Ewe lamb live weight and body condition scores affect reproductive rates in commercial flocks

Abstract

The objective of this study was to determine the relationships between ewe lamb live weight and body condition score immediately prior to breeding with fertility (ewes pregnant per 100 ewes bred) and reproductive rates (fetuses per 100 ewes bred). Over 2 years, ewe lambs on seven commercial sheep farms in the North Island of New Zealand were monitored during the breeding period until pregnancy diagnosis. Live weight and body condition score were recorded prior to breeding. Fertility rates increased with live weight up to 47.4 kg but did not increase above this (P > 0.05). Reproductive rate peaked for ewes in the 47.5–52.4 kg breeding live weight category at 138% (133%–143%). Ewe fertility peaked at a body condition score (BCS) of 3.5 (90% [88%–91%]) while reproductive rates peaked at a BCS of 3.0 (130% [127%–133%]). The apparent plateauing of the relationship indicates there is a point at which no further benefits are gained from increasing live weight or BCS.

Keywords:

• body condition score
• ewe lambs
• fertility rate
• live weight
• reproduction
• reproductive rate

Introduction

In an extensive pastoral production system, breeding ewe lambs at 7–9 months of age has a number of advantages (Kenyon et al. in press). It allows the utilisation of an animal class that may not otherwise be productive, thereby increasing the potential lifetime production of the ewe (McCall & Hight 1981). In addition, ewe lamb breeding can increase the total number of lambs born on the farm, thus providing a means of increasing profits and the rate of genetic gain (Tyrrell 1976; Kenyon & Proctor et al. 2008). There are, however, a number of limitations including low and variable fertility and reproductive rates (Hight 1982; Kenyon & Pinchbeck et al. 2004). Fertility rates of ewe lambs have been reported to be between 47% and 82% compared with 85%–97% in mature multiparous ewes (Donald et al. 1968; Forrest & Bichard 1974; Annett & Carson 2006; Mulvaney et al. 2013).

Fertility rates in ewe lambs are limited due to a number of factors including failure to attain puberty prior to the breeding period, silent oestrus (oestrus without ovulation), shorter and less intense oestrus, impaired breeding behaviour, insemination failure and fertilisation failure (Dyrmundsson 1973; Allison et al. 1975; Keane 1976; Quirke et al. 1981; Quirke & Hanrahan 1983; Smith & Knight 1998). The threshold breeding live weight for ewe lambs to reach puberty is between 40% and 70% of their mature live weight (Dyrmundsson 1973; Jainudeen et al. 2000). Live weight of mature ewes during breeding is known to be positively associated with increased ovulation rates in adult ewes (Coop 1962; Smith 1991; Scaramuzzi et al. 2006). Therefore, it would be expected that heavier live weights at the start of the breeding period would result in improved reproductive rates. A series of studies conducted in New Zealand have shown that ewe lambs bred early in the breeding period or that were identified with twin fetuses were heavier, on average, than ewe lambs bred later in the breeding period, were not pregnant, or had only one fetus (Kenyon & Morris et al. 2008; Kenyon et al. 2009, 2010). Studies in Australia and the USA have also shown a positive relationship with ewe lamb live weight and reproductive performance (Gaskins et al. 2005; Rosales Nieto et al. 2013). However, a limitation across these studies is the relatively small number of animals involved and that they were conducted under experimental conditions rather than on commercial farms.

In mature ewes, body condition score (BCS) has a positive effect on reproductive performance to at least a BCS of 3.0 (Gunn & Maxwell et al. 1991; Kenyon & Morel et al. 2004). However, few studies have examined the effect of BCS on ewe lamb reproductive performance. Ewe lambs bred early in the breeding period had, on average, higher condition scores than ewe lambs bred later or that were not bred (Kenyon et al. 2009). In addition, a greater ewe lamb BCS has been associated with a greater number of fetuses identified at pregnancy diagnosis (Cave et al. 2012). Again these studies have included relatively small numbers of animals. Therefore, the aim of the current study was to determine the relationship of ewe lamb breeding live weight and BCS on fertility and reproductive rates of large numbers of ewe lambs on commercial sheep farms within the North Island of New Zealand.

Materials and methods

The current study was undertaken over 2 years (2011 and 2012). In 2011, ewe lambs on two properties—farm A located in the Waikato region (latitude 37.78° S, longitude 175.28° E) and farm B in the Wairarapa region (latitude 40.97° S, longitude 175.65° E) of the North Island of New Zealand—were enrolled. In the following year (2012), four additional farms (farms C, D, E and F) located in the Wairarapa region and one farm (farm G) in the Manawatu region (latitude 40.36° S, longitude 175.61° E) were added to the study along with a new cohort of ewe lambs enrolled on farms A and B. Ewe lambs on each farm were managed under commercial farming conditions and according to the normal practice for that farm from breeding to pregnancy diagnosis (on average 94 days after introduction of the ram).

The predominant breed over the seven farms was the Highlander^® composite (½ Romney, ¼ Texel, ¼ Finnish Landrace) on farms B, C, D and E. Farm A was in the process of transitioning from a Romney × Finnish Landrace to Coopworth × Finnish Landrace breed. Farms F and G had straight bred Romney and Coopworth ewes, respectively.

A summary of the breeding programme on each farm for each year is given in Table 1. Prior to breeding, five of the seven farms (B, C, D, E and G) introduced vasectomised rams for 17 days which were then removed at the start of the breeding period. All seven farms utilised a 34-day breeding period.

Table 1 Summary of breeding of ewe lambs on each property (farms A, B, C, D, E, F and G) and year (2011 and 2012) including the number of ewe lambs, minimum ewe lamb live weight (kg) for breeding, breeding start date, ratio of rams to ewe lambs, duration of the breeding period and the pregnancy diagnosis date.

Farm	Year	Ewe lambs (n)	Minimum breeding live weight (kg)	Breeding start date	Ram to ewe lamb ratio	Breeding length (days)	Pregnancy diagnosis date
A	2011	3389	No limit	11 April 11	1:50	34	14 July 11
A	2012	4305	No limit	11 April 12	1:50	34	23 July 12
B	2011	712	34 kg	29 April 11	1:65	34	26 July 11
B	2012	927	40 kg	1 May 12	1:65	34	25 July 12
C	2012	2073	39 kg	17 April 12	1:50	34	2 August 12
D	2012	1581	No limit	2 May 12	1:80	34	26 July 12
E	2012	1666	No limit	4 May 12	1:75	34	1 August 12
F	2012	567	38 kg	27 April 12	1:50	34	7 August 12
G	2012	290	No limit	23 April 12	1:60	34	27 July 12

Measurements

Within 7 days of breeding, ewe lamb live weights and body condition scores (BCS) were recorded. BCS was recorded by a single operator on all sevens farms using a scale of 1 to 5 with increments of 0.5 (1.0 = emaciated and 5.0 = grossly fat [Jefferies 1961; Kenyon et al. 2014]). Pregnancy diagnosis using trans-abdominal ultrasound was used a minimum of 45 days after the completion of the breeding period to determine, first, if a ewe lamb was pregnant (yes or no) and, second, the number of fetuses present (0 or 1 or 2). The pregnancy diagnosis on each farm was conducted by a commercial technician regularly contracted to that property.

Statistical analyses

All statistical analyses were conducted using SAS (SAS Institute Inc, Cary, NC, USA). The analyses included all ewe lambs identified as bearing 0, 1 or 2 fetuses that had either breeding live weight or BCS recorded prior to breeding and that also had pregnancy diagnosis results. Ewe lambs identified as bearing triplet fetuses were excluded from the analyses due to small total numbers (n = 16) and uneven distribution across farms. The analyses included a small number of ewe lambs that had breeding live weights but no BCS (n = 70) and ewe lambs that had BCS but no breeding live weights (n = 40).The total number of ewe lambs included in the analyses was 15,510. Farms A and B had data collected over 2 years, and the remainder of the farms (farms C, D, E, F and G) had data from only 1 year, therefore, each farm and year was treated as a single unit (n = 9; FarmA_2011, FarmA_2012, FarmB_2011, FarmB_2012, FarmC_2012, FarmD_2012, FarmE_2012, FarmF_2012 and FarmG_2012). Due to differences in ewe lamb breed, management practices and environment between farms, the interaction of farm and live weight or BCS was not tested.

Comparison between farms

The ewe lamb breeding live weight was compared between farms using a general linear model (Proc Mixed; SAS) with the fixed effect of farm. Breeding BCS and reproductive rates between farms were compared using a generalised linear model (Proc Genmod; SAS) based on a Poisson distribution and a logit transformation and included farm as a fixed effect. Fertility rate was analysed using the same generalised linear model, but was based on a binomial distribution and a logit transformation.

Breeding live weight

The breeding live weight of the ewe lambs was divided into categories as follows: <32.5 kg (n = 100), 32.5–37.4 kg (n = 947), 37.5–42.4 kg (n = 5423), 42.5–47.4 kg (n = 6122), 47.5–52.4 kg (n = 2471) and ≥52.5 kg (n = 377). The effect of the breeding live weight category on ewe lamb’s fertility (ewes pregnant per 100 ewes presented for breeding) and reproductive rate (fetuses per 100 ewes presented for breeding) at pregnancy diagnosis was analysed using a general model (Proc Genmod; SAS). Fertility (pregnant or non-pregnant) was analysed with a binomial distribution and logit link function with farm and live weight category fitted as fixed effects. Reproductive rate (0, 1 or 2 fetuses) was analysed with a Poisson distribution and logit link function with farm and breeding live weight category fitted as fixed effects.

Breeding BCS

Due to low numbers of ewe lambs with breeding BCS scores of less than 2.0 or greater than 4.0 the following BCS categories were created: ≤2.0 (n = 935), 2.5 (n = 5437), 3.0 (n = 6912), 3.5 (n = 1932) and ≥4.0 (n = 254). The effect of the breeding BCS category of the ewe lamb on fertility and reproductive rate at pregnancy diagnosis was analysed using a general model (Proc Genmod; SAS). Fertility rate was analysed with a binomial distribution and logit link function with farm and BCS category fitted as fixed effects. Reproductive rate was analysed with a Poisson distribution and logit link function with farm and BCS category fitted as fixed effects. Each model was run both with, and without, ewe lamb breeding live weight as a covariate in the model.

Results

Description of farm and year

The breeding live weight and BCS of ewe lambs varied widely between farms and between years (Table 2). Fertility and reproductive rates also varied between farms and within years on farms that were recorded over 2 years.

Table 2 Summary statistics of breeding live weight (kg; ± s.e.), body condition score, fertility rate (ewes pregnant per 100 ewes bred) and reproductive rate (fetuses per 100 ewes bred) of ewe lambs by farm and year.

Farm	Year	n	Breeding live weight (kg)	Breeding BCS *	Fertility rate * (ewes pregnant per 100 ewes bred)	Reproductive rate * (fetuses per 100 ewes bred)
A	2011	3389	41.3 ± 0.07^a	2.49 (2.44–2.55)^a	84 (83–85)^b	123 (120–127)^b
A	2012	4305	42.9 ± 0.06^c	2.93 (2.88–2.98)^c	88 (87–89)^d	135 (131–138)^de
B	2011	712	45.1 ± 0.15^f	3.16 (3.03–3.29)^d	86 (83–88)^bcd	126 (118–134)^bcd
B	2012	927	43.8 ± 0.13^d	2.93 (2.82–3.04)^bc	92 (90–93)^e	145 (137–153)^f
C	2012	2073	41.9 ± 0.09^b	2.94 (2.87–3.02)^c	77 (759–79)^a	108 (103–112)^a
D	2012	1581	47.0 ± 0.10^gh	2.87 (2.79–2.96)^bc	90 (89–926)^e	148 (143–155)^g
E	2012	1666	44.2 ± 0.10^e	2.83 (2.75–2.91)^b	88 (86–89)^cd	119 (114–124)^c
F	2012	567	46.7 ± 0.17^g	3.17 (3.03–3.32)^d	85 (82–87)^bc	118 (109–127)^b
G	2012	290	47.5 ± 0.23^h	3.07 (2.87–3.28)^d	89 (85–92)^cde	142 (129–156)^ef

^{a,b,c,d,e,f,g,h} means within columns with differing superscripts are significantly different (P < 0.05).

* Back transformed logit mean and 95% confidence interval.

Effect of ewe lamb live weight

The percentage of ewes identified as pregnant increased as ewe lamb breeding live weight increased from <32.5 to 47.5–52.4 kg (P < 0.05, Fig. 1). Fertility rates of ewes that weighed ≥52.5 kg did not differ from that of ewes weighing 47.5–52.4 kg (P > 0.05).

Figure 1 The effect of breeding live weight category on ewe lamb fertility rate (ewes pregnant per 100 ewes presented for breeding), back transformed logit mean ± 95% confidence interval. Bars with different letters are significantly different (P < 0.05).

The reproductive rate showed a similar pattern to fertility rate. Reproductive rate increased as live weight increased from <32.5 to 47.5–52.5 kg (P < 0.05, Fig. 2). The reproductive rate of ewes that weighed 52.5–57.4 kg did not differ from ewes that weighed 47.5–52.4 kg (P > 0.05).

Figure 2 The effect of breeding live weight category on ewe lamb reproductive rate (number of fetuses per 100 ewes presented for breeding), back transformed logit mean ± 95% confidence interval. Bars with different letters are significantly different (P < 0.05).

Effect of ewe lamb condition score

Fertility rate increased as breeding BCS increased to 3.5 (P < 0.05, Fig. 3). However, the fertility rate of ewes with BCS >4.0 did not differ from that of those with BCS greater than 3.0 (P > 0.05). The addition of ewe live weight as a covariate in the fertility rate model resulted in changes in the pattern of differences observed. The fertility rate of ewes with a BCS ≤2.0 remained lower than those in the BCS 2.5, 3.0, 3.5 and ≥4.0 categories (78% vs 85%, 88%, 88% and 89%, respectively). While the fertility rate of BCS 2.5 ewes was lower (P < 0.05) than that of the BCS 3.0 and 3.5 ewes, it did not differ from BCS ≥4.0 ewes (P > 0.05) and there were no significant differences in fertility rate between BCS 3.0, 3.5 and ≥4.0 ewes (P > 0.05).

Figure 3 The effect of ewe lamb breeding BCS (≤2.0, 2.5, 3.0, 3.5 or ≥4.0) on fertility rate (ewes pregnant per 100 ewes presented for breeding), back transformed logit mean ± 95% confidence interval. Bars with different letters are significantly different (P < 0.05).

Reproductive rates increased as the breeding BCS category increased to 3.0 (P < 0.05, Fig. 4), however, ewes with a BCS of 3.5 and ≥4.0 did not differ from those with BCS of 3.0 (P > 0.05). The inclusion of ewe lamb live weight as a covariate in the model resulted in changes in the pattern of differences observed. The reproductive rate of ewes in the BCS ≤2.0 category remained lower than those with BCS 2.5, 3.0, 3.5 and ≥4.0 (1.09 vs 1.22, 1.27, 127 and 1.31, respectively, P < 0.05). However, the reproductive rates of ewes in the BCS 2.5, 3.0, 3.5 and ≥4 categories no longer differed (P > 0.05).

Figure 4 The effect of ewe lamb breeding BCS (≤2.0, 2.5, 3.0, 3.5 or ≥4.0) on reproductive rate (fetuses per 100 ewes presented for breeding), back transformed logit mean ± 95% confidence interval. Bars with different letters are significantly different (P < 0.05).

Discussion

The aim of the current experiment was to determine the effect of ewe lamb live weight and body condition score at the start of the breeding period on their subsequent reproductive and fertility rates under commercial conditions in New Zealand. Previously in New Zealand, the relationships of live weight and BCS with reproductive performance have only been examined as part of smaller studies designed to investigate other aspects of ewe lamb management. Therefore this study set out to examine these relationships with large numbers of ewe lambs on commercial farms. On the farms in this study, fertility rates ranged from 77% to 92% and reproductive rates were between 108% and 145% which were in the upper range of those previously reported by other researchers both in New Zealand (Kenyon et al. 2010; Mulvaney et al. 2013) and in the Northern Hemisphere (Donald et al. 1968; Gaskins et al. 2005; Annett & Carson 2006). The differences in fertility and reproductive rates between farms could not be examined due to variation and confounding due to breed, environment and farming systems.

Improvements in both fertility and reproductive rates were observed with increases in breeding live weight up to 47.5 kg, with further increases in live weight resulting in no additional gains. This positive relationship, at least up to 47.5 kg, is in agreement with the previous findings of Kenyon & Morris et al. (2008) and Kenyon et al. (2009, 2010) in Romney ewe lambs in New Zealand and Rosales Nieto et al. (2013) in Merino ewe lambs in Australia. If the current industry guidelines were followed—that is, to breed only ewe lambs with live weights greater than 40 kg—reproductive rates of 121% could potentially be achieved. However, if farmers were to use a minimum live weight of 42 kg, this could result in reproduction rates of 130%. For mature ewes it has been reported previously that the proportion of multiple bearing ewes increases at a diminishing rate of return with live weight until plateauing (Smith 1991). Therefore it is unsurprising that there appears to be a plateauing of reproductive rate in heavier hoggets.

Ewe lamb fertility and reproductive rates also improved with increasing breeding BCS up to BCS 3.0 for fertility and BCS 3.5 for reproductive rate. This finding is somewhat in agreement with Kenyon et al. (2010) and Smith & Knight (1998), who reported that ewe lambs with a BCS of >2.5 had higher fertility rates than those with a BCS of 2.0 which in turn had higher rates than BCS 1.5 ewe lambs. Although there have been few studies examining the effect of BCS on the reproduction of ewe lambs, this relationship is similar to that observed in mature ewes.

In mature ewes, both very low and very high BCS have the potential to affect reproductive rates negatively (Kenyon & Morel et al. 2004; Maurya et al. 2009; Sejian et al. 2009; Yilmaz et al. 2011). Body condition of mature ewes has consistently been reported to have a positive relationship with ovulation rate (Bastiman 1972; Rhind et al. 1984) and a plateauing relationship with fertility rate (Gunn & Maxwell et al. 1991; Gunn & Smith et al. 1991). In addition, reproductive rates of mature ewes showed a positive relationship with BCS (Kleemann & Walker 2005). However, others have reported a relationship that plateaued at moderate condition scores of 2.0, 2.5 and 3.0 (Gunn & Maxwell et al. 1991; Kenyon & Morel et al. 2004).

In the current study, the effect of body condition score on ewe lamb reproductive and fertility rates was partially explained by live weight. The inclusion of ewe lamb breeding live weight in the BCS models for reproductive and fertility rates resulted in no differences in BCS above BCS 2.0 and 2.5, respectively. Therefore in the current study, it appears that poor reproductive performance of ewe lambs with a BCS of 2.0 or less was independent of ewe lamb live weight. Therefore, if ewe lambs have a body condition score of greater than 2.0, differences in fertility and reproductive rate appear to be explained by differences in live weight. Body condition score is closely related to total body fat (Kenyon et al. 2014). Stephenson et al. (1980) concluded that the rate of fat increase was related to onset of sexual maturity, therefore, ewe lambs with small fat deposits may be delayed in reaching sexual maturity resulting in reduced fertility and reproductive rates. This finding illustrates the importance of BCS as a method of determining whether a ewe lamb should be bred.

In conclusion, these data suggest that to maximise both ewe lamb fertility and reproductive rate, farmers should aim either to have ewe lambs at a minimum BCS of 3.0 or a minimum live weight of 47.5 kg. The further that ewe lambs are below these targets, the poorer their reproductive performance will be.

Acknowledgements

The authors wish to acknowledge the help provided by the farm staff on each of the properties and the Massey University technical staff involved in this study. Funding for this study was provided by Beef + Lamb NZ, C. Alma Baker Trust and Massey University. R.A. Corner-Thomas is partly funded by C. Alma Baker Trust and P.R. Kenyon by the National Research Centre for Growth and Development (Gravida).