Does dam parity affect the performance of ewe progeny born to 2-year-old ewes?

Transcription

1 CSIRO PUBLISHING Australian Journal of Experimental Agriculture, 2008, 48, Does dam parity affect the performance of ewe progeny born to 2-year-old ewes? P. R. Kenyon A,B, S. T. Morris A, H. T. Blair A and K. J. Stafford A A Institute of Veterinary, Animal and Biomedical Sciences, Massey University, Private Bag 11222, Palmerston North, New Zealand. B Corresponding author. p.r.kenyon@massey.ac.nz Abstract. An increasing number of ewe lambs are being presented for breeding in New Zealand, resulting in more two-tooth ewes of second parity compared with the more normal first parity. The effect of this on the resulting ewe progeny is unknown but is of interest as they are potential replacement ewes. This study was designed to determine if parity of Romney two-tooth ewes had any effect on their ewe lamb progeny to 20 months of age. Two-hundred and fifty female progeny born to two-tooth dams, which had lambed as a ewe lamb (n = 97), failed to conceive at ewe lamb breeding (n = 79), or had not been presented for breeding as a ewe lamb (n = 74), were weighed at regular intervals. The progeny also had onset of puberty and two-tooth breeding and pregnancy scanning performance measured. There were no effects of dam group on liveweight, condition score, 18-month breeding performance or pregnancy diagnosis. However, ewe lambs born to dams which failed to conceive as ewe lambs themselves were less likely (P < 0.05) to achieve puberty. In comparison with single-born lambs, twin-born ewes were lighter (P < 0.05) during their first year of age only. Birth rank had no effect on puberty onset or 18-month breeding performance. These findings indicate that ewe progeny born to second parity two-tooths or two-tooths, which were not presented for breeding as a ewe lamb, do not differ in their productive performance and that either group are suitable as replacement ewes. However, progeny born to two-tooth dams, which failed to conceive as a ewe lamb, would be less suitable as replacement ewes in production systems where ewe lamb breeding is an integral component of the system. Further studies are required to determine if similar relationships would be observed under differing environmental conditions and with different breeds. Additional keywords: growth, selection. Introduction Breeding ewe lambs at 8--9 months of age (hoggets) has several advantages including: additional lambs born, improved lifetime performance, increased efficiency and genetic gain, higher utilisation of spring feed and higher net profit (Dyrmundsson 1973; Tyrrell 1976; Baker et al. 1978; McCall and Hight 1981; Hight 1982; Gavigan and Rattray 2002). In response to these advantages, the percentage of ewe lambs being bred in New Zealand has risen from 10% in 1994 to 33% in 2004 (Anon. 2007) resulting in a greater proportion of second parity two-tooth ewes (~2 years of age at lambing). A flow-on effect of this change in management is that more lambs are being born to second parity two-tooth ewes and the resulting ewe lambs will make up a greater percentage of the New Zealand flock in the future. The potential impact of this is unknown. In addition, some two-tooth ewes are ewes that failed to conceive as a ewe lamb. How their progeny perform is also unknown. Lamb birthweight and weaning weight have been reported to increase with the age of the ewe before declining in older ewes (Notter et al. 2005; Gardner et al. 2007; Gootwine et al. 2007). However, no studies were found that have examined the effects of parity, independent of ewe age, on the performance of the resulting offspring. Dwyer et al. (2005) reported that lamb birthweight was not affected by parity although total litter weight and placental weight and efficiency increased with parity. Interestingly, Symonds et al. (2004) reported that young animals born to adult primiparous ewes had a greater level of adiposity compared with those born to multiparious ewes. In humans, parity is known to affect birthweight (Wilcox et al. 1996) and growth to 5 years of age (Ong et al. 2002), but not the fertility of the offspring (Joffe and Barnes 2000). Wu et al. (2006) stated that it is maturity of the dam and not parity that affects intrauterine growth. This suggests that any potential negative effects of first parity are less likely to be observed in offspring born to well grown dams. The aim of the present study was to determine if the growth and reproductive performance levels of ewe offspring born to first or second parity two-tooth dams differed. Methods and materials Background In April 2005, 18-month Romney ewes that had either been bred and lambed themselves as a ewe lamb, presented for breeding as a ewe lamb but failed to get pregnant, or were not presented for breeding as a ewe lamb, were bred as one group. At 8 months of age the young ewes had been randomly allocated to the groups that were either presented for breeding or were not presented for breeding. Those ewe lambs that were not presented for breeding Ó CSIRO /EA /08/070979

2 980 Australian Journal of Experimental Agriculture P. R. Kenyon et al. did not differ in liveweight from those that were. Their management as ewe lambs and their breeding performance and liveweight to 2 years of age has been previously reported (Kenyon et al. 2008). Study design The present study utilises three groups of female progeny born to the previously mentioned two-tooth ewes, which had either first lambed as a ewe lamb (second parity, n = 97), failed to conceive at ewe lamb breeding (ELB, first parity, n = 79) or had not been presented for breeding as a ewe lamb (first parity, n = 74) from the study of Kenyon et al. (2008). Throughout the present study period all ewe progeny were managed together under commercial grazing conditions from their birth to 657 days after the midpoint of lambing (day 657). The ewe progeny were weighed at birth, day 91, day 187, day 270, day 404 and day 566 and at fortnightly intervals during the puberty measurement period. Ewe progeny were condition scored at day 566 (Jefferies 1961). To determine the onset of puberty, four crayon-harnessed vasectomised rams were joined with the ewe lambs beginning 21 February until 26 June 2006 (day ). Ram harnesscrayon marks were recorded and the colour of the crayon changed every 14 days. Crayon marks were also recorded the day after each crayon colour change, to identify the period in which the ewe lamb was in oestrus. Hence, a ewe lamb that was marked the day after the crayon change and that also displayed the previous colour was recorded as being in oestrus in the previous 2 weeks. Occurrence of puberty was defined as being the time in which the lamb had her first clear ram harness-crayon mark. On 4 April 2007 (day 566) four entire crayon-harnessed rams were introduced for a 34-day breeding period with colours changed after 17 days. Using this information, the two-tooth ewes were characterised as being mated to the first 17-day period only (first crayon colour only on their rumps), mated during both 17-day periods (both crayon colours on their rumps), mated during the second mating period only (second crayon colour only on their rumps), or not mated (no crayon colour on their rumps). Pregnancy status was diagnosed by ultrasound pregnancy scanning on day 657. Ewes were diagnosed as non-pregnant, single-, twin- or triplet-bearing. Twelve two-tooths (7, 1 and 4 from second parity, first parity ELB and first parity groups, respectively) had either died or lost their individual tags between the period of day 270 and day 404 and were removed from the remainder of the study. The study was undertaken with approval from the Massey University Animal Ethics committee. Statistical analysis A general linear model with the fixed effects of dam group and birth rank were fitted to the liveweight data using the statistical package Minitab 13.1 (Minitab Inc., State College, PA, USA). Rearing rank was not fitted in any model as only 14 of the 137 twin-born animals were reared as singletons. The liveweight at breeding of the 18-month dam and date of birth were used as covariates in the models used to partition variation in liveweight at birth, day 91, day 187 and day 270 in the ewe progeny. Liveweight of the dam was used as a covariate in the models used to partition variation in liveweight at day 404 and 566. Interactions between the fixed effects were tested and removed if not significant (P > 0.05). In the model used to partition variation in the age of puberty, date of birth and liveweight of the dam were used as covariates in the model. and birth rank were used as fixed effects in all the categorical data models. Interactions between the fixed effects were tested and removed if not significant (P > 0.05). To determine the proportion of two-tooths that were pregnant based on the first 17 days of breeding only, two-tooths that were not mated at all, those mated during the second 17 days of breeding only, or mated in both the first and second 17 days (returned to service), or determined non-pregnant at pregnancy scanning were assigned a nominal pregnancy value of 0 (indicating a non-pregnancy event to the first 17 days of breeding). Those two-tooths that were mated in the first 17 days of breeding only and diagnosed pregnant were assigned a value of 1. These data were then analysed using the GENMOD procedure in SAS (SAS Institute Inc., Cary, NC, USA) for categorical data modelling (GENMOD). The data is presented as logic values s.e. and back-transformed as percentages. To determine the proportion of two-tooths that were pregnant to the entire 34-day breeding period, two-tooths diagnosed as non-pregnant at pregnancy scanning were assigned a 0 value (indicating non-pregnancy). Those two-tooths that were pregnant were assigned a value of 1. These data were then analysed using GENMOD. Two-tooth pregnancy percentage was determined by dividing the number of fetuses identified at pregnancy diagnosis by the number of 18-month ewes presented for joining. This number was then multiplied by 100. In the model used to partition variation in pregnancy, percentage liveweight at day 566 was used as a covariate. Results Liveweight and condition score Singleton-born ewes were heavier (P < 0.05) than their twinborn counterparts at birth, day 91, day 187 and day 270 (Table 1). had no effect on the young ewes liveweight at birth, day 91, day 197 and day 270. At day 404, there was a significant interaction between birth rank and dam group such that within singletons, those born to dams which had lambed as a ewe lamb themselves were heavier (P < 0.05) than those born to dams who failed to conceive as a ewe lamb. However, this relationship was not observed in their twinborn counterparts. At day 466, there was also a significant (P < 0.05) interaction between birth rank and dam group, although pairwise analysis indicated no significant differences between the groups.

3 Dam parity and the performance of ewe progeny Australian Journal of Experimental Agriculture 981 Table 1. The effect of birth rank and dam group on the mean (þ s.e.) liveweight (kg) of ewe progeny at birth, and 91, 187, 270, 404 and 566 days after the midpoint of lambing Within treatments and columns, means followed by a different letter are significantly different at P = 0.05 n Birth Day 91 Day 187 Day 270 n Day 404 Day 566 Birth rank (Br) b b b b a a a a Second parity A First parity ewe lamb breeding (ELB) B First parity C Br dam group 1 second parity b first parity ELB a first parity ab second parity ab first parity ELB ab first parity ab B Ewes born to two-tooth dams, which had been presented for ELB but failed to conceive. There were no differences in condition scores of ewe progeny between birth ranks or dam groups at day 566 (data not shown). Puberty data Ewe lambs born to dams which failed to conceive as a ewe lamb themselves were less likely (P < 0.05) to achieve puberty during the measurement period (Table 2). Ewe lambs born to dams which failed to conceive as a ewe lamb, and that did achieve puberty, did so at a younger age (P < 0.05) than their counterparts in the other two groups. had no effect on liveweight at puberty or number of oestrus events observed in those that achieved puberty. Two-tooth reproduction Neither birth rank nor dam group had any effect on two-tooth breeding performance or pregnancy percentage (Table 3). Discussion The aim of this experiment was to investigate the effect of parity of the two-tooth dam (18 months at breeding) on the performance of resulting female offspring. Two types of first parity two-tooth ewes can potentially be found on sheep farms, first, those that were bred for the first time at 18 months of age and second, those that were presented for breeding as a ewe lamb but failed to lamb. Presenting ewe Table 2. The effect of birth rank and dam group on the percentage (logit-transformed mean þ s.e., and back-transformed % in parentheses) of ewe lambs, which reached puberty and the age (days) (mean þ s.e.) and weight (kg) (mean þ s.e.) at puberty and number of oestrus events observed in those that achieved puberty in the measurement period (mean þ s.e.) Within treatments and columns, means followed by a different letter are significantly different at P = 0.05 n Reached puberty (%) Age at puberty (days) Liveweight at puberty (kg) No. of oestrus events Birth rank (65.0) (67.5) Second parity A b (73.3) 250.0b First parity ewe lamb breeding B a (54.2) 238.9a First parity C b (70.0) 249.8b B Ewes born to two-tooth dams, which had been presented for ewe lamb breeding but failed to conceive.

4 982 Australian Journal of Experimental Agriculture P. R. Kenyon et al. Table 3. The effect of birth rank and dam group on the percentage pregnant to the first 17 days of breeding (logit-transformed mean þ s.e., back-transformed % in parentheses), percentage pregnant to the entire 34-day breeding period and pregnancy percentage (mean þ s.e.) Within treatments and columns, means followed by different letters are significantly different at P = Within rows, means with different superscripts are significantly different at P = 0.05 n Percentage pregnant to the first 17 days of breeding (%) Pregnant to the 34-day breeding period (%) Pregnancy percentage (%) D Birth rank (78.9) (93.0) (87.1) (95.5) Second parity A (82.9) (92.5) First parity ewe lamb breeding B (80.1) (94.9) First parity C (86.6) (95.6) B Ewes born to two-tooth dams, which had been presented for ewe lamb breeding but failed to conceive. D Number of fetuses identified at pregnancy diagnosis divided by number of ewes presented for breeding multiplied by 100. lambs for breeding will result in a proportion of these failing to conceive (Kenyon et al. 2004a, 2005) but they may still be maintained as replacement ewes. Therefore, this group is also of interest. Breeding ewe lambs can negatively affect their two-tooth liveweight and performance (Kenyon et al. 2008) and this may have negative affects on their progeny. Alternatively, given that parity has a positive relationship with total litter weight, placental weight and placental efficiency (Dywer et al. 2005) it could be suggested that second parity offspring develop in an enhanced uterine environment, which may have positive flow-on effects in post-natal life. In the present study, ewe offspring born to second parity two-tooth dams did not differ in reproductive performance or liveweight from those born to first parity dams which had not been presented for breeding as a ewe lamb. Given the lack of liveweight difference between the groups it is not surprising that there was no difference in reproductive performance, as liveweight is known to affect puberty (Dyrmundsson 1973; Jainudeen et al. 2000) and two-tooth reproductive performance (Kenyon et al. 2004b). Therefore, when farmers are making decisions on which replacement ewes to retain, those born to second parity two-tooths dams are just as suitable as those born to the two-tooth dams which were presented for breeding for the first time at 18 months of age. Ewe lamb offspring born to two-tooths which failed themselves to conceive as a ewe lamb were less likely to display puberty, although they were not lighter than ewe lambs in the other two groups. No difference in two-tooth breeding performance was observed. These results indicate that in production systems in which ELB is an integral aspect, progeny born to two-tooth ewes that had previously failed to conceive as a ewe lamb are not the most suitable replacements. Occurrence of hogget oestrus is known to be under genetic control (Ch ang and Rae 1970). Alternatively, if ELB is not an integral component of the farming system, these animals, based on two-tooth breeding performance only, appear to be just as suitable for replacement ewes as those born in the other two groups. However, the present data do not allow for examination of reproductive performance in later years, which may warrant investigation. Although twin-born ewes were lighter than their singleton-orn counterparts in their first year of life there was no difference in liveweights for the remainder of the study, indicating no long-term stunting effect. Similarly, there was no difference in ewe lamb reproductive performance or twotooth breeding performance. Pregnancy percentage was higher in twin-born than in singleton-born ewes, although this was not statistically significant. Therefore, these data suggests that birthrank has little, if any, impact on ewe progeny performance to two-tooth breeding. Although Gonzalez et al. (1986) reported that twin-born ewes compared with singletons displayed higher lambing percentages over their lifetime. In conclusion, in the present study, ewe progeny born to second parity 2-year-old ewes and 2-year-old ewes which were not presented for breeding as a ewe lamb displayed no difference in growth and reproductive performance to 2 years of age. Therefore, the data suggests that farmers do not need to take parity of the dam into account when selecting replacement ewe lambs from these two groups of ewes. However, progeny born to 2-year-old ewes that failed themselves to conceive as ewe lambs were less likely to achieve puberty as a ewe lamb and would, therefore, be less suitable as replacement ewes in production systems where ELB is an integral component. Further studies are required to determine if similar relationships would be observed under a range of environmental conditions and in other breeds as it is possible that they may be different. Acknowledgements The authors wish to acknowledge the funding provided by Massey University and the National Research Centre for Growth and Development.

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