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Retrospective analysis of dry period length in Italian Holstein cows

Abstract This research studied the relevance of potential sources of variation for dry period length in Italian Holstein cows reared in northern Italy and investigated the effect of days dry (DD) on milk production and calving interval in the subsequent lactation. Field data of individual cow DD were merged: a) with information from the previous lactation (6832 lactations, 87 herds) and analysed to investigate factors influencing DD; b) with milk yield (MY) and calving interval (CI) of the subsequent lactation (≈4000 lactations, >80 herds) and analysed to investigate the effect of DD on subsequent lactation performance. Individual cow DD averaged 67±27d, with a median of 62d; nearly 20% of DD were greater than 80 d. Herd had the greatest impact on DD; average herd DD was 71 d and >50% of herds had a mean dry period length >70d. Longer DD were associated with second or later parity cows, lower daily MY at dry-off, and extremes in the length of the previous lactation, either short or long. After an adjustment for MY genetic merit, dry periods <35 d resulted in losses in subsequent lactation yield of ≈500kg and DD ranging between 35 and 55d were associated with a 200kg decrease when compared to DD that averaged 65d. DD>80d resulted in a 100kg increase in subsequent MY. However, yield lost in the subsequent lactation caused by average DD<65 d might be offset completely by the higher yield obtained in the previous lactation due to its longer length. Conversely, dry period length did not significantly affect ensuing CI. Therefore, data from this study and literature analysis suggest that a decrease in the duration of DD could be profitable for most herds considered. A general recommendation towards dry periods between 45 to 60d could be advised.

2003; Grummer and Rastani, 2004;Kuhn et al., 2006a), and the 60-d dry period has been the generally adopted management goal of modern dairy farms (Annen et al., 2004). However, recent studies have reevaluated optimal dry period length, in view of the considerable changes that have occurred in the genetics, management, and feeding of the dairy animal population. There is rising interest towards a reduction in the length of dry period because a reduction in days dry (DD) could increase income from milk production and simplify dry cow management (Grummer and Rastani, 2004). Moreover, a reduction of dry period length may reduce variation in cow feeding that occurs during dry and transition periods, with favourable effects on ruminal microbial population and, as a consequence, on stress and health status postpartum (Goff and Horst, 1997;Gulay et al., 2003).

Introduction
Dairy cows need a dry period before calving to ensure an adequate turnover of mammary tissue and to restore the functionality of mammary parenchyma (Capuco et al., 1997;Annen et al., 2004). Indeed, several studies showed that omission of the dry period caused a 20 to 40% decrease in milk yield (MY) during the subsequent lactation relative to a 60d dry period independently of the milk production potential of the cows (Remond et al., 1997;Andersen et al., 2005;Rastani et al., 2005).
Parole chiave: Vacche da latte, Durata dell'asciutta, Produzione di latte, Intervallo interparto. Bachman and Schairer (2003) reviewing results from a number of studies highlighted that a reduction of dry period from 60 to 30d caused variable responses in subsequent MY that ranged from -10% to +1%. In recent trials, a reduction of dry period length from 60 to 30 d did not significantly affect milk production, dry matter intake, and body condition score in the subsequent lactation (Gulay et al., 2003) and, when combined with feeding one high energy diet, improved energy balance and decreased the amount of body reserves mobilized in early lactation (Rastani et al., 2005). Conversely, other studies found lower MY for cows with fewer than 50-60 DD (Gulay et al., 2005;Kuhn et al., 2005a), although Gulay et al. (2005) cautioned that the half-udder model used in their study was of questionable value for the comparison of dry period lengths.
This research studied the relevance of potential sources of variation for dry period length in Italian Holstein cows reared in northern of Italy and investigated the effect of DD on milk production and calving interval in the subsequent lactation.

Material and methods
Sources of data Data used in this study were extracted from a regional database arranged by the Department of Animal Science at the University of Padova in cooperation with provincial cattle breeder associations of Veneto. This database extended the routine scheme of milk recording by collecting additional data related to functional and health traits. In this study information concerning dry off, calving date, parity, test day milk yield, somatic cell score, body condition score, and the occurrence of clinical mastitis and lameness for a large Holstein Friesian dairy cows and herds sample were considered. Lactations with valid dry-off and sub-sequent calving dates were extracted from the database in order to compute individual cow DD. The resultant 11,150 lactation records were edited to create the data sets described below.

Editing procedures and statistical analysis a) Data set from lactations preceding the dry period
Records of dry period length were merged with information concerning the previous lactation. Only records that fulfilled the following requirements were retained: calving date recorded; first test day ≤60 DIM; ≥3 test days in the first 150DIM; distance between the date of the last test day and the date of dry off ≤60d. Moreover, if a herd had less than 10 eligible cow records, the entire herd was eliminated. After this edit, 6832 lactations fulfilling the above conditions (2687-1 st parity dry period; 1939-2 nd parity dry period; 2206-3 rd parity dry period) from 87 herds were available.
Before statistical analysis, lactations were classified as follows: 1) length of previous lactation, 4 classes: <280 d (n=1019); 281-330 (n=2463); 331-400 (n=2041); and >400DIM (n=1309); 2) cumulated MY at 100DIM; MY at the last test day before dry off; average lactation somatic cell score (SCS); SCS at the last test day before dry off; and BCS at the last test day before dry off, 3 classes for each trait: low, medium, high. The definitions of classes were based on residual standard deviations estimated by analysis of variance performed for all variables with a model that considered herd, calving year, calving season, parity, and lactation length and class size ranged between 834 to 915, 4722 to 5146 and 845 to 1004 lactations for low, medium and high classes, respectively; and 3) mastitis and lameness occurrence, 2 classes: no or yes, if clinical mastitis (n=1215) or lameness (n=845) required specific treatment during the lactation.
Dry period length was analysed as a dependent variable with a linear model which included the following effects: herd (n=87); dry-off year (n=3); dry-off season (n=4); calving season (n=4); parity (PA, n=3, dry period after a 1 st , 2 nd and 3 rd or later lactation); length of previous lactation (PLL, n=4, as defined above); PAxPLL; cumulated MY at 100DIM; MY at the last test day before dry off (L_Milk); average lactation SCS; SCS at the last test day before dry off; BCS at the last test day before dry off (n=3, as defined above); PAxL_Milk; and first mastitis and/or lameness occurrence in the previous lactation (n=2, as described above).

b) Data set from lactations after the dry period
Records of dry period length were merged with information related to the subsequent lactation. Only records that fulfilled the following requirements were retained: lactation ended with DIM at the last test day ≥220 d and <450d; first test day ≤60DIM; ≥6 test days available in the lactation; cow estimated breeding value for MY available; and subsequent calving date recorded (only for data set concerning calving interval). Moreover, if a herd had less than 10 eligible records, the entire herd was eliminated. Thereafter, cumulated MY at the last test day available and calving interval were calculated.
Cumulated milk yield at the last test day before the lactation ended (CMY) and CI were analysed as dependent variables with a linear model which included the following effects: herd (n=85 for CMY; n=81 for CI); calving year (n=3); calving season (n=4); dry-off season (n=4); parity (PA, n=2); class of dry period length (DP, n=4); and PAxDP. Moreover, DIM at the last test day before dry off and estimated breeding value for MY were considered as covariates in the analysis of CMY. To evaluate the effect of dry period length, the means estimated for the short, medium-short, and long DD classes were contrasted against the mean estimated for the traditional DD class. Table 1 shows the distribution of dry period lengths. Average length of individual cow dry periods was 67d with a standard deviation of 27d; median dry period length approached 62d. The highest frequency class for DD length was between 56 to 60d which comprised nearly 15% of the sample while nearly 45% of all dry periods were between 50 and 70d. Fewer than 4% of dry periods were ≤30d, and nearly 20% of dry periods were >80d. In a recent study which characterized DD for US Holstein herds, Kuhn et al. (2005b) found a shorter individual cow dry period length (60.5d) and a less variable distribution of DD in that about 65% of all dry periods were between 50 to 70d, 2% were <30d, and 10% were longer than 80d.

Descriptive statistics and variation in days dry
Descriptive statistics for production and functional traits taken into consideration from the lactations that preceded and followed the dry period are given in Table 2.
Average length of the lactation preceding the dry period was 343d. Daily milk yield at the last test date prior to dry-off was close to 20kg. However, for nearly 15% of cows this value was close to 30kg, a value that might lead to increased risk of udder problems (Bertilsson et al., 1997). Cumulated MY at the last test day before dry-off (314±53 DIM) in the lactation that followed dry off exceeded 10,000kg; CI was 415±74d. Production and functional traits taken into account depicted a sample representative of the well-managed and modern Holstein herds of northern Italy (AIA, 2006). Table 3 shows the analysis of variance of dry period length in relation to the traits of the preceding lactation. Despite the inclusion of multiple potential sources of variation, the coefficient of determina-tion of the model did not exceed 0.2, which confirmed the difficulty to fully characterize, and therefore to control, the variation sources for DD (Bachman and Schairer, 2003).
Herd effect explained >50% of the model sum of squares. The overall mean DD was 71d, and the standard deviation among herd means was 9d. On average, herds studied had an average DD nearly 10d longer and standard deviation among herd means that was 3 days greater than the corresponding values reported by Kuhn et al. (2005b) for US Holstein herds. The lowest and highest herd mean values for DD were 47 and 89d. Only 4 herds had a mean for DD shorter than 55d, whereas the majority (56%) had mean DD values that exceeded 70d. The mean of the withinherd standard deviations for DD, which ranged from 10 to 50d, was 25d. Kuhn et al. (2005b) also found a significant range in within-herd standard deviation values. They attributed this variation to differences in management of the dry period, distinguishing between herd managers who dry off all cows at a specific gestation length and others who regulate DD on an individual cow basis by taking into account other factors in addition to the days until calving.
Parity and preceding lactation length significantly affected DD (P<0.01), and these effects had a significant interaction (P<0.05). As depicted in Figure 1, on average cows had shorter DD (9 to 10d) after their first than after their second or later lactations. Shorter DD for first parity cows than for second or later parity cows has been found also in US Holstein (Kuhn et al., 2005b) and Jersey cows (Kuhn et al., 2007). Longer dry periods (8 to 10d) were observed after short (<280d) and long (>400d) lactations. Also Kuhn et al. (2005b) found that longer lactations resulted in longer dry periods, whereas DD length did not appear different after short or medium lactations. The association between long lactations and long dry periods is probably related to days open in the previous lactation: in their analysis Kuhn et al. (2005b) reported that days open accounted for most variation in DD, and lactation length was largely confounded with days open. In the present study days open has not been taken into account as a source of variation of DD mainly because a reliable measure of days open was not available. Variation due to previous lactation length appeared stronger for cows dried off after their second or later parity than for cows dried off after their first parity, especially if a long lactation was experienced. This finding can probably be attributed to the higher persistency of first lactation than later parity cows, which could lead managers to keep them milking longer (Kuhn et al., 2005b). This behaviour is in contrast with the general recommendation that cows completing their first lactation benefit more from greater DD than do older cows (Bachman and Schairer, 2003;Grummer and Rastani, 2004). However, average DD value observed for 1 st parity cows always exceeded 60d (Figure 1), thus fitting with DD length generally recommended for cows completing their first lactation.
To investigate the effect of previous MY on DD, two production traits were used: the cumulated MY in the first 100 DIM (Milk100) and the MY at the last test day before dry-off (L_Milk). Milk100 was used instead of lactational yield because in a Average somatic cell score during the lactation. 5 Body condition score at the last test day before dry-off. 6 Cumulated milk yield at the last test day before dry-off (average DIM: 314±53).
preliminary analysis of our data it showed to be a reliable predictor of lactational yield and was available for a higher number of lactations than lactational yield. Results from analysis showed that only L_milk significantly affected DD (P<0.01), and it interacted with parity. As depicted in Figure 2, shorter dry periods were associated with higher last test-day MY, but this effect was stronger in second or later parity dry periods than in first parity dry periods. Average differences in DD between lowest and highest last test day milk yield was 4-5d for the dry period that followed a first lactation and 10-12d for dry periods that followed second or later lactations. Kuhn et al. (2005b) also found a significant relationship between MY in the previous lactation and dry period length, but in their study the association with DD was stronger for lactational yield than for last test day MY.
Dry period length increased as average SCS in previous lactation increased. A 3DD difference existed between the lowest and highest SCS classes, whereas DD did not appear to be associated with SCS on last test day. Conversely, Kuhn et al. (2005b) found that last SCS accounted for more variation of DD than lactational SCS. BCS before dry off was associated with DD (P<0.01). Dry period lengths of 67, 71 and 75d were found for cows with a BCS at the last test day that averaged 2.75, 3.50 and 4.25, respectively. Occurrence of clinical mastitis or lameness in the previous lactation did not affect dry period length.

Relationship of dry period length with milk yield and calving interval in the subsequent lactation
As determined by the analysis of variance given in Table 4, dry period length significantly affected the cumulated MY of the subsequent lactation (P<0.01). Estimates of this effect have been expressed relative to the MY obtained after a "traditional" (65DD) dry period (Figure 3). Subsequent Daily milk yield at the last test day before dry-off. 3 Average somatic cell score in the previous lactation. 4 Somatic cell score at the last test day before dry-off. 5 Body condition score at the last test day before dry-off. 6 Occurrence of clinical mastitis in the previous lactation. 7 Occurrence of clinical lameness in the previous lactation. df: degree of freedom; ns: not significant.
milk yield appeared to be positively associated with DD; however, the increase in milk production for a given increase in DD did not appear to be linear over the range of DD considered. Dry period lengths <35d (average length 26±8d) were associated with a ≈500kg loss of MY in the subsequent lactation, whereas medium-short DD (36 to 55d, average 48d) had a 200kg decrease. DD>80d showed a slight increase (100kg) in MY during the following lactation. Estimates were corrected for lactation length, and this adjustment influenced the magnitude, but not the pattern, of DD effect. Indeed, in a preliminary analysis of our data where actual unadjusted milk yields were  Figure 2. Effect of parity (PA1, PA2, PA3+ for 1 st , 2 nd and 3 rd or more parity, respectively) and of milk yield at the last test day before dry-off on dry period length.
processed, the loss in milk yield resulted slightly higher for DD shorter and increase in milk yield appeared slightly lower for DD longer than traditional 65DD, respectively, when compared to results obtained after an adjustment for lactation length. Estimates were corrected for the producing ability of the cows also, by including the individual EBV as a covariate in the model to thereby avoid possible biases due to nonrandom assignment of cows to each dryperiod category (Bachman and Schairer, 2003;Kuhn and Hutchison, 2005).
Effects of shortening DD on subsequent MY found in this study are in general agreement with previous studies (Bachman and Schairer, 2003;Grummer and Rastani, 2004), which frequently found a decrease in MY following dry periods shorter than 60d. From a recent observational study on US Holstein cows, Kuhn et al. (2005a) found that MY in the subsequent lactation was maximised with a 60 to 65d dry period regardless of parity; however, estimates of losses due to reduced DD in that study were higher than those observed in the present research, particularly for dry periods close to or shorter than 30d. Moreover, a decrease in production has been reported for dry periods longer than 60d for both Holstein (Kuhn et al., 2005a) and Jersey cows (Kuhn et al., 2007). No explanation other than possible over-conditioning has been proposed for this observation (Kuhn et al., 2005a).
Dry period length did not significantly affect reproductive performance expressed in terms of calving interval (Table 4), albeit P value approached significance (0.07). Days open of 411, 416, 422 and 421 were found for short, medium short, traditional and long dry period length, respectively. Although an important consideration, studies on the relationship between DD and subsequent reproductive performance are still scarce (Grummer and Rastani, 2004). From studies on Holstein cows, also Kuhn et al. (2006b) found that short dry periods resulted in fewer days open in the subsequent lactation; however, this relationship was entirely due to the lower MY associated with shortened dry period. When adjusted for MY, short dry periods resulted in Response has been computed as difference between least squares means of cumulated milk yield at the last test day of each dry period length class and that of the traditional -65±7d -dry period length. **: P<0.01; *: P<0.05.
poorer fertility in the subsequent lactation, particularly for DD<30d.

Conclusions
Data from this study provided a characterisation of dry period lengths of Holstein dairy cows and herds in northern Italy. Cows averaged 67DD, and nearly 20% of cows had a dry period longer than 80d. Among the factors considered in this study, herd had the greatest impact on DD; average herd DD was 71d and 56% of herds had a mean dry period length exceeding 70d. Longer DD were associated with second or later parity cows, lower daily milk yield at dry-off and extreme length of previous lactation, either short or long.
Dry periods <35 d resulted in a 5% loss in subsequent lactational milk yield while for DD that ranged between 35 to 55d a slight (-2%) decrease in MY occurred relative to the MY obtained after a traditional 65d dry period (10,000kg). Conversely, extending DD over 80d resulted in a very limited 1% increase in subsequent MY. Dry period length did not affect CI in the following lactation.
Based on results of the present research, the highest MY in the following lactation seems to be obtained by scheduling a dry period close to 60d. However, yield lost in the subsequent lactation caused by DD<60 d might be offset by the higher yield obtained in the previous lactation due to its longer length. From this perspective, given a daily MY at dry off of 20kg/d, which was the average daily yield at the last test day before drying cows observed in this study, the extra production obtained in the previous lactation from cows that experience short or medium-short DD would be sufficient to compensate for the milk yield lost in subsequent lactation as a result of the reduced dry period. In this regard Kuhn et al. (2006) found that the minimum DD required to maximise production across adjacent lactations were 40 to 45d for first parity and 55 to 65d for later parity dry periods. Furthermore, lifetime production was maximised with 40 to 50 and 30 to 40DD, respectively, and dry periods longer than 70 to 80d were even more costly to lifetime yield than very short DD. Although the relationship between days dry and lifetime production has not been explored in the Italian Holstein population, data from this study and previous literature analysis suggest that a decrease in the duration of DD could be profitable for most herds considered, and a general recommendation towards dry period lengths between 45 to 60d could be advised.

Provincial Breeders Associations (APA) of
Padova, Treviso and Venezia are gratefully acknowledged for their cooperation in data collection.