this study was to estimate genetic parameters for major milk fatty acids and milk production traits. One morning milk sample was collected from 1,918 Holstein-Friesian heifers located in 398 commercial herds in the Netherlands. Each sample was analyzed for total percentages of fat and protein, and for detailed fatty acid percentages (computed as fatty acid weight as a proportion of total fat weight). Intraherd heritabilities were high for C4:0 to C16:0, ranging from 0.42 for C4:0 to 0.71 for C10:0. Saturated and unsaturated C18 fatty acids had intraherd heritability estimates of approximately 0.25, except for C18:2 cis-9, trans-11, which was 0.42. Standard errors of the heritabilities were between 0.07 and 0.12. Genetic correlations were high and positive among C4:0 to C14:0, as well as among unsaturated C18, but correlations of C4:0 to C14:0 with unsaturated C18 were generally weak. The genetic correlation of C16:0 with fat percentage was positive (0.65), implying that selection for fat percentage should result in a correlated increase of C16:0, whereas unsaturated C18 fatty acids decreased with increasing fat percentage (−0.74). Milk fat composition can be changed by means of selective breeding, which offers opportunities to meet consumer demands regarding health and technological aspects.

Literature with data from dairy cows on pasture was reviewed to evaluate the effects of supplementation on intake, milk production and composition, and ruminal and postruminal digestion. Low dry matter intake (DMI) of pasture has been identified as a major factor limiting milk production by high producing dairy cows. Pasture DMI in grazing cows is a function of grazing time, biting rate, and bite mass. Concentrate supplementation did not affect biting rate (58 bites/min) or bite mass (0.47g of DM/bite) but reduced grazing time 12 min/d per kilogram of concentrate compared with unsupplemented cows (574 min/d). Substitution rate, or the reduction in pasture DMI per kilogram of concentrate, is a factor which may explain the variation in milk response to supplementation. A negative relationship exists between substitution rate and milk response; the lower the substitution rate the higher the milk response to supplements. Milk production increases linearly as the amount of concentrate increases from 1.2 to 10kg DM/d, with an overall milk response of 1kg milk/kg concentrate. Compared with pasture-only diets, increasing the amount of concentrate supplementation up to 10kg DM/d increased total DMI 24%, milk production 22%, and milk protein percentage 4%, but reduced milk fat percentage 6%. Compared with dry ground corn, supplementation with nonforage fiber sources or processed corn did not affect total DMI, milk production, or milk composition. Replacing ruminal degradable protein sources with ruminal undegradable protein sources in concentrates did not consistently affect milk production or composition. Forage supplementation did not affect production when substitution rate was high. Fat supplementation increased milk production by 6%, without affecting milk fat and protein content. Increasing concentrate from 1.1 to 10kg DM/d reduced ruminal pH 0.08 and NH3-N concentration 6.59mg/dl, compared with pasture-only diets. Replacing dry corn by high moisture corn, steam-flaked or steam-rolled corn, barley, or fiber-based concentrates reduced ruminal NH3-N concentration 4.36mg/dl. Supplementation did not affect in situ pasture digestion, except for a reduction in rate of degradation when high amounts of concentrate were supplemented. Supplementation with energy concentrates reduced digestibility of neutral detergent fiber and intake of N but did not affect digestibility of organic matter or flow of microbial N.

The daily milk yield (DMY) is defined as the sum of milk produced during morning, midday, and evening milkings. This metric is the key parameter for quantifying a cow’s absolute production level and serves as the foundation for calculating economic traits such as 305-day total milk yield, fat yield, and protein yield, making it essential for evaluating lactation performance. Based on the data cleaning pipeline, 2992 valid records were retained from 3332 initial records through rigorous quality-control screening, this study systematically evaluated three critical lactation traits—daily milk yield and its milk components (milk fat percentage and milk protein percentage)—using complete lactation records from Huaxi cows and estimated their phenotypic and genetic parameters. Non-genetic factors (parity, season, and mature body weight) were corrected via the MIXED procedure coupled with a multi-trait animal model. The results showed that all considered non-genetic factors significantly influenced the lactation traits (p < 0.05), while daily milk yield (DMY), milk fat percentage (FP), and milk protein percentage (PP) all exhibited moderate to high heritability (0.29–0.38) (p < 0.01). Genetic evaluation of DMY and its milk components provides quantitative evidence for precision selection and optimized mating decisions in the Huaxi dairy population, thereby accelerating genetic progress in milk production, improving herd profitability, and promoting the development of the regional dairy industry.

The objective of this research was to estimate heritabilities of milk urea nitrogen (MUN) and lactose in the first 3 parities and their genetic relationships with milk, fat, protein, and SCS in Canadian Holsteins. Data were a random sample of complete herds (60,645 test day records of 5,022 cows from 91 herds) extracted from the edited data set, which included 892,039 test-day records of 144,622 Holstein cows from 4,570 herds. A test-day animal model with multiple-trait random regression and the Gibbs sampling method were used for parameter estimation. Regression curves were modeled using Legendre polynomials of order 4. A total of 6 separate 4-trait analyses, which included MUN, lactose, or both (yield or percentage) with different combinations of production traits (milk, fat and protein yield, fat and protein percentages, and somatic cell score) were performed. Average daily heritabilities were moderately high for MUN (from 0.384 to 0.414), lactose kilograms (from 0.466 to 0.539), and lactose percentage (from 0.478 to 0.508). Lactose yield was highly correlated with milk yield (0.979). Lactose percentage and MUN were not genetically correlated with milk yield. However, lactose percentage was significantly correlated with somatic cell score (−0.202). The MUN was correlated with fat (0.425) and protein percentages (0.20). Genetic correlations among parities were high for MUN, lactose percentage, and yield. Estimated breeding values (EBV) of bulls for MUN were correlated with fat percentage EBV (0.287) and EBV of lactose percentage were correlated with lactation persistency EBV (0.329). Correlations between lactose percentage and MUN with fertility traits were close to zero, thus diminishing the potential of using those traits as possible indicators of fertility.