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ANIMAL PRODUCTION |
Department of Animal Science, University of California, Davis 95616
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Abstract |
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 Top Abstract INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: age • birth weight • calving interval • cattle • pregnancy rate • weaning weight
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INTRODUCTION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Kress et al., 1990). However, because of the difficulty in obtaining adequate numbers of cows at older ages, those attempting to examine effects of age have resorted to grouping older cows or have restricted this comparison to multi- vs. primiparous. Because herd age profiles may vary from year to year, good herd management should include an understanding of the effects of age on pregnancy rate, calving interval, birth weight, and weaning weight.
Browning et al. (1994) and Strauch et al. (2001) showed that primiparous cows have longer postpartum intervals and lower pregnancy rates than multiparous cows. Kress et al. (1990) and van Oijen et al. (1993) showed that cows older than 4 yr had a greater calving rate than younger cows.
Notter et al. (1978) and Doornbos et al. (1984) indicated that multiparous cows birthed heavier calves than heifers. Moreover, Nelson and Beavers (1982) and Marshall et al. (1990) showed that 3-yr-old cows gave birth to lighter calves than older cows.
It is well accepted that heifers wean lighter calves than multiparous cows (Bellows and Short, 1978; Doornbos et al., 1984; Browning et al., 1994) and that 3-yr-old cows wean lighter calves than older cows. Yet, little evidence suggests an effect among cows older than 3 yr.
The objectives of this study were 2-fold. The first objective was to evaluate effects of age on pregnancy rate, calving interval, birth weight, weaning weight, and BCS within a multiparous cow herd. In addition, because most studies that have evaluated effects of age have dismissed the correlation between BCS and age, our second objective was to determine whether the association of age with pregnancy rate, calving interval, birth weight, and weaning weight was the result of correlated changes in BCS.
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MATERIALS AND METHODS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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. The study was initiated with 260 multiparous cows of British breed types (119, 67, 15, 18, 12, 12, 9, and 8 cows of age 3 to 10 yr, respectively). Cow herd management was consistent with typical production practices in California. Animals were culled when they failed to become pregnant, weaned a calf at 10 yr of age, or were transferred to another study. Replacement animals entered the study at first calving (2 yr of age), and 45, 54, 27, and 68 animals were added in yr 2 through 5, respectively.
Data collection began at weaning in May 1994. Subsequently, all animals were monitored for BW and BCS. Quarterly measurements were taken on the same date for all animals. These dates coincided with the production calendar to include times immediately before calving (late October to early November), during the breeding season (late January to early February), at weaning (mid May to mid June), and midway through the second trimester of pregnancy (August). Two observers estimated BCS using palpation (1 = severely emaciated to 9 = extremely fat; Richards et al., 1986). Both observations were averaged, rounded to the nearest 0.5 units, and recorded. The same observers were used throughout the entire study to avoid possible bias.
During the first 3 yr of the study, backfat thickness was measured by ultrasound at the same time as the quarterly measurements of BW. Ultrasound was performed at the 12th intercostal space using an Aloka 500V with a 17-cm, 3.5-MHz probe (Aloka Co. Ltd., Wallingford, CT).
Birth weight was taken within 48 h of parturition, at which time the calf was tagged and the navel dipped in iodine. Weaning weight, measured in mid May to mid June, was adjusted by sex and for age to 205 d (BIF, 2002). Pregnancy was determined by rectal palpation 60 d after removal of the bull. Calving interval was calculated as the difference between calving dates.
Statistical Analysis
Because age profiles among nutritional treatments (i.e., supplementation strategy) were not uniform, nutritional treatment was included in the model as a blocking variable. Pregnancy rate was analyzed using the CATMOD procedure of SAS (SAS Inst. Inc., Cary, NC) with a 2-way classification model, using age and supplementation strategy as the independent variables. Subsequently, BCS at breeding (shown to most significantly affect pregnancy rate, data not shown) was added to the model as a third independent variable to test age effects, independent of BCS.
Two repeated measures models, employing PROC MIXED of SAS, were used to analyze the effects of age on birth weight, weaning weight, and calving interval. The first model was also used to evaluate BW, BCS, and backfat thickness, as follows:
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where Yijklmn is an observation n, on the individual m (m = 1, 2, ..., nijklm) of the age i (i = 3, ..., 10), in the production year j (j = 1, 2, ...7), with the stocking rate k (k = moderate, high), and the supplementation strategy l (l = standard, none, strategic); µ is a constant common to all observations; 
i is the fixed effect common to cows of the age i; ßj is the random effect common to cows of the production year j; 
k is the fixed effect common to cows of the stocking rate k; 
l is the fixed effect common to cows of the supplementation strategy l; 
kl is the fixed interaction between the stocking rate k and the supplementation strategy l; 
ij is the random interaction of the age i and the production year j; 
m(kl) is the random effect common to the cow m of the stocking rate k and supplementation strategy l; and eijklmn is a random residual specific to the observation jklmn with null mean and variance 
2.
The second model included BCS as a blocking variable to evaluate the fixed age effect on calving interval, birth weight, and weaning weight, independent of BCS differences among age groups. Because BCS at calving most significantly affected calving interval and birth weight, and BCS at breeding most affected weaning weight (data not shown), BCS at calving, calving, and breeding were used for analysis of calving interval, birth weight, and weaning weight, respectively. If the main effect of age was significant, differences among least squares means were analyzed by t-test using the PDIFF option in PROC MIXED.
Body condition score and production measurements for each cow were corrected for the random effect of production year and the fixed effect of supplementation strategy before regression analyses. Adjustment factors for production year and supplementation strategy were determined using the solutions function in PROC MIXED. PROC REG in SAS was used to perform quadratic and linear ridge regressions of the adjusted values for BCS, calving interval, birth weight, and weaning weight on age. Regression equations are reported for those with an R2 > 0.20.
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RESULTS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Age affected cow BW at each of the quarterly measurements (P < 0.001). Body weight (kg) at calving (Y1) and breeding (Y2) were greatest at 8 yr old (X) and lowest at 3 yr (Y1 = –5.18X2 + 73.85X + 302.53, R2 = 0.369 and Y2 = –4.32X2 + 64.41X + 241.07, R2 = 0.384). At calving, 4-yr-old cows weighed 36 ± 2.5 kg more and 33 ± 2.6 kg less than 3- and 5-yr-old cows, respectively (Table 1). Six-, seven-, eight-, and nine-year-old cows weighed more than 5-yr-old cows (P = 0.003). However, BW loss as animals reached 10 yr old resulted in no difference in BW between 5- and 10-yr-old cows (P = 0.47). Eight-year-old cows weighed 16 ± 4.4, 11 ± 4.2, and 21 ± 10.1 kg more than 6-, 7-, and 10-yr-old cows (P < 0.001, P = 0.01, and P = 0.03, respectively). Because animals of all age groups lost BW from calving to breeding, the weight-age profile observed at calving remained similar at breeding. At weaning, 10-yr-old cows weighed the most numerically (555 kg), whereas 3-yr-old cows numerically weighed the least (493 kg, BW = –4.95X2 + 70.07X + 326.1, R2 = 0.304). Growth from 3 to 5 yr old caused differences of 28 ± 2.5, 46 ± 3.1, and 18 ± 2.8 kg between 3- and 4-, 3- and 5-, and 4- and 5-yr-old cows, respectively. Six- and seven-year-old cows weighed 12 ± 3.8 kg more than 5-yr-old cows (P = 0.001), but contrasts of BW between dams ages 6 to 10 yr were nonsignificant (P > 0.20). In August, 8-yr-old cows numerically weighed the most (558 kg) with a minimum BW for 3-yr-old cows (482 kg, BW = –4.37X2 + 65.63X + 323.64, R2 = 0.337). Again 3-, 4-, and 5-yr-old cows were all of different BW (P < 0.001). Nine- and ten-year-old cows had BW not different than 5-yr-old cows (P = 0.08 and 0.53, respectively).
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Ultrasound backfat thickness followed a pattern nearly identical to that seen with BCS. The trend was for backfat thickness to be lower in 3-and 4-yr-old cows, peak when cows were 8 yr old, and decline from 8 to 10 yr old. However, BCS and backfat thickness at weaning tended to reach a maximum earlier (5 and 6 yr old, respectively) and subsequently plateau to 10 yr old.
Because of the colinearity of age and BCS, production parameters were evaluated using 2 models. The first model evaluated age effects without adjustment; the second used BCS as a blocking variable to test age effects on production parameters independent of BCS.
In the first model, when age and supplementation strategy were the only main effects in the model, pregnancy rate was related to age (P = 0.07; Table 2) in a quadratic fashion with a maximal pregnancy rate at 6 yr old (0.87 ± 0.03) and a minimum at 10 yr old (0.57 ± 0.14). Pregnancy rate rose linearly from 3 to 5 yr old, remained above 80% to 8 yr old, and declined linearly in 8- to 10-yr-old cows. This effect of age on pregnancy rate seems to have been the result of the differences in BCS among the age groups; the inclusion of BCS at breeding in the model diminished the effect of age on pregnancy rate (P = 0.42).
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. The calving interval was longer for 3-yr-old cows than cows 4- to 9-yr-old (P = 0.02). Calving interval was 7 ± 3.2 and 6 ± 3.2 d shorter for 5- and 6-yr-old cows than for 7-yr-old cows. No other differences in calving interval were manifest through cow age. It should be mentioned that the inclusion of BCS in the model removed the differences between 3-yr-old cows and cows older than 6 yr (P > 0.09).
Birth weight of calves from 3-yr-old cows was not different to those from 9- and 10-yr-old cows (P = 0.17 and 0.34, respectively) but different from all other ages (P = 0.002). Calves of 4-yr-old cows also had lower birth weights than 5-, 6-, 7-, and 8-yr-old cows (P = 0.01). Eight-year-old cows birthed the numerically heaviest calves; calves born to 9-yr-old cows weighed 2 ± 0.9 kg less (P = 0.02).
Ten-year-old cows weaned lighter calves than cows of all other age groups (P = 0.003), 18 ± 6.6 kg less than calves of 3-yr-old cows (P = 0.006), and 27 ± 6.6 kg less than calves of 4-yr-old cows (P < 0.001). Three-year-old cows weaned calves 9 ± 2.1 and 14 ± 2.4 kg less than 4- and 5-yr-old cows, respectively (P < 0.001 and P < 0.001), and weaning weight was 5 ± 2.3 kg greater in calves born to 5-yr-old cows than in calves of 4-yr-old cows (P = 0.02). However, 4-yr-old cows weaned calves not different in weight to those weaned from 6-, 7-, 8-, and 9-yr-old cows (P > 0.10).
Both the linear and quadratic terms were significant in each of the regression models of production parameters on age (P = 0.04). Yet, because of a poor fit, these regressions were of little predictive value (R2 = 0.06, 0.11, and 0.07 for calving interval, weaning weight, and birth weight, respectively).
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DISCUSSION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Cow BW at calving, breeding, and in August were numerically lowest at 3-yr-old, peaked at 8-yr-old, and declined thereafter. Many others have shown that BW changes with cow age; however, the age at which mature BW is reached and the existence of a subsequent decline in BW are points of contention. Cundiff et al. (1966), Northcutt et al. (1992), and Tennant et al. (2002) showed that BW plateaued at 5 to 6 yr old. Furthermore, Northcutt et al. (1992) and Tennant et al. (2002) showed that BW declined in cows 11 yr and older. In contrast to the present findings and those of Northcutt et al. (1992) and Tennant et al. (2002), Cundiff et al. (1966) showed no decrease in BW up to 13 yr old. Nadarajah et al. (1984) showed that British breed cattle can reach mature BW as early as 4.5 yr. However, Marlowe and Morrow (1985) and Choy et al. (2002) showed similar results to this study reporting mature BW at 8 and 7 yr old, respectively. However, neither Marlowe and Morrow (1985) nor Choy et al. (2002) described the subsequent decrease in BW observed in this study.
Peak BCS and backfat thickness were closely linked with BW. Body condition score was numerically greatest at age 8 yr at calving, breeding, and in August. Peak BCS at weaning was reached at 7 yr old but was not different among cows age 5 to 10 yr. Despite a similar age to mature BW, Marlowe and Morrow (1985) found that BCS plateaued at 5 yr old. Showing a closer link between BCS and BW, Choy et al. (2002) found BCS plateaued from 6 to 8 yr old. The close link between BW and BCS in our study largely validates the designated BCS. However, the finding by Marlowe and Morrow (1985) that BCS plateaued earlier than BW does not invalidate their BCS measure, but may merely suggest that cows in that study fattened before reaching mature muscle mass.
Because adequate reproductive performance is essential to profitability, the effect of age on pregnancy rate is financially significant. Doornbos et al. (1984), Browning et al. (1994), and Strauch et al. (2001) reported that multiparous cows have greater pregnancy rates than heifers. However, those studies did not evaluate pregnancy rate changes among multiparous cows. Kress et al. (1990) and van Oijen et al. (1993) evaluated calving rates among young multiparous cows and found that cows greater than or equal to 5 yr old had a greater calving rate than younger cows. However, Kress et al. (1990) found no difference in the calving rates of 3- and 4-yr-old cows, whereas van Oijen et al. (1993) showed a small increase in calving rate as cows increased in age from 3 to 4 yr. Unfortunately, few studies have evaluated effects of age on pregnancy rates of older multiparous cows or avoided grouping older cows for analysis. The current study demonstrates a quadratic effect of age on pregnancy rate driven by a dramatic decrease in reproductive performance of 10-yr-old cows and provides evidence that, in this production system, cows beyond 9 yr old should be culled. However, because age was not a significant determinant of pregnancy rate when BCS at breeding was included in the model, our analyses suggest that the effect of age is associated with the concurrent decrease in BCS at breeding of older cows. Therefore, reproductive success may be sustained in older cows if adequate nutrition is provided to maintain BCS.
Age affected calving interval independent of BCS. Cows that were 3 yr old had the longest calving interval. The increased calving interval between the first calving at 2 yr old and calving at 3 yr of age is likely because of the increased dystocia and delayed uterine involution associated with the first birthing. Doornbos et al. (1984) showed that heifers have a pelvic area that is much smaller (80.6 cm2) than that found in mature cows (ages 4 to 7 yr). In turn, calving difficulty is greater in heifers than in cows (Nelson and Beavers, 1982; Doornbos et al., 1984; Gregory et al., 1991). Unfortunately, we did not monitor calving difficulty, but this may explain the much larger calving interval shown by our cows calving at 3 yr old. After 3 yr of age, the results indicate very little change in calving interval. The apparent increase in calving interval between calving at 6 and 7 yr old may be an artifact of a controlled breeding season. Breeding cows consistently earlier at ages 3, 4, and 5 yr would move onset of estrus in these animals earlier into the breeding season. If the previous conception date was early, one could hypothesize that the onset of estrus the following year may occur before the introduction of the bull. Therefore, our measure of an increased calving interval at 7 yr old may be a result of a restrictive breeding season.
Multiple studies looking at the effect of age on birth weight have shown that multiparous cows birth heavier calves than primiparous cows (Notter et al., 1978; Doornbos et al., 1984; van Oijen et al., 1993). However, evidence of an effect amongst multiparous cows has been more equivocal. Trail et al. (1982), Nadarajah et al. (1984), and Kress et al. (1990) showed that multiparous cow age does not affect birth weight. However, 3-yr-old cows have been shown to birth lighter calves than older dams (Nelson and Beavers, 1982; Marshall et al., 1990; Barkhouse et al., 1998). This study shows not only that 3-yr-old cows birth lighter calves than 4- to 8-yr-old cows but also that 4-yr-old cows birth lighter calves than 5-to 8-yr-old cows. Furthermore, this study describes a decrease in birth weight of calves born to 9-and 10-yr-old cows. Although these differences in birth weight are significant, the magnitude of the change is small and likely insignificant to the producer.
Multiparous cows have been shown to wean heavier calves than primiparous cows (Burfening et al., 1978; Doornbos et al., 1984; Barkhouse et al., 1998), and a great deal of evidence suggests that 3-yr-old cows wean lighter calves than older dams (Cardellino and Frahm, 1961; Marshall et al., 1990; Minick et al., 2001). Yet, among calves born to cows of 4 yr or older there is much less evidence of an age relationship with weaning weight. Neither Burfening et al. (1978) nor Kress et al. (1990) showed a difference between calves born to 4-or 5-yr-old cows. Furthermore, Lubritz et al. (1989) showed no weaning weight differences among calves birthed by dams 4 to 10 yr old. However, similar to the results in this study, Trail et al. (1982) and Nadarajah et al. (1984) showed that 4-yr-old cows wean lighter calves than older cows. In addition, Trail et al. (1982) and Nadarajah et al. (1984) showed that 9- and 11-yr-old cows, respectively, weaned lighter calves than cows older than 4 yr. These results compare well with the decreased weaning weight expressed by calves born to 10-yr-old cows in the current study.
Age was highly related to BW, BCS, and backfat thickness in this study. Furthermore, age was correlated with pregnancy rate, calving interval, birth weight, and weaning weight. By evaluating effects of age while controlling for BCS, this study implies an inherent effect of age on calving interval, weaning weight, and birth weight, but not on pregnancy rate. The significant effects of age allude to the importance that herd age profiles can have on year-to-year profitability.
1 Corresponding author: jwoltjen{at}ucdavis.edu
Received for publication December 16, 2005. Accepted for publication February 7, 2006.
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LITERATURE CITED |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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