Lactose on the basolateral side of mammary epithelial cells inhibits milk production concomitantly with signal transducer and activator of transcription 5 inactivation

Mammary epithelial cells (MECs) are the only cells capable of synthesizing lactose. During lactation, alveolar MECs secrete lactose through the apical membrane into the alveolar lumen, whereas alveolar tight junctions (TJs) block the leakage of lactose into the basolateral sides of the MECs. However, lactose leaks from the alveolar lumen into the blood plasma in the mastitis and after weaning. This exposes the basolateral membrane of MECs to lactose. The relationship between lactose in blood plasma and milk production has been suggested. The present study determined whether lactose exposure on the basolateral membrane of mouse MECs adversely affects milk production in vitro. Restricted exposure to lactose on the basolateral side of the MECs was performed using a culture model, in which MECs on the cell culture insert exhibit milk production and less-permeable TJs. The results indicated that lactose exposure on the basolateral side inhibited casein and lipid production in the MECs. Interestingly, lactose exposure on the apical side did not show detectable effects on milk production in the MECs. Basolateral lactose exposure also caused the inactivation of STAT5, a primary transcriptional factor for milk production. Furthermore, p38 and JNK were activated by basolateral lactose exposure. The activation of p38 and JNK following anisomycin treatment reduced phosphorylated STAT5, and inhibitors of p38 blocked the reduction of phosphorylated STAT5 by basolateral lactose exposure. These findings suggest that lactose functions as a partial inhibitor for milk production but only when it directly makes contact with the basolateral membrane of MECs.

Nano-sized zinc addition enhanced mammary zinc translocation without altering health status of dairy cows

This study aimed to evaluate role of nano-sized zinc (Zn) on lactation performance, health status, and mammary permeability of lactating dairy cows. Thirty multiparous dairy cows with similar days in milk (158 ± 43.2) and body weight (694 ± 60.5 kg) were chosen based on parity and milk production and were randomly assigned to 3 treatment groups: basal diet (control, 69.6 mg/kg of Zn adequate in Zn requirement), basal diet additional Zn-methionine (Zn-Met, providing 40 mg/kg of Zn), and basal diet additional nano-sized Zn oxide (nZnO, providing 40 mg/kg of Zn). The study lasted for 10 wk, with the first 2 wk as adaptation. Feed intake, milk yield and the related variables, and plasma variables were determined every other week. Blood hematological profiles were determined in the 8th week of the study. We found that feed intake, milk yield, and milk composition were similar across the 3 groups. The nZnO- and Zn-Met-fed cows had greater milk Zn concentrations in the milk (3.89 mg/L (Zn-Met) and 3.93 mg/L (nZnO)) and plasma (1.25 mg/L (Zn-Met) and 1.29 mg/L (nZnO)) than the control cows (3.79 mg/L in milk and 1.21 mg/L in plasma). The nZnO-fed cows had higher Zn concentrations in plasma but not in milk compared to Zn-Met-fed cows. The Zn appearance in milk was greater in nZnO-fed (area under curve during the first 4 h post-feeding for milk Zn: 16.1 mg/L) and Zn-Met-fed cows (15.7 mg/L) than in control cows (15.0 mg/L). During the first 4 h post-feeding, milk to blood Zn ratio was greater in nZnO-fed animals but lower in Zn-Met-fed cows compared with control cows. Oxidative stress-related variables in plasma, blood hematological profiles, and mammary permeability related variables were not different across treatments. In summary, lactation performance, Zn concentrations in milk and plasma, hematological profiles, mammary permeability were similar in cows fed nZnO and Zn-Met. We therefore suggested that nZnO feeding can improve Zn bioavailability without impairing lactation performance, health status, and mammary gland permeability in dairy cows.

Comparison of sire breeding values for milk yield traits based on daughters milked once or twice daily in New Zealand

Abstract Context In New Zealand, cows are usually milked twice a day (TAD), but in ~8% of herds, cows are milked once a day (OAD) for the entire lactation. If a genetic correlation (rg) of the same trait expressed in two environments (such as TAD and OAD) is substantially <1.0, then the genetic merit assessed from TAD herds may not be reliable for predicting genetic merit in OAD herds. Initial evaluation of sires has been undertaken from progeny test herds with TAD milking, and the best sires have then been made widely available for use in TAD or OAD herds. Aim The study was designed to test formally whether sire re-rankings occur in widely used sires at different milking frequencies. Methods Regression coefficients and rank correlations (rS) for estimated breeding values (EBVs) of 242 widely used sires (86 Holstein-Friesian, 96 Jersey and 60 crossbred) were calculated for yields of milk, fat and protein, and for somatic cell score (SCS). The rS values were contrasted with expected rank correlations (rE) between TAD and OAD EBVs assuming rg = 1.0 between true BVs expressed at the two milking frequencies and accounting for the fact that sires were highly selected. Key results Within and across breeds, regression coefficients of OAD on TAD EBVs for the same traits were <1.0, but rS values between TAD and OAD EBVs of the same sires were strong (>0.75) for milk yield, moderate–strong for fat and protein yield (0.55–0.77), and moderate for SCS (0.41–0.65). Estimates of rS were higher than their critical rE values, indicating no significant sire re-ranking across milking frequencies. Conclusions On the basis of the results, a separate selection program to generate sires for use in OAD herds is not justified. Implications Farmers operating under OAD systems can rely on genetic evaluation of sires evaluated in TAD systems and used in the OAD population. However, producers should recognise that the realised productive and economic advantage of daughters of elite sires born in OAD herds is diminished relative to that expected in TAD herds.

Response profiles of dairy cows to a single 24-h milking interval in relation with milk proteolysis, udder expansion and immune traits

An extended milking interval of 24 h (24-h milking interval (24h-MI)) constitutes the acute phase of cow adaptation to once-daily milking (ODM). A recent trial including 724 24h-MI challenges demonstrated that milk yield responses to this acute phase of ODM are highly variable (from+22% to -52% of milk yield when switching to the 24h-MI, mean=-25.3%) and that factors such as stage of lactation parity and milk yield level influenced cows' responses but did not account for all individual variability. Additional traits related to physiological, immune and behavioural adaptation were measured on a subset (96 observations) of this data set. This study aimed to determine (1) the relationship of these traits with cows' milk yield responses, (2) their ability - combined with previously identified traits - to help predict milk yield responses to 24h-MI (adaptive profiles). The 24h-MI challenge consisted of three successive periods: one control week of twice-daily milking (cTDM), one single day of 24h-MI and then 13 days of TDM (pTDM). Milk yield responses to the 24h-MI (corrected for effects of stage of lactation, parity, milk yield level and milk yield) were related to physiological traits measured during cTDM (milk flow rate, presence or absence of interleukin-8) and to their changes during the 24h-MI (absolute increase in milk flow rate and relative udder distension). Analysis of associations between milk yield responses, stage of lactation, parity, milk yield level, proteolysis, udder expansion and immune traits found three adaptive cow profile clusters. Cows in cluster 1 had a less compliant udder than cows in cluster 2, and they lost more milk during the 24h-MI than cluster-2 and cluster-3 cows. After resuming twice daily-milking (TDM), cluster-2 cows fully recovered the milk they had lost during the 24h-MI. On the opposite, cluster-3 cows did not recover the milk they lost, likely due to udder inflammation during cTDM, as suggested by elevated concentrations of interleukin-8 in their milk. These results combining new traits with stage of lactation, parity and milk yield level constitute a first step towards predicting individual cow responses to a 24h-MI.

Adaptation of dairy cows to increasing degrees of incomplete milk removal during a single milking interval

Milk accumulation in the udder decreases milk secretion and this effect is explained as well by the effects of the quantity of milk stored in the udder as by the duration and repetition of periods of milk stasis. This experiment aimed to better understand the underlying mechanisms of decreased milk yield in response to the specific effects of the quantity of milk stored in the udder, independent from storage duration, on milk yield and composition. Sixteen Holstein cows were assigned to 4 blocks of 4 cows in a 4 × 4 Latin square design using 7-d periods, with a 4-d sampling period and a 3-d washout period. Cows were milked twice daily at approximately 0700 and 1630 h throughout the trial. Treatments consisted of 4 degrees of milk removal (100, 70, 40, and 0%) applied at one morning milking, designated M0. Effects of the quantity accumulated were studied in relation to udder distension, via measurements of the total distance between the ends of the 4 teats, and cisternal capacity, via the evaluation of cisternal area by ultrasonographic scan at 1 and 9 h after M0. The effect of the quantity accumulated was also evaluated in relation to mammary epithelium permeability by determining plasma lactose concentrations 1 h before and 4, 7, and 10 h after M0. Leaving milk in the udder at M0 decreased milk production during the M0-M1 interval in a negative curvilinear manner. As a result, M0+M1 milk yield decreased or tended to decrease significantly by -1.3, -5.3, and -12.8 kg for the 70, 40, and 0% treatments compared with the 100% treatment (41.7 ± 1.26 kg/d), respectively. Negative carry-over effects on milk yield were observed until the M3 milking only for the 40 and 0% treatments, and no differences were observed between the effects of these treatments. The total distance between teats increased significantly but to decreasing degrees during the M0-M1 interval. For the 40 and 0% treatments, cisternal area, which was increased 1 h after M0 milking, exhibited no further increase during the M0-M1 interval, suggesting cisternal distension was close to maximum. Simultaneously, lactose concentrations increased in blood plasma for only these 2 treatments, and this increase occurred earlier for the 0% treatment. It was also observed that cows presenting the earliest increases in plasma lactose concentrations during milk accumulation lost more milk in response to extended milking intervals.

Genetic parameters of milk production traits in response to a short once-daily milking period in crossbred Holstein × Normande dairy cows

Despite its potential utility for predicting cows' milk yield responses to once-daily milking (ODM), the genetic basis of cow milk trait responses to ODM has been scarcely if ever described in the literature, especially for short ODM periods. This study set out to (1) estimate the genetic determinism of milk yield and composition during a 3-wk ODM period, (2) estimate the genetic determinism of milk yield responses (i.e., milk yield loss upon switching cows to ODM and milk yield recovery upon switching them back to twice-daily milking; TDM), and (3) seek predictors of milk yield responses to ODM, in particular using the first day of ODM. Our trial used 430 crossbred Holstein × Normande cows and comprised 3 successive periods: 1 wk of TDM (control), 3 wk of ODM, and 2 wk of TDM. Implementing ODM for 3 wk reduced milk yield by 27.5% on average, and after resuming TDM cows recovered on average 57% of the milk lost. Heritability estimates in the TDM control period and 3-wk ODM period were, respectively, 0.41 and 0.35 for milk yield, 0.66 and 0.61 for milk fat content, 0.60 and 0.80 for milk protein content, 0.66 and 0.36 for milk lactose content, and 0.20 and 0.15 for milk somatic cell score content. Milk yield and composition during 3-wk ODM and TDM periods were genetically close (within-trait genetic correlations between experimental periods all exceeding 0.80) but were genetically closer within the same milking frequency. Heritabilities of milk yield loss observed upon switching cows to ODM (0.39 and 0.34 for milk yield loss in kg/d and %, respectively) were moderate and similar to milk yield heritabilities. Milk yield recovery (kg/d) upon resuming TDM was a trait of high heritability (0.63). Because they are easy to measure, TDM milk yield and composition and milk yield responses on the first day of ODM were investigated as predictors of milk yield responses to a 3-wk ODM to easily detect animals that are well adapted to ODM. Twice-daily milking milk yield and composition were found to be partly genetically correlated with milk yield responses but not closely enough for practical application. With genetic correlations of 0.98 and 0.96 with 3-wk ODM milk yield losses (in kg/d and %, respectively), milk yield losses on the first day of ODM proved to be more accurate in predicting milk yield responses on longer term ODM than TDM milk yield.

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.

Lactation traits associated with short- and long-term once-daily milking performance in New Zealand crossbred dairy cattle

The main objectives of this study were to establish the relative value of milk yields under twice-daily milking (TDM) as a predictor of yield and yield loss under once-daily milking (ODM), and to understand the role of residual milk and udder storage capacity-related traits in regulating yield and yield loss during ODM. A Holstein-Friesian × Jersey crossbred herd was established over 2 seasons (years), as 2 individual cohorts on the same farm, managed on a pasture-based system over 4 lactations. Short-term (1-wk) ODM studies, with a starting total of 690 cows, were undertaken in mid- and late-lactation in lactation 2 and in mid-lactation in lactation 3 for each cohort. A 10-wk study of ODM performance began in mid-lactation in lactation 3, whereas lactation 4 was a full-lactation assessment of ODM. In the short-term studies, milk yield under ODM was well predicted (R(2)=0.7 to 0.8 in 5 of 6 studies) by the daily yield under TDM in the week before ODM. Yield loss (kg/d) increased with increasing milk yield and with increasing somatic cell count (SCC), although predictions were relatively poor (R(2)=0.09 to 0.30). Yield loss (%) decreased with increasing TDM yield in 3 of the 6 studies and was positively correlated with SCC during ODM. Nevertheless, ODM yield loss, in absolute or percentage terms, was a poorly repeatable trait in grazing cows. Part of the variation in yield loss percentage (30%) was positively associated with residual milk (%), measured pretrial, during measurement of functional udder capacity in lactation 3. Total production (kg of milk) over the full-lactation ODM study in lactation 4 was correlated with total production in the 10-wk trial in lactation 3 (r=0.72 and 0.63 for cohorts 1 and 2, respectively). Identifying the highest- and lowest-producing 10% of animals during the full lactation of ODM indicated that poor production was associated with high yields of residual milk (measured in lactation 3) and, conversely, high production was associated with low yields of residual milk, relative to the other 80% of animals. These same "high" and "low" production groups from lactation 4 had similar differences in performance in the earlier short-term studies and a larger or smaller percentage yield loss associated with the residual milk measurement. Breeding strategies for ODM may benefit, therefore, from greater emphasis on selecting for a low residual milk fraction to optimize milking performance. Nevertheless, the level of milk production under TDM is a strong phenotypic predictor of milk production under ODM.

Invited review: reduced milking frequency: milk production and management implications

Most dairy cows throughout the world are milked twice daily. In intensive dairying systems, however, it is not uncommon to increase milking frequency to between 3 and 6 times daily to increase milk production. Reducing milking frequency is much less common; however, once-daily milking of dairy cows, practiced either strategically during certain parts of the lactation or for the entire lactation, is not uncommon in key dairying countries where less emphasis is placed on milk production per cow. The practice fits well with more extensive dairy production systems, particularly those based on grazed pasture. A feature of once-daily milking is that it reduces milk yield by approximately 22%, depending on stage of lactation, breed, and parity, and it may adversely affect lactation length and persistency. However, it can offer several positive farm management options, especially related to labor requirements and farm working expenses. In addition, it may provide a tool to better manage the metabolism and energy balance of cows during early lactation or during periods of pasture deficit, and it may help to improve reproductive performance and animal health and welfare. Once-daily milking, representing one extreme of the mammary function spectrum, has attracted considerable research interest over the years. Consequently, substantial scientific information is available on its effects on mammary function, at both the physiological and molecular levels. This review focuses instead on the management of the cow milked once daily, covering the production response in relation to breed, stage of lactation, and parity, and its effect on energy status, reproduction, health and welfare, as well as on milk composition and processability.