The milk yield response to frequent milking in early lactation of dairy cows is locally regulated

Frequent milking during early lactation of dairy cows increases milk production throughout lactation; however, whether this response is regulated systemically via lactogenic hormones, locally in the mammary gland, or both is unknown. We hypothesized that the effects of frequent milking on milk production during early lactation are regulated via local mechanisms. Ten multiparous cows were assigned at parturition to unilateral frequent milking [UFM; twice daily milking of the left udder half (2×), or 4 times daily milking of the right udder half (4×)] for d 1 to 21 of lactation. After treatment, cows were milked twice daily for the remainder of lactation. At the first milking after calving, milk yield from individual quarters was measured to verify that udder halves produced equal amounts of milk prior to treatment. Thereafter, individual quarters were milked on d 3 and 7, weekly for the first 5 wk of lactation, and once every 3 mo for the remainder of lactation. During UFM, cows produced 3.9 ± 0.7 kg/d more from the side milked 4× than the side milked 2×. Upon cessation of treatment, milk production from the side milked 4× decreased, but remained at 1.8 ± 0.5 kg/d more than the side milked 2× for the remainder of lactation. After milk yield was corrected to the equivalent of a whole-udder basis, acute milk yield responses to frequent milking were found to be consistent with previous reports. Moreover, we observed greater persistency in the milk yield response, which lasted throughout lactation. We conclude that both immediate and persistent effects on milk production of frequent milking during early lactation are regulated at the level of the mammary gland. Our results demonstrate that UFM is a valid and efficient model for investigating the effects of frequent milking during early lactation in dairy cows.     

Hot topic: effects of frequent milking in early lactation on milk yield and udder health

A field study was conducted to evaluate the influence of milking frequency (3 or 6 times/d [3x or 6x, respectively]) during the initial 21 d of lactation on milk and milk component yield and mammary gland health as indicated by somatic cell count. During 2 seasons, spring and fall, multiparous cows were milked 6 times/d until d 21 of lactation and then returned to the 3 times/d frequency for the remainder of lactation (6x; n = 9). Multiparous cows milked 3 times/d from the beginning of lactation served as a control group (3x; n = 17). With the exception of milking frequency, all other aspects of management, including housing, milk harvesting, and feeding, were identical between the groups and were consistent with industry norms. Retrospective analysis of Dairy Herd Improvement Association records was used to evaluate milk yield, milk component yield, and somatic cell scores. As expected, 6x cows produced more milk on the first test day than 3x cows. Compared with 3x cows, higher milk yields persisted for 6x cows from test day 2 through 6, indicating a persistent effect of early lactation milking frequency on milk yield potential for that lactation. Milk component yield followed a similar pattern: 6x cows produced significantly more protein, fat, and total solids than did control cows throughout the study. With regard to udder health, 6x cows had lower somatic cell counts at the first test day relative to 3x cows and had reduced somatic cell scores for the first 3 mo of lactation, which suggests that early lactation milking frequency influences the mammary gland capacity to resist infection in addition to improving milk production efficiency.     

Approaches to the manipulation of mammary involution

Mammary involution is a gradual process that occurs following cessation of milking. Regression of mammary secretory tissue accompanies dramatic changes in secretion composition during the transition from lactation to involution. Conversely, rapid differentiation of secretory tissue and copious accumulation of colostrum occur as parturition approaches. The duration of the nonlactating period, mammary gland health, and secretory cell response to hormones influence subsequent lactational performance in most species. Manipulation of the bovine mammary gland in an attempt to hasten involution has been studied. The primary objective of these studies was to determine if hastened involution would decrease new intramammary infections during the early nonlactating period. Results of these studies have also led to a more fundamental understanding of events that occur during physiological transition of the mammary gland. Adequate regression, proliferation, and differentiation of mammary secretory epithelium during the nonlactating period of ruminants appear to be essential for maximal milk production during lactation. Factors that interfere with these mechanisms can adversely affect mammary function during the impending lactation. A greater understanding of these processes may provide new approaches for increasing milk production in dairy cattle.     

Continuous lactation in dairy cows: effect on milk production and mammary nutrient supply and extraction

Reports over the past decade have indicated that normal lactational performance can be achieved in genetically superior and high-producing dairy cows, even when the dry period between 2 lactations is omitted. The hypothesis tested in this experiment was that normal lactogenesis I and metabolic function may be achievable in continuously milked high-yielding dairy cows as a result of the genetic selection for lactation performance and hence longevity of mammary epithelial cells. The milk production and mammary nutrient uptake in response to omission of the dry period for cows with an expected peak milk yield higher than 45 kg/d were studied in 28 Holstein dairy cows managed without bovine somatotropin. Performance and metabolic parameters were followed in late gestation and in the following early lactation. Fourteen cows were milked continuously throughout late gestation, and another 14 dairy cows experienced a 7-wk prepartum dry period. Continuous milking during the prepartum period reduced milk production in the following early lactation period by >20%. The reduced milk production could not be readily ascribed to inefficiency of the mechanisms responsible for nutrient uptake by the lactating mammary epithelial cells, nor to systemic endocrine changes. This suggests that lowered mammary nutrient uptake must have been associated with reduced mammary blood flow, metabolic activity, or both, most likely as a result of disturbed lactogenesis I prepartum or lactogenesis II postpartum triggered by as yet unknown local mechanisms. Milk protein content was elevated by 0.4 percentage units in the continuously milked cows. The underlying reason is unknown, but given the current pricing system for milk, it deserves to be further investigated.     

Three Times Daily Milking of Cows in California Dairy Herds

Twenty-eight California Holstein dairy herds were on twice daily milking for 3 to 17 mo and for 36 mo on three times daily milking. Effect of three times daily milking on yields of milk, fat-corrected milk, fat, and solids-not-fat, udder health, and reproductive performance was evaluated using Dairy Herd Improvement monthly herd summaries. Data for all cows in the herd and for first lactation cows were analyzed separately. Milk yield for the entire herd increased 12% above previous production on twice daily milking. Fat and solids-not-fat percentages were not affected by three times daily milking. First lactation cows milked three times daily yielded 14% more milk over previous twice daily levels. Fat percent was lower for first lactation cows on three time milking, but solids-not-fat percent did not change. Response to three time milking was not related to herd size or production while on twice daily milking. Udder health, evaluated by California Mastitis Test scores, and reproductive indexes were not affected by milking frequency.     

Mammary growth during lactation: implications for increasing milk yield

Milk yield is greatly influenced by the size of the mammary secretory cell population. In rats, proliferation of cells during early lactation may account for as much as three-quarters of the increase in daily milk yield between parturition and peak lactation; the remainder is due to increased activity of existing cells. Conversely, in goats, all the initial decline in milk yield after peak lactation can be attributed to loss of secretory cells. Increased frequency of milking enhances milk yield and reduces secretory cell loss, whereas goats hemimastectomized at peak lactation undergo compensatory changes in the remaining gland, which include a complete maintenance of cell number for at least 18 wk. Cell proliferation is increased in both cases, showing that mammary growth can occur during established lactation. The significance of this observation is discussed in relation to ways in which the milk yield of dairy animals might be increased.     

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.     

Short communication: genetic selection for milk production increases plasma ghrelin in dairy cows

Ghrelin is an endogenous ligand of the growth hormone secretagogue receptor and a potent orexigenic agent in human and rodent studies, but there is limited information about its effect in dairy cows. Twelve low genetic merit and 9 high genetic merit Holstein-Friesian dairy cows in peak lactation that were offered unrestricted access to fresh pasture were used to determine whether genetic selection for milk production resulted in an associated increase in plasma ghrelin concentration in grazing dairy cows. Blood samples were taken prior to the a.m. milking (i.e., baseline) and following 2 h of grazing after the a.m. milking on 2 consecutive wk during peak lactation. Milk production and dry matter intake were greater in high genetic merit cows compared with low genetic merit cows. Plasma ghrelin and growth hormone concentrations were elevated in high genetic merit cows pre- and postgrazing, and there was no significant interaction between genetic merit and time of sampling. Genetic merit did not affect the plasma nonesterified fatty acid or glucose concentration, but the plasma concentrations of metabolites and hormones measured were diminished 2 h after feeding. Data indicate an increase in plasma ghrelin associated with genetic selection for milk production, and an associated increase in dry matter intake.     

Ovariectomy improves lactation persistency in dairy cows

A current trend in the dairy industry is to reduce milk yield at the peak of lactation and improve lactation persistency. Lactation persistency is influenced by livestock management factors, such as feeding level or milking frequency, or by physiological status, including reproductive status or calving period. These factors modulate mammary gland apoptosis and tissue remodeling, which determine the rate of decline of milk yield after the lactation peak. Previous studies on lactating cows suggested that ovarian steroids have a negative effect on milk yield after the peak of lactation. In the present study, 4 Holstein × Normande crossbred multiparous cows were ovariectomized at the time of the lactation peak, and 5 cows underwent sham operations. All of the cows were maintained in lactation for 14 mo and milk yield was recorded daily. At slaughter, mammary epithelial cell apoptosis and mammary tissue remodeling were assessed. Ovariectomized cows had improved lactation persistency and presented an average daily milk gain of 2.5 kg compared with the sham-operated cows between mo 6 and 14 of lactation. The ovariectomy appears to have limited the decline in the milk yield after the peak of lactation by reducing mammary epithelial cell apoptosis [by reducing poly(adenosine diphosphate-ribose) polymerase expression] and mammary-tissue remodeling (by reducing matrix metalloproteinase activity). In conclusion, removal of ovarian secretion via ovariectomy improved the cows' lactation persistency.     

Effect of somatotropin on the plasminogen and plasmin system in the mammary gland: proposed mechanism of action for somatotropin on the mammary gland

Somatotropin exerts a powerful galactopoietic effect when it is administered to dairy cattle. The mechanism by which somatotropin increases milk yield is currently unknown. This study describes the relationship between long-term exogenous somatotropin injections, milk plasmin concentrations, and milk yield. Plasmin is a serine-protease involved in rodent mammary gland involution. Thirty-five control cows were shown to increase milk plasmin concentrations and decrease milk yield as their lactation period advanced past the 2nd mo. In 42 somatotropin injected cows, milk plasmin was maintained at low concentrations, and milking performance was enhanced. Cessation of somatotropin injections at drying off led to a rapid elevation of milk plasmin to control values. Our hypothesis is that somatotropin suppresses plasmin production within the mammary gland, thereby suppressing involution and allowing a persistence of milk production. During a normal lactation, gradually increasing amounts of plasmin within the mammary gland results in the gradual involution process.     

Effect of modified dry period lengths and bovine somatotropin on yield and composition of milk from dairy cows

Dry periods of 40 to 60 d have been an industry standard because dry periods <40 d have resulted in reduced milk yields in the subsequent lactation by 10 to 30%. However, recent research has demonstrated no production losses for cows given a 30-d dry period. The current study evaluated milk production effects of shortened or omitted dry periods for cows at mature-equivalent production >12,000 kg of milk and treated with bovine somatotropin (bST). The study used 2 commercial dairies and one university dairy and included 4 treatments. Five multiparous and 5 primiparous cows from each farm were assigned to each treatment: 1) 60-d dry period, label use of bST (60DD); 2) 30-d dry period, label use of bST (30DD); 3) continuous milking, label use of bST (CMLST); and 4) continuous milking with continuous use of bST (CMCST). Per label, bST use started at 57 to 70 d in milk and ended 14 d before drying (60DD and 30DD) or expected calving date (CMLST). In primiparous cows, average milk yields during the first 17 wk of lactation were reduced for cows on treatments 30DD, CMLST, and CMCST vs. the 60DD treatment. (38.3, 35.1, and 37.5 vs. 44.1 +/- 1.3 kg/d, respectively). For multiparous cows, respective milk yields did not differ (46.6, 43.4, 46.5, and 47.7 +/- 2.1 kg/d). Shortened or omitted dry periods may impede mammary growth in primiparous cows, resulting in reduced milk yield in the subsequent lactation. In contrast, a shortened or omitted dry period with either bST protocol did not alter production in multiparous cows treated with bST. Quality aspects of prepartum milk and colostrum require additional characterization. For multiparous cows, milk income generated for short dry periods or for continuous milking might increase their profitability. At 17 wk of the subsequent lactation, estimates of the cumulative net margins of multiparous cows on the 30DD treatment and continuous milking treatments exceeded those of cows on the 60DD treatment by 40 dollars to 60 dollars per cow.     

Effect of the prolactin-release inhibitor quinagolide on lactating dairy cows

In most mammals, prolactin (PRL) is essential for maintaining lactation, and yet the short-term suppression of PRL during established lactation by bromocriptine has produced inconsistent effects on milk yield in cows and goats. To assess the effect of the long-term inhibition of PRL release in lactating dairy cows, 5 Holstein cows in early lactation received daily intramuscular injections of 1 mg of the PRL-release inhibitor quinagolide for 9 wk. Four control cows received the vehicle (water) only. During the last week of the treatments, one udder half was milked once a day (1×) and the other twice a day (2×). Blood samples were harvested at milking in wk −1, 1, 4, and 8. The daily injections of quinagolide reduced milking-induced PRL release but not the basal PRL concentration. Quinagolide induced a faster decline in milk production, which was about 5.3 kg/d lower in the quinagolide-treated cows during the last 4 wk of treatment. During wk 9, the inhibition of milk production by quinagolide was maintained in the udder half that was milked 2× but not in the half milked 1×. Milk production was significantly correlated with the quantity of PRL released at milking. Quinagolide did not affect the release of oxytocin at milking. Serum concentration of insulin-like growth factor-1 was not affected by treatment or correlated with milk production. Serum concentrations of leptin and the calciotropic hormone stanniocalcin were not affected by the treatment. In conclusion, the chronic administration of the PRL-release inhibitor quinagolide decreases milk production in dairy cows. The effect is likely the result of the reduced release of milking-induced PRL and is modulated at the level of the gland by milking frequency.     

A 100-Year Review: Total mixed ration feeding of dairy cows

Total mixed rations (TMR) as we know them today did not exist in 1917. In fact, TMR are an invention of primarily the last half of the past 100 yr. Prior to that time many dairy herds were fed only forages, but dairy producers started moving toward TMR feeding as milk production per cow increased, herds became larger, freestall and large-group handling of cows became more common, and milking parlors became more prevalent. The earliest known reports in the Journal of Dairy Science of feeding “complete rations” or TMR may have appeared in the 1950s, but those studies were often reported only as abstracts at annual meetings of the American Dairy Science Association or in extension-type publications. The earliest full-length article on TMR in the journal was published in 1966. An advantage of feeding TMR as opposed to feeding forages supplemented with concentrates is the opportunity to make every bite of feed essentially a complete, nutritionally balanced diet for all cows. Nutritionally related off-feed (e.g., ingredient separation due to poor mixing, feed sorting by the animal, and so on), milk fat depression, and other digestive upsets were less likely to occur with TMR feeding. Feed mixer wagons, feed particle sizes, moisture content of diets, and other factors were not concerns before TMR feeding but are concerns today. Today, most dairy herds, especially larger herds in the United States and elsewhere, feed TMR.     

Association between bovine leukemia virus, production, and population age in Michigan dairy herds

The objective of this study was to determine the herd-level effect of bovine leukemia virus (BLV) infection on dairy production, culling, and cow longevity. During routine herd testing, Dairy Herd Improvement Association technicians collected milk samples from about 40 cows from each of 104 randomly selected Michigan dairy herds averaging ≥120 milking cows and 11,686 kg of milk/yr. Milk samples were analyzed for the presence of anti-BLV antibodies by ELISA, and herd- and lactation-specific estimates of BLV prevalence were computed to determine which were the most predictive of herd milk production, culling rate, and cow longevity (proportion of cows in their third or greater lactation). On this basis, the herd BLV index (an unweighted mean BLV prevalence rate for lactation number 1, 2, 3, and ≥4) was selected as the measure of BLV prevalence that was the most highly associated with BLV economic impact. Step-down multivariate analysis was used to determine the extent to which any of 19 herd-level management variables may have confounded the association of BLV index and measures of herd economic impact (milk production and cow longevity). The BLV index was not associated with the 12-mo culling rate, but was negatively associated in the final multivariable model with the proportion of cows that were ≥third lactation, and was negatively associated with herd milk production. In summary, increased prevalence of BLV within Michigan dairy herds was found to be associated with decreased herd milk production and decreased cow longevity. Our results provide evidence that BLV infection is associated with herd-level economic impacts in high-performing dairy herds.     

Mammary apoptosis and lactation persistency in dairy animals

The decline in milk yield after peak lactation in dairy animals has long been a biological conundrum for the mammary biologist, as well as a cause of considerable lost income for the dairy farmer. Recent advances in understanding the control of the mammary cell population now offer new insights on the former, and a potential means of alleviating the latter. The weight of evidence now indicates that a change in mammary cell number, the result of an imbalance between cell proliferation and cell removal, is a principal cause of declining production. Further, it suggests that the persistency of lactation, the rate of decline in milk yield with stage of lactation, is strongly influenced by the rate of cell death by apoptosis in the lactating gland. Mammary apoptosis was first demonstrated during tissue involution after lactation, but has now been detected during lactation, in mammary tissue of lactating mice, goats and cattle. Those factors that determine the rate of cell death by apoptosis are as yet poorly characterized, but include the frequency of milking in lactating goats. Initial evidence suggests that nutrition also is likely to influence cell survival after peak lactation, an important factor being the degree of oxidative stress imposed by feed and the tissue's ability to deal with, and prevent damage by, reactive oxygen species. Comparison of cows in calf or not pregnant during declining lactation also indicates a likely influence of reproductive hormones, with oestradiol and progesterone acting to preserve mammary ductal and alveolar integrity during the dry period, while allowing a degree of apoptosis and cell replacement. In each case, the molecular mechanisms controlling mammary cell survival (or otherwise) are as yet poorly defined. On the other hand, more persistent lactations are likely to benefit animal welfare through fewer calvings and by placing less emphasis on maximal production at peak lactation, and modelling of persistent lactation with longer calving intervals indicates their likely economic benefits. In these circumstances, there is considerable incentive to elucidate the determinants of mammary apoptosis, and the factors controlling the dynamic balance between cell proliferation and cell death in the lactating mammary gland.     

Relationship of milking rate to somatic cell count

Information on milking rate, monthly bucket somatic cell counts, mastitis treatment, and milk production was obtained from 284 lactations of Holstein cows separated into three lactation groups. Significant correlations between somatic cell count (linear score) and other parameters included production in lactation 1 (-.185), production in lactation 2 (-.267), and percent 2-min milk in lactation 2 (.251). Somatic cell count tended to increase with maximum milking rate in all lactations, but correlations were not statistically significant. Twenty-nine percent of cows with milking rate measurements were treated for clinical mastitis. Treated cows in each lactation group produced less milk than untreated cows. In the second and third lactation groups, treated cows had a shorter total milking time and a higher percent 2-min milk than untreated cows, but differences were not statistically significant. Overall, the data support the concept that faster milking cows tend to have higher cell counts and more mastitis treatments, particularly beyond first lactation. However, the magnitude of the relationship was small.     

ADSA Foundation Scholar Award: A role for serotonin in lactation physiology—Where do we go from here?

Lactation is a physiological event that is exclusive to mammals. Lactation evolved as a strategy to improve the survival of the young by providing them with the complete nutrition that is required for survival upon birth as well as maternal–offspring bonding. Typically, milk production by the dam matches the demand of the young. The dairy cow is a unique exception in which the discoveries and genetic selection related to lactation physiology have been applied and resulted in a dramatic increase in milk yield of dairy cows. Studies on the role of mammary-derived serotonin and the coordination of various aspects of milk production and maternal metabolism have revealed novel mechanisms by which milk production and maternal metabolism can be improved. Furthermore, the investigation into molecular and cellular mechanisms regulating mammary gland function has revealed the importance of epigenetics on mammary gland function. Understanding mammary gland function at the cellular and physiological levels will be important for improving mammary gland control of maternal metabolism during early lactation. The early lactation period is a critical time for a dairy cow as that is when she is most susceptible to disease and metabolic disorders that can lead to negative effects on her productive capacity and overall health. Our research in the area of serotonin physiology has illustrated the importance of serotonin on the regulation of lactation and maternal homeostasis. Future research in the area of lactation physiology should be targeted at improving maternal health and longevity in the herd through manipulation of the signals the mammary gland sends to coordinate maternal metabolism and synthesize milk. Specifically, we believe that serotonin will play a central role in understanding the communication between the mammary gland and the maternal physiology during lactation.     

Drying-off practices on Swiss dairy farms: Status quo and adoption potential of integrating incomplete milking

Drying-off practices to reduce milk production before dry-off are gaining attention because high milk yields at dry-off are becoming more common and increase the risk to cow health and welfare during the dry period. Incomplete milking for the last days before dry-off is one approach for reducing milk production. We conducted an online survey to determine the currently used drying-off practices on Swiss dairy farms and to identify the adoption potential of integrating incomplete milking before dry-off. In March 2021, the online survey was sent to a representative sample of 1,974 Swiss dairy farmers. A total of 518 completed questionnaires were analyzed. The mean number of dairy cows per farm was 39 (range: 11–140 cows). Thirty-five percent of cows produced considerable quantities of milk (>15 kg/d) at dry-off, and milk yield at dry-off increased with increasing annual milk yield. Abrupt dry-off was applied on 45% of the farms. The participants reported observing behavioral changes of cows such as increased vocalizations and decreased lying time associated with dry-off. Selective dry cow therapy was applied on 74% of the farms, and 44% of the participants indicated the use of antibiotics at dry-off as being “rather often,” “often,” or “always.” Correlation analysis revealed that with increasing annual milk yields, the frequency of observed behavioral changes and antibiotic use at dry-off increased as well. Therefore, drying-off approaches that reduce milk production while supporting cow welfare are needed. We found that farmers showed an interest in testing the presented drying-off approach of incomplete milking. In addition, the farmers indicated that they would be more willing to test incomplete milking before dry-off if it became available for automated use in milking parlors or robots. Uncertainties regarding udder health appeared to be the main barrier for the adoption potential of this approach.