* Shortening the dry period to less than 60 days has been promoted recently. These recommendations are supported by research demonstrating minor production losses when the dry period is reduced from approximately 60 to 30 days. However, the decision to implement a short dry period should be more complex than simply comparing extra milk income from extending the current lactation to the potential loss in milk income in the following lactation.
Shortening the dry period may alter grouping strategies or facility requirements, diet formulation, incidence of metabolic disorders and diseases, and/or impact reproductive efficiency. All of these factors should influence the decision to shorten the dry period. Unfortunately, beyond the effects on milk yield, little is known about the consequences of short dry periods. The following discussion is intended to examine some of these issues.
Dry period length
Most research examining the effect of dry period length on subsequent milk yield was done retrospectively with on-farm records. As a result, data were from cows not purposely managed for short dry periods. Therefore, many of the study cows carried twins, had incorrect predicted calving dates, aborted or had abnormally short gestations. These studies generally indicated an optimal dry period length of 50-60 days.
Studies specifically designed to examine the effects of reducing the dry period to approximately 30 days on milk production (Lotan and Adler, 1976; Sorensen and Enevoldsen, 1991; Bachman, 2002; Annen et al., 2003; Gulay et al., 2003; Rastani et al., 2003) have focused on milk production following the treatment period and do not consider additional milk from the lactation prior to calving. Length of time cows were followed after calving varies among studies and ranged from 70 to 305 days.
Of the six studies listed above, two showed a significant drop in milk yield. The study by Sorensen and Enevoldsen (1991) which indicated a significant drop in milk and fat-corrected milk yield was conducted on 8 commercial dairies in Denmark, and included Danish Black and White, Red Danish, and Jersey cattle. A signficant drop in milk yield, but not fat-corrected milk yield, was noted by Rastani et al. (2003). Several of the studies reported a numerical drop in milk yield that was not statistically significant. This likely reflects inadequate replication (cow numbers) to detect a significant difference. Pooling data from all six studies, leads to the conclusion that a 5% drop in milk yield the following lactation could be expected if the dry period is shortened from 50-60 days to approximately 30 days.
These results leave open the question of optimal dry period length. Is a dry period of less than 30 days feasible? If the dry period is shortened to 35 or 40 days, will milk yield in the next lactation remain unchanged? In the early seventies, Cornell researchers conducted a study where cows on commercial farms were assigned to 20, 30, 40, 50 or 60 day dry period for 42 mos. Cows were dried off regardless of treatment when milk production was less than 9 kg/d. Consequently, few cows assigned to the short dry period actually “qualified” for their treatment. For cows that adhered to their assigned days dry, a net milk yield loss (additional milk from the previous lactation was added back) of about 5% occurred when dry period was reduced below 40 days. This data suggests that a 40 day dry period could be beneficial. Most Midwestern (US) dairy producers implementing a shortened dry period have targeted 40-45 days dry to allow a margin of safety for cows calving earlier than expected.
Shortening the dry period to 30 to 45 days eliminates the need to have two diets during the dry period. This creates the potential for a single dry cow group. However, a single dry cow group presumes that all cows in a herd will respond similarly to a shortened dry period.
Parity may alter the impact of a shortened dry period; however, research results are inconsistent. Several studies found a more detrimental effect for shortening thee dry period between the first and second lactation than between later lactations (Sanders, 1928; Wilton et al., 1967; Dias and Allaire, 1982; Annen, 2004). However, other studies have not found differences in response to shortened dry periods based on parity (Keown and Everett, 1986; Funk et al., 1987; Sorensen and Enevoldsen, 1991; Rastani et al., 2005).
It has been speculated that high producing cows need a longer dry period, but there is very little research available to examine this idea. Comparing second lactation milk yields to first lactation milk yields, showed that the advantage of a longer dry period was greater for low producing herds than for high producing herds (Dickerson and Chapman, 1939). These researchers speculated that low producing herds were fed a lower plane of nutrition and that a longer “rest period” was needed when cows were underfed. In contrast, a subsequent study indicated higher producing cows required a longer dry period to reach peak milk the subsequent lactation, but the relationship was only evident for cows between their first and second lactation (Dias and Allaire, 1982). Additional studies are needed to make a firm conclusion.
A very strong interaction has been documented between calving interval and the dry period length required for maximum milk yield the following lactation (Dias and Allaire, 1982). Cows with longer calving intervals required fewer days dry, with this relationship growing stronger as cows got older. If only a portion of a herd will have a shortened dry period, older cows with longer calving intervals should be considered first.
Typical feeding management of dry cows includes a far-off dry cow diet and a prefresh transition diet. The far-off diet is low in energy density and is designed to maintain body condition of the cow during the first five weeks of the dry period. The prefresh/transition diet is fed during the final three weeks of the dry period and is designed to acclimate the cow and rumen microorganisms to the high-energy lactation diet that will be fed following calving.
This traditional strategy involves two grouping changes and two diet changes within a three-week time frame. This may lead to increased stress from grouping and diet changes, larger than desired declines in feed intake, and metabolic complications postpartum.
Avoiding both diet and group changes during the dry period may ease the transition into milk production. However, feeding a transition-type diet for 8 weeks may lead to over-conditioned cows and an increased incidence of metabolic disorders (Rukkwamsuk et al., 1999). Feeding a single high fiber diet during the entire dry period may be successful; however, changing abruptly from a high to a low fiber diet at calving may be disruptive to the cow and/or the rumen microbes.
If one diet could be fed for a 60 day dry period, then multiple dry period lengths can be employed using a single dry cow pen. A compromise strategy may be to shorten the dry period and feed one diet with relatively high energy throughout the dry period. Energy density of the diet would depend on the targeted dry period. In other words, as dry period length decreases, diet energy density could increase because there would be less time to accumulate excess body condition.
In a recent experiment (Rastani et al., 2005) multiparous cows were dried off and assigned to treatments at -56 days prepartum. The 3 treatments were: 1) 56 days dry with a low energy far-off diet from -56 to -29 days prepartum and a close-up transition diet from -28 days to parturition 2) 28 days dry; cows continued on the lactation diet (minus buffer) throughout the dry period; and 3) 0 days dry; cows continued on the lactation diet (minus buffer) until calving. After calving, all animals were fed the same postpartum lactation ration.
Actual days dry for the 56, 28 and 0 days dry treatments were 54, 29 and 5 days. Some cows on the 0 days dry treatment dried themselves off.
Continuation of milking resulted in higher dry matter intakes prior to calving.
However, even cows with 0 days dry experienced a decline in feed intake as calving approached. Differences in feed intake between treatments continued, but to a lesser magnitude, after calving. There was no significant difference in 4% FCM production between 56 and 28 day dry treatments; cows on 0 day treatment produced about 5 kg FCM less per day than those with 28 days dry. Cows on the 28 day treatment produced milk with a higher fat test, resulting in FCM yield differences between cows on the 56 and 28 day dry treatments.
Changes in body condition score and body weight postpartum suggested a more favorable energy balance as days dry decreased. This was accompanied by a reduction in plasma nonesterified fatty acids (NEFA), and liver triglyceride. However, the differences were only significant between cows with 0 and 28 day dry. There were no differences in calf size due to treatment (42.7, 42.9, and 43.1 kg for 56, 28 and 0 day treatments). Incidences of metabolic disorders were too low to allow a statistical analysis of this data.
Dry cow treatment strategies
Limited data suggests dry cow antibiotic therapy may be more effective in cows with shortened dry periods, and there may be a reduced rate of new infections in cows with shortened dry periods (Natzke et al., 1975; Rindsig et al., 1978). However, current dry cow therapies are targeted for a 45 to 60 day dry period. Consequently, if cows receive a dry cow treatment and have a dry period less than 45 days, antibiotic residues may be present in milk post-calving. Implementation of a shortened dry period with standard dry cow antibiotic therapies should be accompanied by postpartum testing of milk for residues. A reasonable approach may be to use a lactating cow antibiotic treatment during the final milkings combined with a teat sealant after the last milking.
Reducing the dry period has resulted in a more rapid resumption of ovarian activity (Gumen et al., 2003). Cows with 0 days dry had higher first service conception rates, fewer services per conception, and fewer days open. However, because these cows were not on experiment beyond 70 d and limited cow numbers were used, these results must be interpreted with caution. These differences in reproductive performance may have resulted from differences in days dry, energy balance, or milk yield. Therefore these results must be verified with additional studies.
Future strategies for reducing dry period length
Available data indicates that a 30-day dry period may be feasible, but a 0 day dry period results in significant milk yield losses during the subsequent lactation (20-25% loss). Will this change in the future? University of Arizona research indicates that milk loss associated with 0 days dry can be avoided in multiparous, but not primiparous cows, by continuous bST treatment. However, this is off-label use and a control group of cows with a 60 d dry period and continuous bST treatment was not included.
Neither prostaglandin E2 (Annen et al., 2004) nor 4x milking postpartum (Fitzgerald et al., 2004) could prevent the yield loss associated with continuous milking of primiparous cows. Preliminary results from University of Wisconsin indicate milking 4 times/day during the final 4 weeks of pregnancy may eliminate the production loss associated with a 0 day dry period in mature cows, but not for those having their second calf. More basic research is needed to investigate the factors that affect lactation persistency, including mammary cell proliferation and mammary cell death, so that future strategies can be developed to shorten or eliminate the dry period.
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