http://orcid.org/0000-0002-7398-2653
ABSTRACT
Aims: To investigate associations between lamb growth to weaning and dam udder and teat scores measured at pre-mating, pre-lambing, docking and weaning.
Methods: Mature Romney ewes (n = 1,009) were enrolled from a commercial sheep flock located near Masterton, in the Wellington region of New Zealand in 2017. A range of udder and teat traits were scored in all ewes, using visual assessment and palpation, at pre-mating, pre-lambing, docking and weaning. During the lambing period, each newborn lamb was matched to its dam and lamb sex, birthweight and birth-rank were recorded. A rearing rank was allocated to each live-lamb at weaning, when all lambs were weighed (n = 1,570), allowing calculation of daily growth rates (g/day). Associations between udder and teat scores and lamb growth rates to weaning were examined using multivariable models for each udder-scoring time.
Results: Growth rates of lambs whose dams had udder palpation scores of hard, or both teats recorded as abnormal, pre-mating were lower than lambs whose dams had normal scores (229.9 (95% CI = 213.2–246.6) vs. 254.5 (95% CI = 245.6–263.5) g/day; p = 0.011) and (227.4 (95% CI = 208.3–246.6) vs. 247.9 (95% CI = 235.7–260.2) g/day; p = 0.024), respectively. Growth rates of lambs whose dams had clinical mastitis at docking or weaning were lower than those without mastitis (215.8 (95% CI = 199.9–231.7) vs. 235.4 (95% CI = 225.4–255.0) g/day; p = 0.007) and (220.0 (95% CI = 205.2–234.8) vs. 254.7 (95% CI = 248.9–260.5) g/day; p < 0.001), respectively. Growth rates of lambs whose dams had asymmetrical udders at docking or weaning were lower than lambs whose dams had symmetrical udders (204.6 (95% CI = 189.7–219.5) vs. 240.2 (95% CI = 225.4–255.0) g/day; p < 0.001) and (223.3 (95% CI = 213.9–232.7) vs. 242.2 (95% CI = 229.4–255.0) g/day; p = 0.014), respectively.
Conclusions and Clinical Relevance: Pre-mating udder palpation and teat palpation scores can be used to identify ewes whose lambs are predicted to have lower growth to weaning. Assuming a mean lamb age at weaning of 84.4 days, lambs born to ewes with a pre-mating score of hard would be expected to have a mean weaning weight that was 2.1 kg less than those whose dams had normal scores. Udder palpation, udder symmetry and clinical mastitis scores during lactation were also associated with lamb growth rates.
Abbreviation: CALW: Conceptus-adjusted liveweight
Introduction
The ability of a commercial New Zealand sheep farmer with a pasture-based production system to maximise the total weight of lamb available for sale directly influences the farm’s ability to generate income (Bohan et al. Citation2018). Both the number and the weight of the individual lambs drive the total weight of lamb available for sale per ewe. Thus improving both lamb survival and growth to weaning improves total weaning weight, which is in turn closely correlated with the total weight of lambs sold. Lambs are dependent on their dam for milk for both survival and growth, with milk remaining an important source of digestible energy and protein to weaning (Hayman et al. Citation1955; Glover Citation1972; Clark Citation1980). The quantity and quality of milk produced by the ewe are known to directly influence lamb growth (Hayman et al. Citation1955; Clark Citation1980). Lambs born to ewes with poor udder health, low milk yield, poor colostrum quality or quantity have poorer growth rates (Hayman et al. Citation1955; Watson and Buswell Citation1984; Arsenault et al. Citation2008). Mastitis has an important impact on udder health and reduces both the quality and quantity of milk produced (Albenzio et al. Citation2002; Leitner et al. Citation2004).
Udder morphology scoring is commonly used in dairy ewes for selection purposes to improve both udder health, and associated milk quality, and machine milkability (Casu et al. Citation2006, Citation2010). In addition, udder morphology scores, such as udder depth, are associated with milk production, with ewes with larger udders generally producing more milk (Labussiere Citation1988). Therefore udder morphology measures may be useful in non-dairy sheep for estimating lamb growth to weaning. Ideally, farmers could select ewes to keep or cull based on the predicted performance of their lambs, i.e. culling those ewes whose lambs are predicted to have poor survival or poor growth to weaning. If such an udder and teat scoring system could be developed related to lamb survival and growth, it would enable farmers to identify ewes that are either unsuitable for retention in the flock, or alternatively require selective treatment.
In our companion article, we report the frequency of different scores for udder traits in ewes on four occasions between mating and weaning (Griffiths et al. Citation2019). We also found that lambs born to ewes with poor udder scores had lower survival to weaning than lambs born to ewes with desirable udder scores. However, these relationships varied between traits and measurement times, indicating the importance of measuring specific udder traits and examining ewes at the most appropriate times.
The aim of the present study was to investigate associations between lamb growth to weaning and dam udder and teat scores measured at pre-mating, pre-lambing, docking and weaning. It was hypothesised that lambs born to ewes with poor udder and teat scores would have lower growth rates to weaning than those born to ewes with more desirable udder and teat scores.
Materials and methods
Farm and animals
The present study utilised data collected during 2017 from mixed-age, mature Romney ewes (n = 1009) that were born in 2013 or 2014 and were part of a commercial sheep flock located near Masterton, in the Wellington region of New Zealand. The enrolment, general and reproductive management of the study flock is described in Griffiths et al. (Citation2019).
Prior to lambing, ewes were weighed and body condition score was assessed to the nearest 0.5 score using a 1–5 scale (1 = thin, 5 = obese; Jefferies Citation1961). In order to eliminate the potential influence of conceptus weight on ewe pre-lambing measured liveweight, predicted conceptus-adjusted liveweights (CALW) of the ewes, calculated as described by Freer et al. (Citation1997), were used in the analyses. Prior to lambing, mean ewe CALW were 65.0 (SD 6.1), 63.1 (SD 5.7) and 62.2 (SD 6.8) kg for triplet, twin and single-bearing ewes, respectively.
During the lambing period (commencing 1 October 2017), intensive lambing observations were conducted twice daily, matching each newborn lamb with its’ dam, and recording the lamb’s birth-date, sex, birth-rank (single, twin or triplet-born) and birth-weight. All dead lambs were collected throughout the lambing and lactation period. These observations were used to assign a rearing rank to each live lamb at weaning. Single-reared indicated the lamb was the only lamb reared to weaning (either a single born lamb, a twin born lamb whose sibling died, or a triplet born lamb where both siblings died), twin-reared indicated the lamb was one of two lambs that were reared to weaning (either a twin born lamb pair where both survived, or a triplet born set in which one lamb died), and triplet-reared indicated the lamb was one of three triplet-born lambs, all of which were reared to weaning. Lamb weights were recorded at weaning, which occurred on 3 January 2018. Lamb growth to weaning was calculated as the mean growth rate (g/day) from birth to weaning.
Ewe udder scores
The udders and teats of all ewes were scored immediately prior to the start of mating (pre-mating), 11 days before the planned start of the lambing (pre-lambing), and at docking and weaning. The system used for scoring is described by Griffiths et al. (Citation2019) and is summarised in . Due to the small number of ewes in some categories, some scores were subsequently combined for analyses, as described in .
Table 1. Description of the traits and scores used to assess udder morphology in ewes at different times between mating and weaning. Due to small numbers of ewes in some categories, scores were subsequently combined for analyses.
Statistical analyses
All statistical analyses were conducted using SAS version 9.4 (SAS Institute Inc., Cary, NC, USA). The main predictor variables for modelling were udder and teat scores, which were measured at the ewe-level. The outcome variable was lamb growth rate from birth to weaning, which was measured at the lamb-level and was the unit of analysis. Scores from each recording time were analysed separately, with pre-mating and pre-lambing scores considered as predictive, and docking and weaning scores considered as explanatory of lamb growth to weaning. Firstly, generalised estimating equation models were used to examine the association between each predictor variable and the outcome variable. If variables were associated (p < 0.2) they were included in initial multivariable models.
Multivariable generalised estimating equation models, using an exchangeable correlation structure to account for clustering between lambs born to the same ewe were developed for each time when udder and teat scores were recorded. Forward manual variable selection was used to build the preliminary models, with variables retained where p < 0.05, after which the effect of adding variables in different orders was investigated (but without effect on the chosen model). In addition to the predictor variables determined by univariate analyses, all final models included the variables lamb sex (male or female), lamb birthweight, lamb birth rank (single, twin or triplet-born), lamb rearing rank (single, twin or triplet-reared), ewe pre-lambing body condition score and ewe pre-lambing CALW. Finally, interaction terms that were biologically plausible were considered, but no interactions were retained in final models. Back-transformed logit mean growth rates, with 95% CI, for lambs born to ewes with different scores were calculated from final models.
Results
This study included records from 1570 lambs that survived to weaning, and their respective dams (n = 926). The number of lambs that were alive at weaning that were born to ewes with different scores for udder traits at each recording time is shown in . At weaning, the mean lamb age was 84.4 (SD 5.4) days, and mean weaning weights of triplet, twin and single-reared lambs were 22.4 (SD 5.2), 26.3 (SD 4.4) and 31.0 (SD 5.9) kg, respectively.
Table 2. Number (%) of lambs that survived to weaning in one flock that were born to ewes that were recorded with different scores for udder traits (see ) on four occasions between mating and weaning. Note some ewes did not have all traits recorded on all occasions.
Associations between lamb growth to weaning and ewe udder and teat scores
Univariate analyses
The results of all univariate analyses examining associations between growth rates of lambs to weaning and different scores for udder traits in ewes at each recording time are presented in .
Table 3. Results of univariate analyses showing the mean (95% CI) growth rate (g/day) between birth and weaning of lambs born to ewes in one flock that were recorded with different scores for udder traits (see ) on four occasions between mating and weaning.
Multivariable analyses
Each of the final multivariable models included the lamb-level explanatory variables lamb birth rank, lamb rearing rank and lamb sex. The estimated mean growth rates for different categories of each of these variables from the models for each recording time are presented in . The udder traits that were included in final models for each recording time are shown in .
Table 4. Estimated mean (95% CI) growth rate (g/day) between birth and weaning of lambs that differed in birth rank, lamb rearing rank and sex, from final multivariable models that examined associations between growth rates of lambs and udder traits of ewes recorded on four occasions between mating and weaning.
Table 5. Results of final multivariable models a showing the estimated mean (95% CI) growth rate (g/day) between birth and weaning of lambs born to ewes with different scores for udder traits (see ) recorded on four occasions between mating and weaning.
Pre-mating: growth rates of lambs whose dams had udder palpation scores of hard, or both teats recorded as abnormal, pre-mating were lower compared with lambs whose dams had normal scores ().
Pre-lambing: growth rates of lambs whose dams had udder palpation scores of hard or lump pre-lambing were less than those whose dams had normal scores (). Additionally, the offspring of dams who had clinical mastitis pre-lambing had lower growth rates compared with lambs whose dams did not have clinical mastitis ().
Docking: growth rates of lambs whose dams had udder palpation scores of hard or lump at docking were lower compared with lambs whose dams had normal scores (). The growth rates of lambs were higher if their dams had udder depth scores of 2, 3 or 4 compared with 5 at docking, and were lower for lambs whose dams had clinical mastitis or asymmetrical udders at docking, compared with those whose dams did not have clinical mastitis or had symmetrical udders, respectively ().
Weaning: growth rates of lambs whose dams had udder palpation scores of hard or lump at weaning were lower compared with those whose dams had normal udder scores (). Additionally, lambs whose dams had clinical mastitis or asymmetrical udders at weaning had lower growth rates compared with lambs whose dams did not have clinical mastitis or had symmetrical udders, respectively ().
Discussion
The results of the present study support the hypothesis that lambs born to ewes with poor udder and teat scores would have lower growth rates from birth to weaning than lambs born to ewes with more desirable scores. However these relationships varied between traits and management times. Similarly, we found that the relationship between lamb survival and udder and teat scores varied between traits and management times (Griffiths et al. Citation2019).
In this study, udder palpation score at all observation times was associated with lamb growth to weaning, with lower growth rates in lambs born to ewes with abnormal scores compared to those born to ewes with normal scores. There are limited comparable data available; Hayman et al. (Citation1955) reported lambs born to ewes with defective udders grew up to 34 g/day less than those born to ewes with normal udders, while Grant et al. (Citation2016) reported offspring of ewes with intramammary lumps grew 10 g/day less than those without lumps. Based on the results of the present study, and assuming a mean lamb age at weaning of 84.4 days, lambs born to ewes with abnormal udder palpation scores would on average be 1.0–2.9 kg lighter at weaning than those born to ewes with normal udder palpation scores. For example, lambs born to ewes with a pre-mating score of hard would be expected to have a mean weaning weight that was 2.1 kg less than those whose dams had normal scores, although this would vary between lambs and ewes.
If farmers wish to identify ewes whose lambs are predicted to have poorer performance, the results of the present study indicate pre-mating udder palpation score would be an appropriate trait to assess. When combined with the results of our analysis of traits associated with lamb survival (Griffiths et al. Citation2019), these results indicate that offspring of ewes with pre-mating udder palpation scores of hard have greater odds of failure to survive to weaning, and, should they survive, have lower growth rates to weaning compared with offspring whose dams had normal udder palpation scores.
The present study also identified associations between lamb growth to weaning and udder palpation scores at pre-lambing, docking and weaning. However any effective interventions for ewes with abnormal udder palpation scores at pre-lambing and docking have not been evaluated, and therefore any economic benefit of intervention is also unknown. In addition, there is a lack of New Zealand data to demonstrate whether these ewes would also have undesirable udder traits the following season. Results from Great Britain indicated that abnormal udder palpations were repeated between lactations, with greater odds of a ewe having an intramammary mass if she had previous intramammary masses in pregnancy or the previous lactation (Grant et al. Citation2016). Therefore, additional longitudinal investigation in New Zealand flocks is warranted, including the mechanisms and biology of the abnormal udder scores, the economic benefit of culling ewes with abnormal scores, and management interventions to improve udder health of these ewes in both the current and future seasons.
To the authors’ knowledge, this study is the first to describe an association between teat palpation score and lamb growth. Lambs born to ewes with both teats recorded as abnormal pre-mating were on average approximately 1.7 kg lighter at weaning than lambs from ewes with normal teat scores, based on a mean age at weaning of 84.4 days. However it should be noted that there was no association between teat palpation score pre-mating and lamb survival in the final multivariable model (Griffiths et al. Citation2019); neither was there a relationship between lamb growth rates and abnormal teat palpation scores at pre-lambing, docking or weaning, highlighting the importance of timing of teat palpation.
In the present study, offspring of ewes with clinical mastitis had lower growth rates. This is in agreement with previous studies in which lambs born to ewes with mastitis had reduced growth rates (Hayman et al. Citation1955; Watson and Buswell Citation1984; Arsenault et al. Citation2008). A variety of bacterial species have been identified as being involved in ovine mastitis in New Zealand commercial ewes (Quinlivan Citation1968, Citation1972; Clark Citation1980), with Staphylococcus aureus being of particular importance at lambing (Quinlivan Citation1972). In the present study, ewe mastitis at pre-lambing, docking and weaning was associated with lamb growth to weaning. At pre-lambing and docking treatment may be the most appropriate course of action, if both effective and economically feasible. However bacteriological cure in ewes is difficult to achieve (Gelasakis et al. Citation2015), and it may be best to cull these ewes prior to the next breeding season. Additionally, S. aureus has been associated with permanent udder damage in some cases (Quinlivan Citation1972; Clark Citation1980), and a carrier state has been described in both dairy and non-dairy sheep (Quinlivan Citation1972; Bergonier et al. Citation2003). Therefore, to allow New Zealand farmers to make informed decisions about ewes diagnosed with mastitis, further studies are required examining the likelihood of mastitis in subsequent lactations, economic consequence of mastitis in non-dairy flocks, and longitudinal studies examining both efficacy and economic viability of treatment of mastitis.
The only udder morphology scores associated with lamb growth were udder depth at docking and udder symmetry at both docking and weaning. The offspring of dams with udder depth scores of 5 at docking had lower growth rates compared with lambs born to dams with scores of 4, 3 or 2. This was not unexpected, as a score of 5 represents a very small udder, more typical of that seen in a non-lactating ewe. Also in commercial Romney ewes in New Zealand, a positive correlation was found between ewe udder dimensions and milk yield, although udder dimensions did not give an accurate prediction of lamb growth rates (van der Linden et al. Citation2010). However, in that study, only single-born and reared lambs were included, and the methods used to measure udder dimensions were different to the present study.
The findings of the present study highlight that different traits are associated with lamb growth rate when assessed at different times. Some, such as udder palpation score, was associated when assessed at any time between mating and weaning, while others were only associated at specific times. Future research is required to assess the repeatability of udder scores over time (i.e. in subsequent lactations to allow for prediction of future offspring performance), the economic consequences of keeping (or culling) ewes whose lambs have increased risk of failure to survive or poorer growth to weaning, and to investigate management interventions to improve udder health. In addition, further studies designed to investigate the biological cause for each of the scores at each time would be useful to enhance our understanding of ewe udder and teat health. Combined, this will enable development of a farmer-friendly scoring system to identify ewes suitable for either targeted culling or selective treatment, as appropriate.
It is important to note the limitations of the present study, namely that it was performed on only one farm using ewes from a single commercial flock during one year. However, the farm used can be considered representative of a North Island hill country commercial sheep farm based on farm and flock size and productivity parameters (Griffiths et al. Citation2019). Additionally, the statistical analysis did not allow for comparison of the model estimates between the four management times at which scoring was undertaken, nor the repeatability of scores within ewes within the season. As the study was only conducted during one year, we are also unable to compare between seasons, or to examine the relationship between the present season’s scores and subsequent season’s performance.
In conclusion, udder palpation score, teat palpation score, udder depth, udder symmetry and clinical mastitis scores were all associated with lamb growth to weaning. Farmers can use pre-mating udder palpation scores and teat palpation scores to identify ewes whose lambs are predicted to have lower growth rates to weaning. Farmers can also use udder palpation, udder depth, udder symmetry and clinical mastitis scores during lactation to identify ewes whose lambs may have lower growth rates to weaning.
Acknowledgements
The authors would like to thank the farm manager, shepherd and technical staff at Massey University (particularly Geoff Purchas and Dean Burnham) for their invaluable assistance during the study. In addition, the help of staff and students at Taratahi Agricultural Training Centre was greatly appreciated. This research was funded by Beef + Lamb New Zealand and the C. Alma Baker Trust.
ORCID
RA Corner-Thomas http://orcid.org/0000-0002-7398-2653
PR Kenyon http://orcid.org/0000-0003-1131-0736
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