The udder of the dairy cow requires a nonlactating, or rest period, prior to calving in order to optimize milk production in the subsequent lactation. This phase of the lactation cycle is commonly referred to as the dry period. A 45- to 60-day dry period is recommended, and dry periods of less than 40 days result in less than optimal milk yields. The effect of dry period length on milk yield in the subsequent lactation is provided in Table 1. Although the data is somewhat dated, greatest yields were experienced with dry periods of between 40 and 70 days; dry periods of less than 40 or over 91 days were detrimental to subsequent milk yield. A good rule of thumb is to provide a 55-day dry period for first lactation cows and at least 45 days for older animals.
|Number of Days Dry||Percentage of Cows||Yield Difference, pounds (kg)|
It was once believed that dairy cows needed a dry period to replenish nutritional reserves. However, it is now known that active milk-producing cells need to regress or involute to a nonsecretory, resting state to prepare for the next lactation. As calving approaches, these cells become active again, and new milk-producing cells are formed as well. Thus, the amount of milk-producing tissue increases with each lactation. Milk production will be 25 to 30% less in the next lactation if a dry period is not allowed.
The nonlactating period is related to the dynamics of intramammary infection within a dairy herd. Existing infections from the previous lactation as well as new infections established during the dry period contribute markedly to the increasing number of infected quarters that occur with each successive lactation. This relationship of the dry period with the level of mastitis in dairy herds resulted in the development of dry cow therapy as a necessary component of mastitis control to both eliminate existing infections and prevent new ones.
Physiology of the Dry Period
The physiology of the udder during the nonlactating stage differs markedly from that during lactation. The primary function of the mammary gland during lactation is one of continuous synthesis and secretion of large quantities of milk. However, during the dry period, the mammary gland progresses through three distinct stages, which include: (1) active involution; (2) steady state involution; and (3) colostrum formation.
Based on a 60-day dry period, the period of active involution begins with the termination of regular milking, and is completed by approximately three to four weeks into the dry period. The period of steady state involution does not have a distinct beginning or end but represents the period of time during which the udder is maintained in the resting or nonlactating phase. The length of this stage will increase or decrease proportionally with the length of the dry period. Colostrum formation begins one to three weeks prepartum and is characterized by: (1) development of milk-producing cells; (2) accumulation of antibodies; and (3) onset of copious milk secretion.
Thus, at both the beginning and end of the dry period, the mammary gland is undergoing transition either from or to a state of active milk synthesis and secretion. These periods of transition from one stage to another are periods of heightened susceptibility to new intramammary infection. Details of the three stages of the dry period are presented below.
Although the dry period is initiated by cessation of milking at the end of lactation, the mammary gland continues to synthesize and secrete milk, which accumulates in the udder. Studies have demonstrated that udders accumulate about 75 to 80% of their daily yield at the time of drying off, and that maximum accumulation of fluid occurs within two to three days. Milk volume decreases significantly by four days of involution, and fluid volume continues to decrease through at least day 16 of the dry period, and probably through day 30.
The increase in pressure is believed to trigger the events of active involution, and as a result, milk composition changes markedly. The rate of synthesis of the major milk constituents — that is, milk fat, casein and lactose — decreases by three to four days into the dry period. By day seven, the concentration of serum proteins — albumin and antibodies — that leak into the udder from the blood is significantly elevated and is associated with the period of major reduction in fluid volume. The concentrations of antibody classes IgG1, IgG2, IgA and IgM, as well as the concentration of somatic cells, increase markedly by day seven of the dry period.
A characteristic milk composition change associated with active involution is the dramatic increase in concentration of the iron-binding protein, lactoferrin. This protein is thought to be derived from milk-producing cells and neutrophils. In normal milk during lactation, lactoferrin is found at concentrations of 0.1 to 0.3 milligrams per milliliter, but concentrations increase markedly during active involution from 0.4 mg/ml at drying off to 20 to 30 mg/ml by 30 days into the dry period.
Lactoferrin is partially responsible for the nonspecific defenses of the involuting mammary gland and is of particular importance with regard to prevention of coliform infections. Lactoferrin inhibits growth of these mastitis-causing bacteria because of the ability to bind iron in mammary secretions. Thus, invading bacteria are forced to compete with lactoferrin for iron, a nutrient required for normal growth of most pathogens like coliforms. Lactoferrin may also play an important role in the control of leukocyte activity. For example, lactoferrin promotes adhesiveness of leukocytes, which serves to retain these cells at sites of inflammation, thereby amplifying the inflammatory response. Lactoferrin may also direct the influx of these cell types into mammary tissue during involution and stimulate their activity.
The rate of new infection increases markedly during the process of active involution and is more than six times that observed during lactation. This increased incidence results in an elevated number of infected quarters at calving and is responsible for the high level of infection during lactation in many herds. Without dry cow therapy, approximately 10% of quarters in herds with average infection levels will become infected, causing inflammation, and resulting in production losses.
Factors that favor development of new infections during the early dry period include: (1) the teat canal becomes shortened and dilated due to internal udder pressure, allowing bacterial entry; (2) the contents of the gland are no longer being removed at regular intervals, thus the flushing of colonized bacteria from the teat canal is terminated; (3) teat end disinfection and udder sanitization are discontinued, allowing mastitis-causing bacteria to accumulate on the skin; (4) minimal changes in milk composition occur through the first three days of involution, thus natural defense factors are only marginally elevated over normal milk levels during this vulnerable period; (5) leukocytes, a primary means of eliminating bacteria that gain entrance to the mammary gland, are involved in the removal of degenerated milk-producing cells and milk components rather than bacteria; (6) soluble factors present in secretions of the involuting gland may reduce leukocyte activity; (7) components that are either increased in concentration or produced during the process of involution act as growth stimulants for certain bacteria; and (8) the increasing viscosity of secretions could have an inhibitory effect on leukocytes, resulting in reduced phagocytosis.
The elevation in leukocytes, antibodies, and lactoferrin could be interpreted as changes occurring during involution that should render the gland more resistant to new infection. However, these changes occur too slowly, and, in addition, their antibacterial action is compromised by other normal events of involution. Acceleration of involution to promote development of resistance factors and reduced infection rates are discussed below.
It is well established that dry-cow therapy reduces the new infection rate during the early dry period because antibiotics are used at higher concentrations and in slow release formulations, which persist for three to four weeks. This practice is also the most effective means of reducing the prevalence of infection at calving. In general, therapy is more effective against contagious than against environmental pathogens. Antibiotics approved for use in dairy cows are not as effective against coliforms because of resistant strains. However, therapy is effective against environmental streptococci during early involution.
Treatment of all quarters of all cows is recommended at drying off rather than selective therapy. In this way, all existing infections are treated, new infections arising shortly after drying off are prevented, and laboratory or screening procedures to decide which quarters to treat are eliminated. Research in New York has shown that in a 100-cow herd, the production gain from preventing nine new infections would pay the cost of treating all cows at drying off. Other studies have shown that the new infection rate is higher at calving than at drying off when selective therapy is used rather than treatment of all quarters of all cows.
The following protocol is recommended for sanitary, successful dry treating: 1) Confirm and segregate dry cow. Locate cows to be dried off, mark them for separation, and reconfirm their pregnancy status. Then segregate these animals from the milking herd and process them as a group. 2) Prepare materials. Before treating, soak white surgical gauze pads in 70 percent isopropyl alcohol using a clean container. Stand mastitis tubes up, cannula pointing down, in a clean and dry holding rack, as opposed to laying them down in the parlor where they can become contaminated. 3) Milk the quarters. Milk out each cow using good milking technique. 4) Sanitize teats. After milking, dip all teats in clean, fresh, commercially approved and effective teat dip from a clean container, and allow a minimum of 30 seconds contact time. 5) Scrub teat ends. Starting with the back teat on the far side of the udder, disinfect the teat ends by scrubbing each teat for a few seconds with a separate alcohol-soaked gauze pad. 6) Administer dry cow therapy. Starting with the front teat on the near side of the udder, infuse each quarter with a single-dose syringe of dry cow treatment using the partial insertion method. 7) Re-dip teats. Immediately following treatment, dip all teats again in a teat dip solution. 8) Identify animal. Even if the cow already has been marked once, mark her again with a method recognized by everyone on the dairy; it will reduce the likelihood of a dry treated cow being milked with the milking herd. Place treated cows in an area away from the milking herd with secure gates and fences.
Steady State Involution
Methods of dairy cattle management that stress minimum calving intervals and dry periods of 45 to 60 days, likely result in the process of involution being completed just prior to the stage of colostrum formation, which is hormonally regulated. The overlapping of the two events may lead to a less than optimal hormone-mediated lactogenic response within the udder tissue. Thus, lactations following dry periods of less than 40 days may result in reduced milk yield. As a consequence of having shortened dry periods, the udders of normal, pregnant dairy cows may be maintained in steady state involution for only a very short time. However, in cows that are not pregnant, or when dry periods are 60 days or greater, mammary glands do enter into a period of steady state involution for an extended period of time. During this stage, fluid volume is maintained at very low levels, milk constituents are greatly reduced in concentration, and concentrations of antibodies and lactoferrin are elevated.
The incidence of new infections during this period of steady state involution is the lowest of any stage of the dry period. The degree to which the fully involuted udder can resist the establishment of new infection may vary depending on bacterial species and immune function of the cow. For example, the fully involuted gland is quite resistant to the establishment of coliform infection but is still susceptible to some streptococci. The marked reduction in new infection during steady state involution may relate partially to the high levels of antibacterial factors such as antibodies in the secretion, but may also suggest a reduced rate of bacterial penetration through the teat canal. The latter occurs as a result of: (1) the development of a keratin plug within the teat canal; and (2) a substantially lower level of exposure of teat ends to potential mastitis pathogens. Spontaneous elimination of infection is a frequent event and acute clinical cases of mastitis are seldom observed in the fully involuted udder.
The physiology of the gland during the late dry period is again characterized by transition. In contrast to the active involution of milk-producing tissue of the early dry period, the changes occurring during this time are constructive as the gland prepares to produce milk. In addition, the changes are hormonally mediated. A major function of the mammary gland during this period is the formation and accumulation of colostrum. A unique feature of colostrum is the high concentration of antibodies, which are necessary for the newborn calf.
The concentration of milk components begins to increase approximately two weeks prior to parturition, with marked increases in milk fat, casein, and lactose occurring in the five days preceding parturition. The volume of fluid in the gland cistern increases slowly over the last two weeks and then increases dramatically at one to three days prepartum, with the onset of copious secretion.
At approximately two weeks prepartum, the concentration of lactoferrin declines, which suggests a loss of the antimicrobial properties, particularly with regard to coliform bacteria. The concentration of antibodies, in particular IgG, begins to increase two to three weeks prepartum, and maximum concentration of all antibody classes is achieved approximately five to ten days prepartum. Concentrations decline as fluid accumulates in the gland with the onset of copious milk secretion. However, antibody concentrations in colostrum are approximately 80 times those in milk.
Susceptibility to new infection by the environmental pathogens, such as environmental streptococci and coliforms, is increased just prior to calving, which is due to: (1) increased fluid volume and dilation of the teat canal allowing bacterial entry; (2) reduced leukocyte numbers and loss of their ability to engulf microorganisms; (3) utilization of milk components by mastitis microorganisms for growth; (4) reduced antimicrobial activity of milk; (5) absence or reduced residual antimicrobial activity of nonlactating cow therapy; and (6) stress associated with the impending parturition.
Because of increased susceptibility during this time, the provision of clean and dry environmental conditions is recommended to reduce new infections at calving. Use of straw or inorganic bedding material in maternity pens is preferred to sawdust, which supports the growth of coliforms.
Methods of Drying Cows Off
The method by which cows are dried off may affect the incidence of new udder infections. Basic methods of drying off include the following: (1) abrupt cessation of milking and administering dry cow therapy; and (2) intermittent milking only once a day during the last week of lactation, and administering dry cow therapy after the last milking. When used in conjunction with dry cow therapy and reduced energy intake, either method of drying off is suitable, as there is no difference in the new infection rate. However, among cows not receiving dry cow therapy, one study showed that new infections at calving were more numerous using abrupt cessation of milking. The practice of intermittent milking combined with feeding only free choice hay during the last week of lactation will increase protective factors in milk, such as leukocytes and antibodies, but the effect on new infection rate is questionable. Irrespective of the method used for dry-off, the usual recommendation is to treat all four quarters of all cows; however, some disadvantages of this practice exist and are cited below.
Disadvantages of Conventional Dry Cow Therapy
Dry cow therapy is not always effective in curing existing infections. For example, present formulations are not effective against all species of bacteria, such as coliforms, and they provide no protection against new infections during the late dry period. In addition, elimination of common udder pathogens, such as Staphylococcus species and Corynebacterium bovis via treatment, may render cows more susceptible to less common pathogens such as coliforms. Development of antibiotic resistance is sometimes considered to be a disadvantage; however, routine use of dry cow therapy does not lead to development of resistant microorganisms.
Effect of Prepartum Milking on New Infections at Calving
New infections during the prepartum period may be due, in part, to accumulation of colostrum in the udder and associated structural changes to the teat duct. This may lead to the loss of the keratin seal, allowing entrance of bacteria. In addition, the presence of teat edema may hamper: (1) contraction of the sphincter muscle; (2) milking of the udder to relieve intramammary pressure; and (3) flushing of bacteria from the teat canal. Thus, prepartum milking has been attempted to reduce the risk of infection as well as to stimulate milk production.
Most prepartum milking trials demonstrate that: (1) peak milk yield occurs earlier; (2) frequencies of milk fever and ketosis are not affected; and (3) the stress of the first milking is not combined with the stress of parturition. Research studies in New York involving seven herds revealed that cows milked twice daily starting 14 days prior to expected calving date had fewer cases of mastitis caused by environmental streptococci and Escherichia coli during the first month of lactation than cows milked starting 12 hours after calving. The reduced number of new infection in the cows milked prior to calving was probably due to: (1) twice daily udder preparation; (2) flushing of potential mastitis microorganisms from the teat canal; and (3) teat dipping. There were no effects on milk yield or persistence of milk production for that lactation. It must be kept in mind that prepartum milking removes colostral antibodies; therefore, colostrum should be saved from a cow milked prepartum and fed to the calf.
Dry Period Teat Dipping and Application of Teat Seals
Because mastitis-causing bacteria are known to accumulate on teat ends shortly after dry-off, attempts to reduce these populations and the potential development of new infections have been made by teat dipping during the early dry period. One investigation showed that dipping teats in a 5% tincture of iodine at drying off and again 24 hours later significantly reduced new Staphylococcus aureus infections but not those caused by Streptococcus uberis. Conversely, no protection was found by dipping daily using a 1% iodine dip for seven days after drying off. Likewise, dipping in a 0.5% iodine twice daily for seven days prepartum did not reduce new infections before calving.
Research on latex products demonstrated that the number of new infections at parturition in quarters dipped with a latex teat dip containing germicide was similar to undipped controls, suggesting no benefit of prepartum teat dipping. Other research also showed that there was no advantage in dipping teats of cows in a latex teat dip for the first and last seven days of the dry period.
Studies on the use of a bismuth/paraffin based teat seal, which is infused into the teat cistern, demonstrated a significant reduction in percentage of new dry period infections after experimental bacterial challenge. Inclusion of antibiotic into the seal gave similar results. In general, the seal remained in place in the teat canal and teat cistern for three to four weeks; however, some loss occurred during the dry period via the orifice.
More recently, an oil-based teat seal containing bismuth subnitrate was evaluated after infusion at dry-off. Two treatments were tested: (1) the teat seal alone (control); and (2) the teat seal containing the antibacterial food-grade microorganism Lactococcus lactis (treated). After challenge with Streptococcus dysgalactiae, 61% of control quarters and 6% of treated quarters became infected. Subsequent field trials demonstrated that use of this teat seal at drying off reduced new infections at calving by 50 to 90%. Thus, this nonantibiotic approach to mastitis prevention may not only prevent new infections, but also reduce antibiotic usage.
Other types of sealants have been applied as dips to the surface of teat skin. Such sealants theoretically form a water insoluble barrier over the surface of the teat as well as at the teat end and should persist into the dry period until a keratin plug has formed in the teat canal. In a Canadian study, evaluation of various formulations of a commercially available product showed that the average period that the formulations stayed on teats was 6.3 days, with a range of one to 15 days. Another study in Iowa showed a persistency of at least three days on the majority of teats. In this latter trial, teats were dipped one time in sealant after dry cow therapy, and again at approximately 10 to 14 days prepartum; teats were redipped as necessary to provide protection until calving. Results showed a greater than 50% reduction in new dry period infections when using the sealant, with no harmful effects on teat tissues or the teat end. A similar study in Louisiana showed no benefit of the teat sealant in reducing the new infection rate during the dry period. This sealant appears to adhere to teat surfaces better if it is stored at 40°F (4.4°C) and applied cold so that it is in a more viscous state. In fact, in one study, use of the product under warm and humid conditions was believed to compromise efficacy; thus, climactic conditions may be responsible for conflicting results between the Iowa and Louisiana studies.
The average nonlactating or dry period of the dairy cow lactation cycle ranges from 45 to 60 days in length. During this period, the mammary gland progresses through three distinct phases: active involution, steady state involution and colostrogenesis. During active involution, the mammary gland is highly susceptible to new intramammary infections; however, the gland is very resistant to infection during steady state involution. Resistance at this later stage is due to an increase in activity of antibacterial factors in lacteal secretions and those associated anatomical changes to the teat end. During colostrogenesis, as the mammary gland tissues transition to those synthesizing and secreting copious quantities of lacteal fluids, susceptibility to infection again increases. To prevent new infections during active involution, nonlactating dry-cow therapy and use of external and internal teat seals have been successful. Likewise, the new infection rate during colostrogenesis has been reduced by prepartum milking and providing a clean and dry environment in which cows give birth to calves.
Stephen C. Nickerson
University of Georgia