J. Dairy Sci. 88:1552-1560
© American Dairy Science Association, 2005.
Relationship Between Type Traits and Longevity in Canadian Jerseys and Ayrshires Using a Weibull Proportional Hazards Model
A. Sewalem1,2,
G. J. Kistemaker2 and
B. J. Van Doormaal2
1 Agriculture and Agri-Food Canada, Guelph, ON, Canada N1G 4T2
2 Canadian Dairy Network, Guelph, ON, Canada N1G 4T2
Corresponding author: A. Sewalem; e-mail: sewalem{at}cdn.ca.
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ABSTRACT
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The aim of this study was to use a Weibull proportional hazards model to explore the impact of type traits on the functional survival of Canadian Jersey and Ayrshire cows. The data set consisted of 49,791 registered Jersey cows from 900 herds calving from 1985 to 2003. The corresponding figures for Ayrshire were 77,109 cows and 921 herds. Functional survival was defined as the number of days from first calving to culling, death, or censoring. Type information consisted of phenotypic type scores for 8 composite traits and 19 linear descriptive traits. The statistical model included the effects of stage of lactation; season of production; annual change in herd size; type of milk recording supervision; age at first calving; effects of milk, fat, and protein yields calculated as within herd-year-parity deviations; herd-year-season of calving; each type trait; and the animal’s sire. Analysis was done one trait at a time for each of 27 type traits in each breed. The relative culling risk was calculated for animals in each class after accounting for the previously mentioned effects. Among the composite type traits with the greatest contribution to the likelihood function was final score followed by mammary system for Jersey breed, while in Ayrshire breed feet and legs was the second most important trait next to final score. Cows classified as Poor for final score in both breeds were >5 times more likely to be culled compared with the cows classified as Good Plus. In both breeds, cows classified as Poor for feet and legs were 5 times more likely to be culled than were cows classified as Excellent, and cows classified as Excellent for mammary system were >9 times more likely to survive than were cows classified as Poor.
Key Words: survival • Jersey • Ayrshire • type trait
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INTRODUCTION
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In dairy production, longevity is a highly important trait that considerably affects overall profitability. With increased longevity, the mean production of the herd increases because a greater proportion of the culling decisions are based on production and the proportion of mature cows, which produce more milk than do young cows, is increased (Allaire and Gibson, 1992; VanRaden and Wiggans, 1995). However, genetic improvement of longevity traits is very hard to achieve because of their low heritability. Heritability estimates range from 0.03 to 0.05 (Van Doormaal et al., 1985; Jairath et al., 1998) using a linear model, and estimates from survival analysis using Weibull proportional hazards models range from 0.10 to 0.20 (Ducrocq, 2002; Roxstrom et al., 2003; Sewalem et al., 2005). Moreover, one must wait for the animal or its relatives to leave their respective herds before obtaining a direct measurement of longevity.
Type traits have been used as indirect selection criteria for herd life (Short and Lawlor, 1992; Dekkers et al., 1994; VanRaden and Wiggans, 1995; Weigel et al., 1998; Cruickshank et al., 2002). Type traits are recorded relatively early in life, most often in the first lactation, and are more highly heritable than longevity (Kadarmideen and Wegmann, 2003), which makes selection on them relatively more efficient. To obtain reliable information for sires regarding the longevity of their daughters directly, it is necessary to wait until a minimum number of daughters are culled or die. Normally, these evaluations are available too late to be useful in breeding programs. Hence, genetic evaluations for direct longevity information based on number of culled cows should be combined with indirect information based on early predictors such as type traits. Knowledge of genetic relationships between type and longevity are required and, therefore, a proper identification of type traits to be used as early predictors is essential. In Canada, the breeding goal has emphasized high production combined with superior conformation to support such production levels (Boettcher et al., 1999) and considerable improvement on type traits has been made compared with other countries (Schneider et al., 2003; Sewalem et al., 2004). Because the value of type classification is seen by producers to be important for herd management, sire selection, and marketing purposes, they are collected routinely for these purposes, and their use as an indirect predictor for longevity can also be very cost effective.
Survival analysis using a Weibull proportional hazards model can offer a better fit to survival data than can linear models because of its ability to account for censored records properly (Ducrocq et al., 1988). This feature may increase precision by accounting for differences in days of productive life among cows that survive for the same number of lactations. Survival analysis also accounts for the skewed distribution of survival data. Time-dependent variables can be used for the survival analysis to model the effects of environmental factors accurately.
Jersey and Ayrshire breeds comprise the 2 largest dairy cattle populations in Canada next to the Holstein breed. The type classification system used in these 2 breeds is also different from that of the Holstein breed. Dekkers et al. (1994), Boettcher et al. (1997), Schneider et al. (2003), and Sewalem et al. (2004) have studied the relationship between type traits and survival in Canadian Holstein cows, but no such study exists for Canadian Jersey and Ayrshire breeds. Therefore, the objective of the present study was to evaluate the impact of linear descriptive type traits on functional longevity in the Canadian Jersey and Ayrshire breeds.
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MATERIALS AND METHODS
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Data consisted of 49,791 cows in 900 herds from 2041 sires for Jerseys and 77,109 cows in 921 herds from 2421 sires for Ayrshires. Data were obtained from lactation and type classification records extracted for the Canadian May 2003 genetic evaluation. In both breeds, all animals were herd book-registered and were required to have a type record. Length of productive life was defined as time (days) from one calving to the next calving, death, or culling. Censored records represented cows being sold for dairy purposes, exported, or leased to another herd or cows still in the herd. A lifetime record was considered to be completed (uncensored) if the cow received a termination code, indicating the reason for which it was removed from the herd. For cases in which cows completed a given lactation but failed to initiate the next lactation, cows were considered culled if the latest recorded test day was >6 mo before the end of the study. Records associated with missing sire identification, incorrect calving dates, and age at first calving outside of the range 18 to 40 mo were excluded from the analysis.
Type information consisted of phenotypic type scores of 19 linear descriptive traits (Table 1
) evaluated on 1-to 9-point scale and 8 composite traits. The composite traits were final score, mammary system, feet and legs, dairy character, rump, capacity, fore udder, and rear udder. These traits were evaluated on a scale with 18 categories ranging from Poor-1, Poor-2, and Poor-3 to Excellent-1, Excellent-2, and Excellent-3. However, in the present study only the 6 major categories (Poor, Fair, Good, Good Plus, Very Good, and Excellent) were used because of low numbers of observations in some of the extreme categories. Composite traits were directly calculated from the linear traits by weighing each linear score according to existing relative weights for each breed (Russell, Jersey Canada, 2004, personal communication).
The following model was used:
where 
(t) is the hazard of a cow, i.e., her probability of being culled at time t given she survived up to t; 0,s(t) = 
( t)–1 is the Weibull baseline hazard function with scale parameter and shape parameter ; t is the time in days from one calving to the next calving for each stratum; ß contains the possible time-dependent covariates affecting the hazard, with x’m(t) being the corresponding design vectors; and u is a vector of random variables with associated incidence vector z’m.
The fixed covariates included in the model were as follows: time-dependent effect of stage of lactation in days (1 = 0 to 80; 2 = 81 to 235; 3 >235); effect of year and season of calving (years of calving ranged from 1985 to 2003; seasons of calving were January to March, April to June, July to September, and October to December); effect of season of production with the same definition as seasons of calving; effect of annual change in herd size with 3 classes (decreasing = a decrease in herd size of >5%, nearly unchanged = a change ranging from a decrease of up to 5% to an increase of up to 10%, and increasing = an increase in herd size of >10%); effect of the type of milk recording supervision, with 3 classes (unsupervised, supervised, and unknown, i.e., records that do not fulfill the minimum criteria set by the milk recording agency); effect of age at first calving in months; and effects of milk, fat, and protein yields. The latter effects were calculated as within herd-year-parity deviations with 3 classes for each, low = cows producing >0.4 standard deviations below the herd-year-parity average, average = cows producing between 0.4 standard deviations below and 0.6 standard deviations above the herd-year-parity average, and high = cows producing >0.6 standard deviations of the herd-year-parity average. Each type trait (27 in total) was evaluated separately by including it as a covariate in the model.
The random effects included were the effect of herd-year-season class, which was assumed to follow a log gamma distribution, and the genetic effect of the cow’s sire, which was assumed to follow a multivariate normal distribution with mean zero and variance A
2s, where 2s is the variance among sires and A is the relationship matrix. Sire variance of 0.040 and 0.039 for Jersey and Ayrshire, respectively (Sewalem et al., 2005), were used in the analyses.
The analyses were performed using the Survival Kit Version 3.12 (Ducrocq and Solkner, 1998). One baseline hazard function 0,s(t) was defined for each lactation (subscript 0 designates a baseline hazard and subscript s relates to stratum s). Detailed description of the model and survival analysis of longevity data in dairy cattle on a lactation basis has been provided by Ducrocq (2002), Roxstrom et al. (2003), and Sewalem et al. (2005). The overall influence of each type trait on functional survival was assessed using the likelihood ratio test. To perform this test, the likelihood of the full model (with one particular type trait) was compared with the likelihood of the reduced model (without any of the type traits), following a procedure previously used by Larroque and Ducrocq (2001), Caraviello et al. (2003), and Schneider et al. (2003).
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RESULTS AND DISCUSSION
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Relative Contribution of Each Type Trait
In both breeds, all type traits included in this study had highly statistically significant (P < 0.001) associations with functional longevity. Figure 1 shows the relative contribution of each type trait (composite and linear traits) to the likelihood (–2logL). In all breeds, final score was by far the most important single trait with respect to longevity. In Jersey, final score was followed by mammary system, fore udder, rear udder, and dairy character (Figure 1). In Ayrshire, final score was followed by feet and legs, mammary system, rear udder, and dairy character. Among the linear type traits, udder traits had the strongest effect on the survival of cows (Figure 1). Fore attachment, udder texture, udder depth, rear attachment height, rear attachment width, and median suspensory were udder traits that had the strongest relationship with longevity of cows. Among the feet and legs traits, bone quality and foot angle had the greatest influence on functional survival. Size, thurl width, and stature had the least influence on the survival of Ayrshire cows; rump angle and rear legs side view were the least traits with the least influence on the survival of Jersey cows.
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Figure 1. Contribution of final score and each linear type trait to the likelihood of functional survival (as a percentage of the contribution of final score) for Canadian Jersey and Ayrshire breeds.
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The results for fixed effects were expressed in relative culling risk defined as the ratio of the estimated risk of being culled under the influence of certain environmental factors relative to the average risk (or reference risk), which is usually set to one. Values larger than one indicate higher culling risk associated with that environmental factor. Relative culling risks smaller than one indicate lower culling risks, i.e., a positive effect of that environmental factor on longevity. For example, if the relative culling risk for a given class is 2, it means that a cow in that class has twice the risk of being culled compared with a cow in the reference class for that effect. Conversely, if the relative culling risk for a given class is 0.5, then a cow in that particular class has 50% less chance of being culled than a cow in the reference class.
Composite Traits
Figure 2 shows a clear linear relationship between final score and longevity in Jersey and Ayrshire breeds. Cows classified with final score of Poor had a risk of being culled >5 times of that for cows classified Good Plus, whereas cows with final score of Excellent had a risk of being culled <0.5 times to that of cows classified as Good Plus. Final score was the most important type trait in both breeds. Similar results using the same methodology were also reported by Sewalem et al. (2004) for Canadian Holstein cows and by Caraviello et al. (2004) for US Holstein cows. Boettcher et al. (1997) used a linear regression approach and reported that overall conformation had the largest effect on survival through first lactation of Canadian Holsteins. Caraviello et al. (2003) reported, however, that final score was not the most important type trait that influences the functional longevity of US Jersey cows, which may suggest that the Jersey breeders in the US put less emphasis on final score relative to other type traits, such as udder traits, when compared with their Canadian counterparts.
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Figure 2. Relative risk of culling (RRC) by category for final score (RRC for category Good Plus was set to 1) for Canadian Ayrshires and Jerseys: minimum 25 uncensored failures per subclass.
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Figure 3 shows the relationship between relative risk of culling and mammary system, fore udder, and rear udder in Jersey cows. Mammary system was the second most important trait next to final score for functional longevity in this analysis. Cows classified as Poor were >3.5 times more likely to be culled compared with the reference score of cows classified as Good Plus. On the other hand, cows classified as Excellent had a risk of being culled that was 0.5 times less than that for cows in the reference category. Figure 3 also shows that cows with low scores (classified as Poor) for rear udder and fore udder were >3.5 times more likely to be culled compared with cows with intermediate scores (Good Plus). Figure 4 shows the relationship between relative risk of culling and mammary system, fore udder, and rear udder for Ayrshire cows. The relative risk of culling for these traits follows the same trend of that in Jerseys, except for slight differences in the relative risk of culling in each category. Sewalem et al. (2004), working on Canadian Holsteins, reported that cows classified as Poor for these traits were at greater risk of being culled compared with cows classified higher. A strong positive relationship between mammary system and longevity was also reported by Boettcher et al. (1997), Larroque and Ducrocq (2001), and Schneider et al. (2003).
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Figure 3. Relative risk of culling (RRC) by category for mammary system, fore udder, and rear udder (RRC for category Good Plus was set to 1) for Canadian Jerseys: minimum 25 uncensored failures per subclass.
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Figure 4. Relative risk of culling (RRC) by category for mammary system, fore udder, and rear udder (RRC for category Good Plus was set to 1) for Canadian Ayrshires: minimum 25 uncensored failures per subclass.
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Figures 5 and 6 show the relationship between relative risk of culling and feet and legs, rump, dairy character, and capacity for Jersey and Ayrshire breeds, respectively. Relative risk of culling for dairy character for Ayrhsires showed that cows classified as Poor were >3 times more likely to be culled compared with cows classified as Good Plus. In both breeds, feet and legs, rump, and capacity also showed a clear relationship with a cow’s survival. Cows classified as Excellent were associated with less risk of culling compared with cows classified as Poor. The relative risks of culling for Jersey cows classified as Poor for feet and legs and capacity was >2.7 and 3.3 times, respectively, more likely to be culled compared with that of cows classified as Good Plus (Figure 5). The corresponding ratios for Ayrshire cows were 3.3 and 2.6, respectively. A positive, strong relationship between feet and legs and capacity on longevity was also reported by Boettcher et al. (1997), Larroque and Ducrocq (2001), Schneider et al. (2003), and Sewalem et al. (2004).
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Figure 5. Relative risk of culling (RRC) by category for capacity, dairy character, feet and legs, and rump angle (RRC for Good Plus was set to 1) for Canadian Jerseys: minimum 25 uncensored failures per subclass.
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Figure 6. Relative risk of culling (RRC) by category for capacity, dairy character, feet and legs, and rump angle (RRC for Good Plus was set to 1) for Canadian Ayrshires: minimum 25 uncensored failures per subclass.
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Linear Type Traits
Udder traits.
Relative risk of culling for fore attachment, median suspensory ligament, rear attachment height, rear attachment width, and udder texture show a clear linear relationship with functional longevity, indicating that cows with high scores for these traits were more likely to last longer than were cows with low scores for these traits in both breeds (Table 2). On the other hand, udder depth exhibits an intermediate optimum. Cows with low and high scores for udder depth were more likely to be culled compared with cows with an intermediate score for this trait in both breeds (Table 2). Cows with extremely wide rear teat placement from the center of the quarter were >3.5 and 4.0 times more likely to be culled in Jersey and Ayrshire cows, respectively, compared with cows with center rear teat placement (Table 2). Moreover, cows with a linear score of 9 for this trait, representing teats that are close together, in both breeds, had better chance of survival than did cows with an intermediate score. However, they were more likely to be culled compared with cows with score of 8. Table 2 also shows that, in the Ayrshire breed, cows with nearly central fore teat placement were more likely to survive culling compared with cows with extremely inside or extremely outside fore teat placement, which has also been observed in other studies (Caraviello et al., 2003; Schneider et al., 2003; Sewalem et al., 2004). However, in the Jersey breed, fore teat placement showed a clear linear association with longevity, indicating that cows with wide fore teat placement were more likely to be culled compared with cows with ideal placement or teats that are close together.
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Table 2. Relative culling rates of cows for udder traits (relative culling rate for score 5 was set to 1) for Ayrshire (AY) and Jersey (JE) breeds: minimum 25 uncensored failures per subclass.
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In both breeds, udder traits had an important influence on the culling decisions. This may be due to their influence on the reduced susceptibility to mastitis and other infectious diseases. Cows with extremely closely placed rear teats were more likely to be culled compared with cows with extremely wide rear teats, a trend that was also found by Buenger et al. (2001), Larroque and Ducrocq (2001), Schneider et al. (2003), and Sewalem et al. (2004). Cows with extremely close rear teats may be problematic for farmers because they present more difficulty in placing the teat cups. In extreme cases, the rear teats can often be so close that they may cross each other. In addition, Caraviello et al. (2003) reported an intermediate optimum for rear udder height and rear udder width that was not found in this study. This might be due to breed differences or trait definitions. Low scores for median suspensory or depth of cleft were associated with higher risk of culling than the other scores. Cows with extremely fleshy udders were also more likely to be culled compared with cows with soft, pliable, elastic udders that collapse well after milking. Cows with fleshy udders are more difficult to achieve complete milkout. This may be related to a higher incidence of mastitis or other disease. Cows with extremely deep or extremely shallow udder depth from hock to the floor of the udder were also associated with higher risk of being culled.
Body and dairy character traits.
Results regarding the impact of body and dairy character traits (style, chest width, stature, size, and head) on the functional longevity of cows are presented in Table 3. In both breeds, cows with extremely desired smooth blending of parts and breed character showed a better chance of surviving than those with extremely undesired blending of parts and breed character. Moreover, cows that were extremely short and small with a narrow chest had a higher risk of being culled compared with cows with an intermediate class and, hence, a shorter length of productive life. Sewalem et al. (2004) observed that Holstein cows with score of 9 were essentially equivalent to a score of 2, indicating that extremely large cows have almost a similar relative risk of culling as extremely small cows. This trend was not observed in Jersey and Ayrshire cows. A moderate positive relationship between longevity and body type traits was reported by Boettcher et al. (1997). Size and stature did not have a strong relationship with functional survival in studies by Buenger et al. (2001) and Caraviello et al. (2003). Mahoney et al. (1986) also reported that larger cows were more predisposed to displaced abomasums than small cows. Rogers et al. (1999) did not find a significant difference that indicated genetically larger body size or more strength would be advantageous from a disease perspective. On the other hand, Schneider et al. (2003) found taller and bigger cows had better chances of surviving than cows in other classes. Table 3 also shows a clear linear relationship between head and functional longevity for both breeds. Cows with narrow and short heads were >2 times more likely to be culled compared with cows with wide and long heads with desired breed character.
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Table 3. Relative culling rate of cows for body type traits (relative culling rate for score 5 was set to 1) for Ayrshire (AY) and Jersey (JE) breeds: minimum 25 uncensored failures per subclass.
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Rump traits.
The relationships between risk of relative culling and linear score for loin strength, thurl width, and rump angle are shown in Table 4. In both breeds, cows with weak loins showed a higher risk of being culled compared with cows with strong strength of vertebrae between back and rump. Sewalem et al. (2004) also reported that Holstein cows with strong loins had a better chance of surviving compared with cows with weak loins. Table 4 also shows that cows with extremely narrow thurls were at higher risk of being culled compared with cows with intermediate thurl width. However, cows with extremely wide thurls (score 9) did not have a better chance of surviving than did cows with score of 7 or 8. Sewalem et al. (2004) did not find a strong relationship between pin width and relative culling risk in Holstein cows, except that cows with extremely narrow pins were 1.18 times more likely to be culled compared with cows with score 5. With respect to rump angle, cows with low or high scores for rump angle (representing extremely high or low pin bones relative to the height of hook bones, respectively) were more likely to be culled compared with the intermediate score of 5, indicating that rump angle is an intermediate optimum trait. Rump traits may be associated with calving difficulty, and cows with intermediate rump angle are preferred (Cue et al., 1990). Similar findings were reported by Buenger et al. (2001) and Schneider et al. (2003). Moreover, Rogers et al. (1999) found a favorable, moderately positive genetic correlation of 0.39 between rump angle and foot and leg diseases. However, Caraviello et al. (2003, 2004) did not find strong relationships between rump traits and longevity in US Holstein and Jersey cows.
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Table 4. Relative culling rate of cows for rump and feet and legs traits (relative culling rate for score 5 was set to 1 for Ayrshire (AY) and Jersey (JE) breeds: minimum 25 uncensored failures per subclass.
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Feet and legs.
Table 4
also shows the relationship between the risk of relative culling and linear score for feet and legs traits. There was a clear linear relationship for bone quality and foot angle with relative culling risk in both breeds. Cows with extremely coarse bone structure were more likely to be culled compared with cows with extremely flat bones. The relationship between degree of curvature of the rear legs viewed from side with longevity showed an intermediate optimum, indicating that cows with extremely straight legs or extremely curved legs were more likely to be culled compared to the intermediate cows with respect to this trait. The importance of feet and legs traits to the functional survival of cows found in the present study corroborates the findings of Boettcher et al. (1997), Buenger et al. (2001), and Caraviello et al. (2004). Extremely coarse bone, extremely low foot angle, and extremely straight or curved rear legs led to a decreased functional survival of cows.
The present study also showed differences in the relative importance of type traits to functional longevity between Canadian Holsteins (Sewalem et al., 2004) and Canadian Jerseys and Ayrshires. For instance, in Jersey and Ayrshire cows, breed characteristics and style had more influence on survival of cows than observed in Holstein cows. Size and stature had nearly negligible influence on the survival of Holstein cows, but in Jersey and Ayrshire cows, these traits contribute a significant amount of information to the culling decisions of cows. Even so, in both breeds, final score and udder traits were the most important traits that influenced functional longevity of Canadian dairy breeds.
Results from this study are from a model that includes a sire genetic effect in both breeds. Buenger et al. (2001) and Larroque and Ducrocq (2001) argued that inclusion of the genetic effect in the model would bias the effect of type trait’s phenotype effect on culling risks. However, in our opinion, similar to analysis of other traits such as production, correcting known causes of effects for a trait is a worthwhile consideration, and inclusion of the genetic effects in the model eliminates any sort of bias that might arise in the process of culling. However, Schneider et al. (2003) and Sewalem et al. (2004) found no differences between phenotypic and genetic analysis of type traits and its impact on functional survival of cows. In this study, a phenotypic analysis of each trait (excluding the sire effect in the model) was also analyzed, and results were compared with the present study (not shown). There were no changes in the significance tests. The absence of clear differences between the 2 models (excluding and including the genetic effect in the model) might have been due to a dominance of the phenotypic influence on survival; thus, including the genetic effect did not have much impact.
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CONCLUSIONS
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Survival analysis was used to investigate the effect of type traits on functional longevity in Canadian Jersey and Ayrshire cows. Based on the results presented herein, using herd book-registered cows, final score had the greatest contribution to the hazard function, or instantaneous risk of culling, in both breeds. Mammary system and feet and legs also had strong relationships with functional longevity. All composite type traits showed a linear relationship with functional longevity, indicating that cows classified as Excellent for these traits were found to survive longer than cows classified lower. Among the linear type traits, udder traits such as fore attachment, udder texture, and udder depth were the most important with a strong relationship with functional survival of cows. Udder depth, rump angle, and rear legs side view showed an intermediate optimum with regard to functional longevity. Improvement of type traits either through genetics or management would have a positive influence on the functional longevity of cows. Therefore, by choosing the right type traits, the functional herd life of cows can be effectively predicted prior to the availability of actual longevity data for daughters of progeny test bulls.
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ACKNOWLEDGEMENTS
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Appreciation is extended to Vincent Ducrocq for providing the Survival Kit V3.12 software and his valuable assistance throughout the study
Received for publication August 12, 2004.
Accepted for publication November 25, 2004.
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REFERENCES
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Allaire, F. R., and J. P. Gibson. 1992. Genetic value of herd life adjusted for milk production. J. Dairy Sci. 75:1349–1355.[Abstract]
Boettcher, P. J., L. K. Jairath, and J. C. M. Dekkers. 1999. Genetic evaluation of herd life in Canada: Current status and future outlook. In Proc. Workshop on Genetic Improvement of Functional Traits in Cattle (GIFT), Jouyen-Josas, France. Interbull Bull. 21:23–30.
Boettcher, P. J., L. K. Jairath, K. R. Koots, and J. C. M. Dekkers. 1997. Effects of interactions between type and milk production on survival traits of Canadian Holsteins. J. Dairy Sci. 80:2984–2995.[Abstract]
Buenger, A., V. Ducrocq, and H. H. Swalve. 2001. Analysis of survival in dairy cows with supplementary data on type scores and housing systems from a region of northwest Germany. J. Dairy Sci. 84:1531–1541.[Abstract]
Caraviello, D. Z., K. A. Weigel, and D. Gianola. 2003. Analysis of the relationship between type traits, inbreeding, and functional survival in Jersey cattle using a Weibull proportional hazards model. J. Dairy Sci. 86:2984–2989.[Abstract/Free Full Text]
Caraviello, D. Z., K. A. Weigel, and D. Gianola. 2004. Analysis of the relationship between type traits and functional survival in Holstein cattle using a Weibull proportional hazard model. J. Dairy Sci. 87:2677–2686.[Abstract/Free Full Text]
Cruickshank, J., K. A. Weigel, M. R. Dentine, and B. W. Kirkpatrick. 2002. Indirect prediction of herd life in Guernsey dairy cattle. J. Dairy Sci. 85:1307–1313.[Abstract]
Cue, R., H. Monardes, and J. F. Hayes. 1990. Relationships of calving ease with type traits. J. Dairy Sci. 73:3586–3590.[Abstract]
Dekkers, J. C. M., L. K. Jairath, and B. H. Lawrence. 1994. Relationships between sire genetic evaluations for conformation and functional herd life of daughters. J. Dairy Sci. 77:844–854.[Abstract]
Ducrocq, V. 2002. A piecewise Weibull mixed model for the analysis of length of productive life of dairy cows. 7th World Congr. Genet. Appl. Livest. Prod., August 19–23, 2002, Montpellier, France. Communication No. 20–04.
Ducrocq, V. P., R. L. Quaas, E. J. Pollak, and G. Casella. 1988. Length of productive life of dairy cows. 2. Variance component estimation and sire evaluation. J. Dairy Sci. 71:3071–3079.[Abstract/Free Full Text]
Ducrocq, V., and J. Solkner. 1998. The Survival Kit version (V3.12)—A Fortran package for the analysis of survival data. Proc. 6th World Congr. Genet. Appl. Livest. Prod., Armidale, Australia, 27:447–448.
Jairath, L., J. C. M. Dekkers, L. R. Schaeffer, Z. Liu, E. B. Burnside, and B. Kolstad. 1998. Genetic evaluation for herd life in Canada. J. Dairy Sci. 81:550–562.[Abstract]
Kadarmideen, H. N., and S. Wegmann. 2003. Genetic parameters for body condition score and its relationship with type and production traits in Swiss Holsteins. J. Dairy Sci. 86:3685–3693.[Abstract/Free Full Text]
Larroque, H., and V. Ducrocq. 2001. Relationship between type and longevity in the Holstein breed. Genet. Sel. Evol. 33:39–59.[Medline]
Mahoney, C. B., L. B. Hansen, C. W. Young, G. D. Marx, and J. K. Reneau. 1986. Health care of Holsteins selected for larger and smaller body size. J. Dairy Sci. 69:1922–1931.
Rogers, G. W., G. Banos, and U. Sander-Nielsen. 1999. Genetic correlations among protein yield, productive life, and type traits from the United States and diseases other than mastitis from Denmark and Sweden. J. Dairy Sci. 82:1331–1338.[Abstract]
Roxstrom, A., V. Ducrocq, and E. Strandberg. 2003. Survival analysis of longevity in dairy cattle on a lactation basis. Genet. Sel. Evol. 35:305–318.[Medline]
Schneider, M. del P., J. W. Durr, R. I. Cue, and H. G. Monardes. 2003. Impact of type traits on functional herd life of Quebec Holsteins assessed by survival analysis. J. Dairy Sci. 86:4083–4089.[Abstract/Free Full Text]
Sewalem, A., G. J. Kistemaker, V. Ducrocq, and B. J. Van Doormaal. 2005. Genetic analysis of herd life in Canadian dairy cattle on a lactation basis using a Weibull Proportional Hazard Model. J. Dairy Sci. 88:368–375.[Abstract/Free Full Text]
Sewalem, A., G. J. Kistemaker, F. Miglior, and B. J. Van Doormaal. 2004. Analysis of the relationship between type traits and functional survival in Canadian Holstein dairy cattle. J. Dairy Sci. 87:3938–3946.[Abstract/Free Full Text]
Short, T. H., and T. J. Lawlor. 1992. Genetic parameters for conformation traits, milk yield and herd life in Holsteins. J. Dairy Sci. 75:1987–1998.[Abstract]
Van Doormaal, B. J., L. R. Schaeffer, and B. W. Kennedy. 1985. Estimation of genetic parameters for stayability in Canadian Holsteins. J. Dairy Sci. 68:1763–1769.
VanRaden, P. M., and G. R. Wiggans. 1995. Productive life evaluations: Calculation, accuracy and economic value. J. Dairy Sci. 78:631–638.[Abstract]
Weigel, K. A., T. J. Lawlor, Jr., P. M. VanRaden, and G. R. Wiggans. 1998. Use of linear type and production data to supplement early predicted transmitting abilities for productive life. J. Dairy Sci. 81:2040–2044.[Abstract]
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ABSTRACT
INTRODUCTION
