Effect of milking interval on milk secretion and mammary tight junction permeability in dairy ewes

Twenty-four lactating ewes (Manchega, n = 12; Lacaune, n = 12) in mid lactation were used to assess the short-term effects of different machine milking intervals (4, 8, 12, 16, 20, and 24 h) on milk yield, milk composition, and tight junction (TJ) permeability of mammary epithelia. Milk samples were analyzed for chemical composition, somatic cell count (SCC), and plasmin activity. Plasma lactose, and milk Na and K concentrations were used as indicators of TJ permeability. Milk accumulated linearly for up to 24 h, showing a different rate according to the milk yield of the breed (Manchega, 38 mL/h; Lacaune, 87 mL/h). Milking interval affected milk fat content, which decreased markedly from 4 to 24 h in both breeds, but no differences were observed in milk protein content. The milk contents of casein, true protein, lactose, and total solids also varied according to milking interval. Values of SCC did not vary by breed (175 × 10³ cells/mL, on average), showing the lowest log₁₀ values for the 4-and 24-h milking intervals in both breeds. Plasmin activity in milk increased with milking interval until 20 h of udder filling in both breeds, and was poorly but positively correlated with SCC content (r = 0.39). Plasma lactose increased dramatically after 20 h of milk accumulation, indicating enhanced permeability of mammary TJ. As a result, an increase in Na concentration and in the Na:K ratio, and a decrease in K concentration, were observed in the milk of Manchega ewes. On the contrary, no differences in Na and K concentrations in milk were detected in Lacaune ewes. In conclusion, our results proved that Manchega and Lacaune dairy sheep could maintain high rates of milk secretion during extended milking intervals in the short term, with no effects on udder health and few negative effects on milk yield. Increased TJ permeability, caused by the effect of udder filling, induced changes in milk composition that were more marked in Manchega than in Lacaune ewes.     

Effect of omitting one or two milkings weekly on lactational performance in dairy ewes

We investigated the effects of omitting one or two milkings weekly on milk yield and milk composition in dairy sheep. Ninety Spanish Assaf ewes were allocated to three experimental treatments: T0 (no milking omission; 14 milkings/week), T1 (omission of one evening milking per week; 13 milkings/week) and T2 (omission of two evening milkings per week; 12 milkings/week). The experiment was extended for 11 weeks, from week 7 after lambing to week 17 of lactation. Once a week, daily milk production was recorded just before and after the omission. Milk quality and composition was analysed in lactation weeks 9, 13 and 16. Omission of one or two evening milkings each week resulted in an increase in milk production recorded the day immediately after the omission (27% for T1 and 16% for T2), which can be attributed to accumulation of milk in the udder cisterns. This increase did not compensate completely for the loss of yield in the omitted milking. The lack of significant differences among the three milking strategies in milk production (on average 1903, 2062 and 1833 ml/d for T0, T1 and T2, respectively) recorded before omission throughout the trial, would indicate the absence of residual effects of the omission on production in subsequent weeks. Treatment T2 resulted in a loss of milk production of approximately 39% during those days when milkings were omitted, representing a decrease of approximately 10% of the estimated weekly milk production. In relation to milk composition, milking omission led to significant increases in the fat and protein concentrations of the milk collected after the omission, whereas differences were not significant for the milk obtained before the omission. Despite the slight increase observed in treatment T2 after the milking omission, the somatic cell count was always far below that considered as indicative of possible pathologies. In conclusion, at least one evening milking could be omitted each week in high-producing dairy sheep without adversely affecting milk yield and milk composition.     

Mammary epithelium disruption and mammary epithelial cell exfoliation during milking in dairy cows

The presence of mammary epithelial cells (MEC) in the milk of ruminants indicates that some MEC are shed from the mammary epithelium; however, the mechanisms that regulate the MEC exfoliation process are not known. Through the release of oxytocin, prolactin, and cortisol and through oxytocin-induced mechanical forces on the mammary epithelium, milking could participate in regulating the MEC exfoliation process. The aims of the present study were to determine the rate of MEC exfoliation throughout milking and to investigate its relationship to mammary epithelium integrity and milking-induced hormone release. Milk samples from 9 Holstein dairy cows producing 40.6 ± 1.36 kg of milk/d were collected at the beginning (after 1 and 2 min), in the middle, and at the end of milking. Milk MEC were purified using an immunomagnetic method. Blood samples were collected before, during, and after milking, and the oxytocin, prolactin, and cortisol concentrations in the samples were measured. Tight junction opening was assessed by plasma lactose concentration and the Na⁺:K⁺ ratio in milk. The somatic cell count in milk varied during the course of milking; it decreased at the beginning of milking and then increased, reaching the highest values at the end of milking. Exfoliated MEC were present in all milk samples collected. The presence of MEC in the milk sample collected during min 1 of milking, likely corresponding to the cisternal milk fraction, suggests that MEC were exfoliated between milkings. The observed increase in the Na⁺:K⁺ ratio in milk and in the plasma concentration of lactose indicated that disruption of mammary epithelium integrity occurred during milking. The MEC exfoliation rate at milking was not correlated with the variables describing milking-induced prolactin release but was negatively correlated with cortisol release, suggesting that cortisol may play a role in limiting exfoliation. In conclusion, milking induced a disruption of the mammary epithelial barrier. Mammary epithelial cells may be continuously exfoliated between milkings or exfoliated during milking as a consequence of the oxytocin-induced mechanical forces and the disruption of mammary epithelium integrity.     

Increasing milking intervals decreases the mammary blood flow and mammary uptake of nutrients in dairy cows

Increasing the milking intervals reduces milk yield. The aims of this study were to determine whether the reduction in milk yield could be explained by a decrease in mammary uptake of the nutrients or a decrease in the efficiency of the mammary gland in using the milk precursors to synthesize milk components, or both. In a Latin square design with 5 periods, 4 multiparous lactating dairy cows in midlactation were milked at 8-, 12-, 16-, or 24-h intervals over a period of 7 d. The cows were surgically prepared to estimate the net mammary balance of nutrient precursors of milk components (glucose, α-amino nitrogen, acetate, β-hydroxybutyrate, and total glycerol). The efficiency of the mammary gland in synthesizing milk components was estimated by the mammary uptake:milk output ratio. After 7 d of treatment, the decrease in milk yield of 6.1 kg/d between 8- and 24-h milking intervals was associated with a reduction in the uptake of nutrients by the mammary gland, whereas the efficiency of the mammary gland in synthesizing milk components remained relatively unchanged. The mammary uptake decreased by 26% for glucose, 32% for α-amino nitrogen, 18% for acetate, 24% for total glycerol, and 24% for β-hydroxybutyrate, respectively. These reductions in nutrient uptake were due to a decrease in the mammary blood flow (1.23 ± 0.24 L/min). For milk fat precursors (acetate, β-hydroxybutyrate, and total glycerol), the decrease in mammary blood flow explained the entire reduction in the mammary uptake. For glucose and the milk protein precursors, the reduction in the mammary blood flow explained 60% of the decrease in the mammary uptake, with the other 40% being accounted for by a reduction in the mammary extraction of nutrients. The nutrient uptake was altered as milk yield decreased. These decreases began with the 16-h milking interval and were higher at the 24-h milking interval.     

Changes in alveolar and cisternal compartments induced by milking interval in the udder of dairy ewes

The aim of this work was to evaluate the effects of milking interval (4, 8, 12, 16, 20, and 24 h) on cisternal size and milk partitioning (cisternal and alveolar) in the udders of dairy ewes. Twenty-four dairy ewes (Manchega, n = 12; Lacaune, n = 12) were used in a 2-wk experiment during mid-lactation. Cisternal and alveolar milk yields were measured and milk samples from each udder fraction were collected for analysis. Cisternal milk was obtained after i.v. injection of an oxytocin receptor antagonist, and alveolar milk was obtained after i.v. injection of oxytocin. Enlargement of the cisternal compartment due to milking intervals was measured by ultrasonography for each half udder. Volumes of cisternal and alveolar milk differed according to breed, being greater in Lacaune (888 ± 43 and 338 ± 25 mL, respectively) than in Manchega ewes (316 ± 43 and 218 ± 25 mL, respectively). Alveolar milk increased linearly to 16 h in Manchega and 20 h in Lacaune and remained constant thereafter. Cisternal milk accumulated linearly to 24-h milking intervals in both breeds. Cisternal area (values per udder half) increased as milking interval increased, reaching a plateau at 20 h in Manchega (21 ± 1 cm²) and 16 h in Lacaune (37 ± 1 cm²). Correlation between cisternal area and cisternal milk was the greatest at 8 h (Manchega: r = 0.70 and Lacaune: r = 0.56). Cisternal area correlated with total milk (r = 0.80). Milk fat content varied markedly with milking intervals, increasing in alveolar milk (until 12 h in Manchega, 8.90 ± 0.18%; and 20 h in Lacaune, 8.67 ± 0.19%) and decreasing until 24 h in cisternal milk (5.74 ± 0.29% and 4.85 ± 0.29%, respectively). Milk protein content increased in alveolar milk until 24 h (Manchega, 6.46 ± 0.11%; Lacaune, 5.95 ± 0.11%), but did not vary in cisternal milk. Milk lactose content only decreased at the 24-h milking interval in the cisternal milk of Manchega ewes (4.60 ± 0.04%). In conclusion, our results suggest that cisterns play an important role in accommodating secreted milk during extended milking intervals. Thus, long milking intervals could be a recommended strategy for large-cisterned dairy sheep. Evidence indicates that ultrasonography provides accurate estimations of udder cistern size and could be used as an indicator for selecting large-cisterned dairy ewes.     

Effects of increased milking frequency on metabolism and mammary cell proliferation in Holstein dairy cows

Results of previous studies have shown that increased milking frequency (IMF) during early lactation results in increased milk yield not only during the period of IMF but also after cows have returned to a decreased milking frequency. The cellular mechanisms underpinning this increased milk yield and the overall effects of IMF on metabolism have not been well characterized. The objective of this study was to determine the effects of IMF on metabolism and mammary epithelial cell proliferation in dairy cows. Thirty primiparous and 30 multiparous Holstein cows were assigned randomly at calving to 1 of 2 treatments. The control group was milked twice daily (2×) for 119 d, whereas the IMF group was milked 4 times daily (4×) from d 2 postcalving until d 21 and then 2× from d 22 until d 119. Overall milk yield did not differ between treatments throughout the 119 d monitored; however, the interaction of treatment by week was significant in that IMF cows yielded 4.8 kg/d more milk than control cows during wk 2 and 3 and had similar levels of milk yield during the remainder of the study period. Reanalysis of data excluding data from cows subjected to mammary biopsy suggested that the mammary biopsy procedure contributed to the lack of overall responses of milk yield, but that responses overall to IMF were greater in primiparous cows compared with multiparous cows. Plasma nonesterified fatty acid concentrations were elevated in multiparous cows subjected to IMF during the period of IMF, but were not influenced by treatment in primiparous cows. Plasma β-hydroxybutyrate concentrations were not affected by treatment. Mammary tissue was collected by biopsy in a subset of cows (n = 8 cows per parity and treatment) at calving and at d 21 and 75 postpartum and used for immunohistochemical localization of the cell proliferation antigen, Ki67. Effects of treatment on mammary epithelial cell proliferation were not significant, suggesting that other mechanisms must be responsible for carryover effects of IMF on lactational performance.     

Longer milking intervals alter mammary epithelial permeability and the Udder's ability to extract nutrients

Increasing the milking interval decreases milk yield and modifies milk composition. To gain a clearer understanding of the regulation of milk yield and composition, a study was conducted to establish the response curves of nutrient extraction by the mammary gland and mammary epithelial permeability in response to increasing milking intervals. Four multiparous lactating dairy cows were milked at 8-, 12-, 16-, or 24-h intervals over a period of 7 d using a Latin square design. Between the 8- and 24-h milking intervals, milk yield and milk protein levels fell curvilinearly from 38.2 to 29.2 kg/d and from 1,086 to 827 g/d, respectively. Milk fat yield decreased linearly from 1,475 to 1,235 g/d. Indicators of the opening of tight junctions increased linearly with increasing milking intervals: milk BSA increased from 148 to 207 mg/L and plasma lactose increased from 22.9 to 32.0 mg/L. The mammary gland's ability to extract nutrients decreased with increasing milking intervals. Extraction rates of glucose, β-hydroxybutyrate, and total glycerol decreased significantly (from 27.2 to 23.3%, from 42.3 to 34.4%, from 36.6 to 30.8% between 8- and 24-h milking intervals, respectively), and not significantly for α-amino nitrogen (from 23.2 to 20.0%). The extraction rate of acetate remained constant. Moreover, the extraction of milk fat precursors appeared to be less regulated than those of the precursors of milk protein and lactose, which could partly explain why milk yield and milk protein yield decreased more than milk fat yield. The arteriovenous differences of β-hydroxybutyrate and total glycerol remained constant, whereas those of glucose decreased significantly from 0.98 to 0.87 ± 0.05 mmol/L and not significantly from 0.74 to 0.64 ± 0.12 mmol/L for α-amino nitrogen. As a result, the mammary gland's ability to extract nutrients appears to be downregulated explaining partly the decrease in daily milk yield observed in response to increased milking intervals.     

Infusions of casein hydrolyzates into the mammary gland disrupt tight junction integrity and induce involution in cows

Milk stasis triggers local stimuli, which make the tight junctions leak and trigger involution. The aim of the study was to test the hypothesis that casein hydrolyzates compromise tight junction integrity and dry-off milk secretion in dairy cows. Six repeated doses of casein hydrolyzates after each milking during 3 d caused drastic changes in mammary secretion and composition, which were associated with irreversible cessation of milk secretion. No such changes were recorded in the control glands that had been treated with nonhydrolyzed casein. Treatment with casein hydrolyzates disturbed tight junction integrity within 8 h (as indicated by changes in Na+ and K+ concentrations), reduced the concentrations of lactose precipitously, activated the plasmin activator-plasminogen-plasmin system, and induced the secretion of immunoglobulin type G and lactoferrin. At the end of the 3-d treatments, we stopped milking the experimental and control glands. Milk composition 19 d later was similar in the experimental and control glands and was consistent with the composition expected in fully involuted glands. We conclude that casein hydrolyzates are among the milk-borne factors that cause the disruption of tight junction integrity and induce involution in cows. The process induced by casein hydrolyzate was more rapid and synchronized than the involution induced at drying-off.     

Effect of omitting one milking weekly on lactational performances and morphological udder changes in dairy cows

The response of dairy cows to omitting one milking weekly was investigated in two successive experiments conducted with Holstein cows milked twice daily. Experiment 1 compared the lactational performances and udder changes in eight cows (31.2 L/d, 201 d in milk) in the 5 wk before and the 5 wk after introducing the suppression of one milking weekly. Milk yield was recorded daily and milk composition twice weekly. Milk partitioning in the udder (alveolar and cisternal milk) and cisternal size (ultrasonography), 8 h after milking, were also measured at the start and the end of the experiment. Although daily milk yield decreased 32% during the experiment (10 wk), linear regression analysis revealed a loss of milk yield of 1.1 L/d (3.7%) as a consequence of the omission of one milking weekly. Milk composition, lactation persistency, and somatic cell count (SCC) were unaffected by milking omission. Milk partitioning in the udder decreased by 38% in alveolar milk volume and showed a tendency to decrease in cisternal milk volume (15%) and cisternal size (7%), as a result of milking omission and advancing lactation. Loss in total milk yield was negatively related with cisternal milk volume (r = -0.77) and cisternal size (r = -0.70) indicating smaller losses in the udders with large cisterns. In Experiment 2, five cows (21.0 L/d, 227 d in milk) previously adapted to the milking omission schedule were used to study the daily effects of milking omission on milk yield, milk composition and udder health during 10 wk. Milk yield and milk composition were approximately constant but SCC increased with lactation stage. The omission of one milking caused an important decrease in milk yield, fat content and SCC on the omission day and a compensatory increase over the following 2 d, but milk protein and lactose did not vary. All variables reached the average weekly value three days after the milking omission (six milkings). In conclusion, under the conditions used, omitting one milking weekly slightly reduced milk yield and did not affect milk composition when healthy cows were used. Milk losses by milking omission depend on udder cistern characteristics; evaluating cistern size by ultrasonography may be a useful tool for choosing cows that are better adapted to a reduced milking frequency.     

Effects of supplementation and stage of lactation on performance of grazing dairy ewes

The majority of dairy sheep in the world are fed pasture and supplemental grain during lactation; however, no trials have reported the effects of supplementation of dairy ewes grazing improved pastures in North America. In trial 1, 56 three-year-old grazing dairy ewes in early [21 ± 10 d in milk (DIM)] or late (136 ± 9 DIM) lactation were fed 0 or 0.82 kg of dry matter/d per ewe of supplement (16.5% crude protein mixture of corn and a soybean meal-based high-protein pellet) in a 2 × 2 factorial arrangement of treatments. There were no significant interactions between stage of lactation and supplementation treatments. Average test-day milk production was higher in early-lactation ewes than in late-lactation ewes (1.74 vs. 1.21 kg/d, respectively). Although test-day milk protein percentage was higher in late-lactation ewes than in early-lactation ewes (5.02 vs. 4.86%, respectively), there was no difference in milk fat percentage between stages of lactation. Supplemented ewes had higher milk production (1.59 vs. 1.36 kg/d, respectively), lower milk fat percentage (5.75 vs. 6.00%, respectively), and lower milk protein percentage (4.84 vs. 5.04%, respectively) than unsupplemented ewes. Milk urea N levels were similar between the 2 stages of lactation and between the 2 supplementation treatments and were above recommended levels for dairy sheep, indicating an excess intake or inefficient utilization of protein for both supplementation treatments. In trial 2, 96 two-, three-, and four-year-old grazing dairy ewes in midlactation (112 ± 21 DIM) were randomly assigned to 4 treatments of 0, 0.41, 0.82, or 1.24 kg of dry matter/d per ewe of whole corn. Average test-day milk production increased linearly and milk fat percentage decreased quadratically with increasing amounts of corn supplementation. Milk protein yield increased linearly, and milk urea N levels decreased quadratically with increasing amounts of corn supplementation, suggesting an improvement in the utilization of pasture protein with increasing dietary energy intake.     

Mammary cell turnover and enzyme activity in dairy cows: effects of milking frequency and diet energy density

The aim of the study was to investigate the effects of diet energy density and milking frequency on mammary cell turnover and synthetic capacity in dairy cows. Experiment 1 used 20 dairy cows. From d 4 to wk 16 postpartum, the cows were fed either a low-energy density or a high-energy density diet. From d 4 through wk 8, half of the cows in each group were milked 3 times (3x) or 2 times (2x) daily. From wk 9 to 16, all cows were milked 2 times daily. Mammary biopsies were obtained at wk 8 and 16 postpartum. In experiment 2, udders of 18 individual cows were milked diagonally 2x and 4x, and biopsies were obtained after 7 d. In experiment 1, cows on the low-energy density diet yielded 17 and 24% less milk during wk 3 to 8 and wk 11 to 16 postpartum, respectively. Furthermore, at 8 wk postpartum, mammary enzyme activities tended to be lower and mammary cell proliferation was lower in cows on the low-energy density diet. Three times daily milking during the first 8 wk postpartum resulted in 11% higher milk yield. Mammary cell turnover or enzyme activities were not significantly affected at 8 wk. The 3x milking for 8 wk resulted only in a transient carryover effect on milk yield and neither cell turnover nor enzyme activities were significantly affected at 16 wk postpartum. In experiment 2, mammary cell turnover and enzyme activity were unaffected after 7 d of 4x milking although milk yield increased by 18%. We conclude that nutrient restriction affects mammary cell turnover and possible enzyme activity, and that tuning of negative feedback loops in response to filling of the gland may be the dominating effects of changes in milking frequency.     

Adaptations of mammary uptake and nutrient use to once-daily milking and feed restriction in dairy cows

The aim of this study was to gain a clearer understanding of the different levels of regulation involved in the reduction in milk yield in response to once-daily milking and feed restriction. The treatments were designed as a 2 × 2 factorial arrangement of 2 milking frequencies (once- or twice-daily milking) and 2 feeding levels (70 or 98% of requirements determined 1 wk before the trial). The cows were surgically prepared to study the net mammary balance of the nutrients that are precursors of milk components. Mammary efficiency in synthesizing milk components was estimated using a milk output:mammary uptake ratio. No interaction was observed between the effects of milking frequency and feeding level on milk and blood parameters except for milk protein yield, milk fatty acid profile, and nonesterified fatty acids metabolism. Once-daily milking and feed restriction reduced milk yield by 5.1 and 2.9 kg/d and fat-corrected milk yield by 4.2 and 4.1 kg/d, respectively. Both treatments induced a decrease in mammary blood flow. Once-daily milking led to a reduction in the extraction rate of glucose but no changes to the lactose output:glucose uptake ratio. Feed restriction did not change the glucose extraction rate but tended to improve the lactose output:glucose uptake ratio. Under once-daily milking, the slight increase in milk fat content (0.34 percentage units) was linked to a depressed uptake of glucose and acetate but without any variations in the uptake of β-hydroxybutyrate and total glycerol and in the efficiency of acetate and β-hydroxybutyrate conversion to short- and medium-chain fatty acids in milk. The decline in milk fat and protein contents (−0.43 and −0.23 percentage units, respectively) under feed restriction was associated with relatively similar reductions in the mammary uptake of all nutrients and with enhanced conversion of the glucose taken up by the mammary gland and used for lactose synthesis. As a result, once-daily milking and feed restriction seem to affect milk yield through mechanisms that may be different and relatively independent.     

The effects of increased milking frequency during early lactation on milk yield and milk composition on commercial dairy farms

Increased milking frequency (IMF) during early lactation has the potential for carryover responses following the return to normal herd milking frequency. The objective was to determine the consistency of response of cows in commercial dairy farms to IMF during early lactation. Cows (n = 398) were assigned randomly at calving within each of the 4 participating farms to 1 of 2 treatments. The control group was milked twice-daily (2×) during the entire lactation. The IMF group was milked 4-times daily (4×) starting on d 1 to 7, depending on farm, until d 21 postcalving and 2× thereafter. Cows in the IMF group were milked at the beginning and again at the end of the normal milking routine. Milking intervals differed across the farms for the 4× cows with a minimum interval of 3.5, 4.0, 5.0, and 6 h for each of the 4 farms, respectively. The milk yield of cows subjected to IMF increased by 2.2 ± 0.4 kg/d during the first 7 mo of lactation. Interactions of treatment with lactation group (primiparous vs. multiparous) were not significant. Although percentages of fat and protein in milk were decreased by early lactation IMF (3.69% ± 0.03 fat and 3.05% ± 0.02 true protein for control vs. 3.57% ± 0.03 fat and 2.99% ± 0.02 true protein for IMF), overall yields of protein were increased by IMF (1.02 ± 0.01 vs. 0.98 ± 0.01 kg/d). Early lactation IMF did not affect udder health as assessed by somatic cell count linear score. Cows subjected to IMF were 1.4 times more likely classified as subclinically ketotic than the control cows. Early lactation IMF has the potential to increase milk yield on commercial dairy farms. Although the direction of response was the same on all farms, the magnitude of the response was different among farms and appears influenced by management practices specific to each farm, which included, but were not limited to, housing system, stocking density, nutrition, genetics, and other covariates differing among farms.     

Effect of milking frequency on the behavior and productivity of lactating dairy cows

The objective of this study was to determine the effect of milking frequency on the behavioral patterns and productivity of lactating dairy cows. Twelve freestall-housed, lactating Holstein dairy cows (7 primiparous and 5 multiparous) were exposed to each of 2 treatments (over 21-d periods) in a replicated crossover design. Treatments were milking frequency of 2×/d (at 0600 and 1800 h) or 3×/d (at 0600, 1400, and 2200 h). Milk production, feeding, lying, and rumination behavior were monitored for each animal for the last 7 d of each treatment period. Milk samples were collected for the last 3 d of each period for milk component analysis. The results indicated that cows milked 3×/d produced 2.9 kg/d more milk than those milked 2×/d. Primiparous cows consumed 3.9 kg/d less dry matter (DM) than did multiparous cows. The extra time (14.6 min/d) required for milking 3×/d altered the distribution of cow behavioral activity throughout the day. Although this did not affect total daily lying or rumination time, we observed a tendency for cows milked 2×/d to spend less time (224.6 vs. 237.5 min/d) feeding and, thus, those cows tended to consume their feed at a faster rate (0.13 vs. 0.12 kg of DM/min) than cows milked 3×/d. For multiparous cows, the increase in feeding activity was facilitated through having longer (40.1 vs. 36.8 min/meal) and numerically larger meals (4.8 vs. 4.6 kg of DM/meal) when milked 3×/d. Alternatively, primiparous cows consumed smaller (2.9 vs. 3.2 kg of DM/meal) and more frequent meals (9.1 vs. 7.7 meals/d) throughout the day when milked 3×/d, resulting in a tendency for greater DM intake (24.7 vs. 23.6 kg/d) compared with primiparous cows milked 2×/d. These results indicate that under 3×/d milking schedules, primiparous cows will positively adjust their feeding behavior to achieve similar production increases as multiparous cows. In summary, milking 3×/d can be used to improve production; however, greater milking frequency elicits varying effects on the behavior of primiparous and multiparous cows, suggesting that grouping and management of cows based on parity may be beneficial.     

Modeling the interaction of milking frequency and nutrition on mammary gland growth and lactation

A mathematical model of biological mechanisms regulating lactation is constructed. In particular, the model allows prediction of the effect of milking frequency on milk yield and mammary regression, and the interaction of nutrition and milking frequency in determining yield. Possible interactions of nutrition with milking frequency on alveolar dynamics are highlighted. The model is based upon the association of prolonged engorgement (as a consequence of milk accumulation) of active secretory alveoli with changes in gene expression that result in impairment and, ultimately, cessation of milk secretion. The emptying of alveoli at milking, following alveolar contraction induced by oxytocin, prevents this process and also allows quiescent alveoli to reactivate. Prolonged engorgement results in apoptosis of the secretory cells and, hence, regression of the mammary gland. Milk yield is linked to alveolar populations, with secretion rates being modulated by nutrition and udder fill effects. The model was used to investigate different management scenarios, and is in agreement with experimental results. The model shows that while milking frequency drives alveolar population, and therefore potential milk production, actual production varies considerably with nutrition. A significant portion of the loss associated with once-daily milking was due to udder fill rather than loss of secretory tissue. The model showed qualitative agreement with experimental data, on the acute and chronic effects of temporary once-daily milking.     

Effects of heat stress and dietary zinc source on performance and mammary epithelial integrity of lactating dairy cows

Dietary Zn and heat stress alter gut integrity in monogastric animals. However, effects of Zn on mammary epithelial integrity in heat-stressed lactating dairy cows have not been studied. Multiparous lactating Holstein cows (n = 72) were randomly assigned to 1 of 4 treatments with a 2 × 2 factorial arrangement to study the effects of environment and Zn source on performance and mammary epithelial integrity. Treatments included 2 environments [cooled (CL) or not cooled (NC)] and 2 Zn sources [75 mg/kg of supplemental Zn as Zn hydroxychloride (IOZ) or 35 mg/kg of Zn hydroxychloride + 40 mg/kg of Zn-Met complex (ZMC)]. The experiment was divided into baseline and environmental challenge phases of 84 d each. All cows were cooled during the baseline phase (temperature-humidity index = 72.5), whereas NC cows were not cooled during environmental challenge (temperature-humidity index = 77.7). Mammary biopsies were collected on d 7 and 56 relative to the onset of environmental challenge to analyze gene expression of claudin 1, 4, and 8, zonula occludens 1, 2, and 3, occludin, and E-cadherin and protein expression of occludin and E-cadherin. Deprivation of cooling increased respiration rate (64.8 vs. 73.9 breaths/min) and vaginal temperature (39.03 vs. 39.94°C) and decreased dry matter intake (26.7 vs. 21.6 kg/d). Energy-corrected milk yield decreased for NC cows relative to CL cows (24.5 vs. 34.1 kg/d). An interaction between environment and Zn source occurred for milk fat content as CL cows fed ZMC had lower milk fat percentage than other groups. Relative to CL cows, NC cows had lower concentrations of lactose (4.69 vs. 4.56%) and solids-not-fat (8.46 vs. 8.32%) but a higher concentration of milk urea nitrogen (9.07 vs. 11.02 mg/mL). Compared with IOZ, cows fed ZMC had lower plasma lactose concentration during baseline and tended to have lower plasma lactose concentration during environmental challenge. Plasma lactose concentration tended to increase at 3, 5, and 41 d after the onset of environmental challenge in NC cows relative to CL cows. Treatment had no effect on milk BSA concentration. Cows fed ZMC tended to have higher gene expression of E-cadherin relative to IOZ. Compared with CL, NC cows had increased gene expression of occludin and E-cadherin and tended to have increased claudin 1 and zonula occludens 1 and 2 gene expression in the mammary gland. Protein expression of occludin and E-cadherin was unchanged. In conclusion, removing active cooling impairs lactation performance and affects gene expression of proteins involved in the mammary epithelial barrier, and feeding a portion of dietary zinc as ZMC improves the integrity of the mammary epithelium.     

Effect of rumen-undegradable protein supplementation and fresh forage composition on nitrogen utilization of dairy ewes

Previous trials with dairy ewes fed stored feeds indicate a positive effect of rumen-undegradable protein (RUP) supplementation on milk yield. However, dairy sheep production in the United States is primarily based on grazing mixed grass-legume pastures, which contain a high proportion of rumen-degradable protein. Two trials were conducted to evaluate the effects of high-RUP protein supplementation and fresh forage composition on milk yield and N utilization of lactating dairy ewes fed in confinement or on pasture. In a cut-and-carry trial, 16 multiparous dairy ewes in mid-lactation were randomly assigned to 8 pens of 2 ewes each. Pens were randomly assigned 1 of 2 protein supplementation treatments, receiving either 0.0 or 0.3 kg of a high-RUP protein supplement (Soy Pass, LignoTech USA Inc., Rothschild, WI) per day. Within supplementation treatment, pens were randomly assigned to 1 of 4 forage treatments, which were applied in a 4 × 4 Latin square design for 10-d periods. Forage treatments included the following percentages of orchardgrass:alfalfa dry matter: 25:75, 50:50, 75:25, and 100:0. No interactions were observed between supplement and forage treatments. Supplementation with a high-RUP source tended to increase milk yield by 9%. Milk yield, milk protein yield, milk urea N, and urinary urea N excretion increased linearly with increased percentage of alfalfa. Milk N efficiency was greatest on the 100% orchardgrass diet. In a grazing trial, 12 multiparous dairy ewes in mid lactation were randomly assigned to 3 groups of 4 ewes each. Within group, 2 ewes were randomly assigned to receive either 0.0 or 0.3 kg of a high-RUP protein supplement (SoyPlus, West Central Cooperative, Ralston, IA) per day. Grazing treatments were arranged in a 3 × 3 Latin square design and applied to groups for 10-d periods. Ewes grazed paddocks that contained the following percentages of surface area of pure stands of orchardgrass:alfalfa: 50:50, 75:25, and 100:0. No interactions were found between supplement and forage treatments. Milk yield, milk protein yield, and milk urea N increased linearly with increased percentage of alfalfa in the paddock. In conclusion, supplementing with high-RUP protein tended to increase milk yield and increasing the proportion of alfalfa in the diet increased dry matter intake, milk yield, and protein yield of lactating dairy ewes fed or grazing fresh forage.     

Effect of prepartum photoperiod on milk production and prolactin concentration of dairy ewes

Long photoperiods during established lactation increase milk production in dairy cattle and dairy sheep, but recent research in cattle and dairy goats suggests an additional influence of prepartum day length on milk yield in the subsequent lactation. The proposed mechanism of function is the level and role of circulating prolactin in mammary development. The objectives of this study were to evaluate the effect of prepartum photoperiod on milk production, milk composition, and prolactin concentration of 22 multiparous dairy ewes exposed to short day prepartum photoperiod (SDPP; 8 h of light:16 h of dark) or long day prepartum photoperiod (LDPP; 16 h of light:8 h of dark) for at least 6 wk prepartum. During the first 8 wk of lactation, SDPP ewes tended to produce more milk than LDPP ewes (2.43 vs. 2.29 kg/d, respectively), and the milk of SDPP ewes had a greater fat percentage than that of LDPP ewes (6.04 vs. 5.51%, respectively). Due to daily milk yield and greater fat content, SDPP ewes produced more 6.5% fat-corrected milk (+0.30 ± 0.08 kg/d) and 6.5% fat- and 5.8% protein-corrected milk (+0.28 ± 0.08 kg/d) than LDPP ewes. For the lactation period of 180 d, SDPP ewes produced more test day milk than LDPP ewes (1.76 vs. 1.60 ± 0.05 kg/d, respectively), but there were no differences in milk fat or protein percentages. Ewes in both treatments experienced a prolactin surge at lambing, but SDPP ewes had lower circulating prolactin concentration than LDPP ewes from 4 to 0.5 wk before lambing (14.7 vs. 51.3 ± 4.2 mg/dL, respectively). These data suggest that decreased prepartum photoperiod may be important for increasing milk production in dairy ewes and may provide a management strategy for dairy sheep producers to increase milk yield.     

The effects of milk removal or four-times-daily milking on mammary expression of genes involved in the insulin-like growth factor-I axis

Frequent milking of dairy cows during early lactation elicits both an immediate increase in milk yield and a partial carryover effect that persists to the end of lactation. We hypothesized that the immediate response would be associated with a local increase in insulin-like growth factor (IGF)-I signaling and a consequent increase in mammary growth. Four multiparous cows were assigned at parturition to unilateral frequent milking [UFM; milking of the left udder half twice daily (2×; 0230 and 1430 h); milking of the right udder half 4 times daily (4×; 0230, 0530, 1430, and 1730 h)]. Mammary biopsies were obtained from both udder halves at 5 d in milk at 0530 h (immediately after 4× glands were milked). Incorporation of [³H]-thymidine into DNA and mammary cell apoptosis were not affected by UFM. Because biopsies were obtained when udder halves were at different postmilking intervals, our results reflected both the acute, transient mammary response to milking and the sustained mammary response to frequent milking treatment. We further hypothesized that the acute, transient response involves mechanisms distinct from those regulating the sustained response to frequent milking. To test that hypothesis, mammary biopsies were obtained from UFM cows (n = 5) at 0500 h, when time postmilking was the same for both udder halves. Mammary cell apoptosis was not affected by UFM. Expression of genes involved in the IGF-I axis was analyzed to identify acute responses associated with milking, per se, versus sustained responses to frequent milking treatment. Removal of milk from 4× glands was associated with an acute increase in expression of IGF binding protein-1, -3, and -4 mRNA in 2× glands, whereas IGF-I expression was increased by frequent milking treatment. These effects, however, were significant only for expression of IGF binding protein-3. Expression of IGF-I receptor did not differ because of milking frequency but was higher in both udder halves immediately postmilking, indicating a systemic effect. We conclude that several genes of the IGF-I axis respond to milking, per se, or frequent milking treatment, via at least 3 distinct patterns. Increased milking frequency does not alter mammary cell proliferation or apoptosis at 5 d in milk; however, it may increase the bioavailability of IGF-I in the mammary gland. Moreover, the increase in local expression of IGF-I in 4× udder halves indicates a role for this gene in the immediate milk yield response to frequent milking during early lactation.