On-station comparative analysis of reproductive and survival performance between Red Maasai, Dorper, and Merino sheep breeds

The reproductive performance of ewes and the survivability of lambs to weaning have a critical economic impact on sheep farming worldwide. Further, knowledge of major mortality causes allows an opportunity for improved flock management to evade financial losses. The maximum likelihood estimates for generalised linear mixed models and chi-square test methods were used to examine 971 mating records, 839 and 763 lambs born and weaned (singles or twins) from the Naivasha Sheep and Goats station in Kenya for the years 2011 to 2020 consisting of Dorper, Red Maasai (RedM), and Merino breeds. The RedM (P < 0.05) outperformed Dorper and Merino in weaning rate, whereas reproductive performance between the three breeds was not significantly different (P > 0.05) in litter size and multiple lambings per ewe lambing. On the one hand, Dorper significantly (P < 0.05) outperformed the other two breeds only in weaning weight per lamb born. In addition, among all the major causes of death, pneumonia appeared to be the one to which Dorper breeds were most susceptible (chi-square test, P < 0.05). According to the findings of this study, neither the Dorper nor the Merino sheep breeds were reproductively superior to the RedM in an extensive semi-arid production environment. In addition, Dorper's susceptibility to the leading causes of mortality, particularly pneumonia and sheep pox, were relatively high compared to other breeds and could be a precursor to massive economic losses for Dorper sheep producers. In contrast to the indigenous RedM breed, imported sheep breeds appeared to be more susceptible to major mortality-related under an extensive production system. Therefore, regardless of weaning weight, RedM breed production appears to be a more viable investment for small-scale farmers, particularly in semi-arid regions.

Long term genetic selection for reproductive success affects neonatal lamb vitality across cold stress conditions

Adverse weather conditions are important contributors to mortality in new-born lambs. Previous studies have shown variation between lambs in their ability to cope with circumstances of cold stress, and genetic selection could be a viable option for improving animal robustness. The Elsenburg Merino flock was divergently selected on number of lambs weaned (NLW). This resulted in divergent responses in reproduction and lamb survival. This study evaluated lamb vitality and mortality of positively selected H-Line relative to the negatively selected L-Line in response to cold stress. Traits included lamb rectal temperature (RT), surface temperature (ST), shiver score (SS), lamb vigor score (LVS), breaths per minute (BPM), mortality to three days of age (M3) and to weaning (TM). Cold stress was described by a chill index derived from daily rainfall, wind speed and ambient temperature, and represented as the mean of the one (CI), two (CI-2) or three (CI-3) days since parturition. Overall, H-Line lambs had a higher neonatal RT and were less likely to succumb than L-Line contemporaries. In a significant (P < 0.05) interaction, the predicted RT of L-Line showed a non-linear decline with increased levels of CI-2, while H-Line lambs better maintained their core temperature. M3 was also affected by a significant interaction between CI-3 and selection line, further suggesting that observed lower mortality rates in the H-Line depends on H-Line lambs' improved ability to cope with stressful environments. Long term selection for NLW in the H-Line led to improvements in both adaptations associated with lower lamb losses. The continued recording of viability traits to produce larger datasets amenable to genetic analysis is recommended, specifically for rectal temperature.

Genetic parameters and trends for lamb survival following long-term divergent selection for number of lambs weaned in the Elsenburg Merino flock

Context Mortality of new-born lambs is well known to have important implications for both animal production and welfare. Improving survival rates by genetic selection is very desirable, but the low heritability of survival traits challenges the prospect of useful genetic gain. Aim This study aimed to derive genetic and environmental parameters for lamb survival in the Elsenburg Merino resource flock. It also investigated correlations to possible indicator traits birthweight and birth coat score and reported genetic progress in breeding values for lamb survival following 33 years of divergent selection. Methods The flock was divergently selected for number of lambs weaned and was separated by the High (positive) and Low (negative) selection groups. The pedigree file identified 8138 lambs as the progeny of 273 sires and 2062 dams. The study considered total survival from birth to weaning (Tsv) that was also partitioned into perinatal survival to 3 days of age, and the remaining period. Variance components were derived by linear mixed models by using the ASREML® program. Genetic trends were derived by predicting mean breeding values for selection groups within each year and evaluated by fitting linear and broken-stick regression models. Results Predicted Tsv of H-line lambs (0.81 ± 0.01) was higher (P &lt; 0.01) than that of L-line lambs (0.68 ± 0.01). Heritability was significant but low for survival traits (0.03–0.07), moderate for birthweight (0.16) and high for birth coat score (0.54). Genetic trends of the H-line trended divergently (P &lt; 0.01) to the L-line for survival traits, but a changepoint (P &lt; 0.01) in trend suggested that the H-line reached a selection plateau following 19–22 years of selection. Preceding this period, the rate of genetic change equalled ~1% of the mean for Tsv. Conclusions Despite the low heritability of survival, the genetic trends reported in this study contradicted the premise that genetic selection is not a worthwhile method to reduce incidences of lamb mortality. Implications It is recommended that lamb survival phenotypes should be recorded and incorporated into indices individually where possible, but composite traits are a viable alternative.

Genetic parameter estimates for reproductive traits in Chokla ewes of India

Early selection of ewes for better reproductive performance may reduce generation interval and increase genetic gains. Considering this, the present study aimed to derive the genetic parameter estimates of ten reproductive traits in 1172 maiden Chokla ewes over a period of 22 years. Initially, the traits, namely age at first service (AFS), weight at first service (WFS), age at first successful service (ASS), weight at first successful service (WSS), age at first lambing (AFL), ewe's weight at first lambing (WFL), first lambing interval (LI1), first service period (SP1), first parity lamb survival (SUR1) and first service fertility rate (FER1) were analysed for the effects of genetic and non-genetic factors. The significant effects were fitted into six animal models to estimate the genetic parameters. The heritability estimates were low for the age traits and FER1 (0.06-0.09) but moderate for the rest of the traits (0.16-0.23), indicating the suitability of these traits for improvement through direct selection. The maternal direct and permanent effects were meagre for all the traits. The ewe weights had strongly negative correlations with the age of sexual maturity and positive correlations with lamb survival. The results emphasise the need to optimize the weights of maiden ewes to improve reproductive performance.

Thermogenesis and physiological maturity in neonatal lambs: a unifying concept in lamb survival

Lamb mortality represents reproductive wastage and an animal welfare concern. While lambs are thought to be at a thermogenic advantage following birth in comparison to other species, death from exposure can still be a major contributor to lamb mortality, largely because of the inclement conditions often prevailing at lambing. For this reason, thermogenesis has been studied extensively in neonatal lambs. Heat is produced in the neonatal lamb by shivering and non-shivering thermogenesis. The latter is heat generated by metabolism of brown adipose tissue (BAT) found largely in the thorax and peri-renal areas of the newborn lamb. Brown adipose tissue differs from normal adipose tissue in that it contains densely packed mitochondria, a high cytochrome c content and a vast vascular network. Heat is generated in BAT by uncoupling of the proton conductance mechanism from ATP production, resulting in heat production instead of stored energy. The ability of lambs to resist cooling differs among individuals and this is likely to be due to both genetic and phenotypic factors. The heritability of cold resistance is moderate-to-high and polymorphic gene markers associated with energy homeostasis and cold-related mortality have been identified. Additionally, several aspects of the phenotype of the lamb have been associated with cold resistance. Most relate to properties of the coat, skin and bodyweight, the latter being particularly important, presumably through effects on surface area to volume ratios and subsequent heat loss. The ability of the neonate to achieve the transition from intra- to extra-uterine life has been termed physiological maturity and is associated with the ability to activate appropriate neuro-endocrinological and behavioural changes that are consistent with homeostasis of energy metabolism. Ways to alter physiological maturity of the lamb, such as nutrition, pharmacology and genetic selection, have been identified, and while these show promising results with regards to thermoregulation, a key limitation of their application has been the lack of a repeatable, representative model of neonatal cold stress. An estimation of the non-shivering component potential of a lamb’s ability to thermoregulate can be derived from norepinephrine challenges, but more useful models of real-world cold stress are climate chambers or controlled water bath tests. Further use of repeatable test models such as these with appropriate neuroendocrine and metabolic metrics will identify key components and markers of physiological maturity associated with lamb thermogenesis and survival.

Variation in physiological profiles may explain breed differences in neonatal lamb thermoregulation

Ability to adapt rapidly from the uterine environment to self-thermoregulation following birth is a vital requirement for neonatal lamb survival. This investigation reports factors that could explain differences in thermoregulation among breeds that differ in lamb survival. Breeds such as the Merino and Border Leicester have previously been shown to be divergent for birthweight, cold resistance and lamb survival. Cross-bred (Poll Dorset Border Leicester (PDBL, n = 9) and Poll Dorset Merino (PDM, n = 25)) and pure-bred (Border Leicester (BL, n = 35) and Merino (M, n = 46)) lambs were recorded for the thermogenic measures rectal temperature at birth, cold resistance (time for rectal temperature to fall to 35°C while in a cooled water bath) and cold recovery (time to restore rectal temperature after cold exposure) at 1 day of age. In pure-bred lambs, 1 kg increase in weight resulted in a 0.25°C increase in rectal temperature at birth (P < 0.001) and 4.2 min increase in cold resistance (P < 0.001). In contrast, cross-bred lambs did not exhibit any relationship between birthweight and rectal temperature at birth, although they displayed a 3.2 min greater cold resistance for every 1 kg increase in birthweight (P < 0.001). BL-derived lambs were more cold resistant than M lambs (cross-bred: PDBL, 67.1 ± 2.5 min; PDM, 56.4 ± 1.6 min; P < 0.01; and pure-bred: BL, 58.1 ± 1.5 min; M, 53.2 ± 1.3 min; P < 0.01). The quadratic relationship of glucose concentration over time during cold exposure differed with lamb breed. PDBL exhibited higher peak glucose concentrations than did PDM (11.0 mmol/L and 8.9 mmol/L, respectively; P < 0.01). BL took longer to reach peak glucose concentration (50 min) than did M (40 min) and this peak value was higher (BL, 9.4 mmol/L; M, 7.7 mmol/L; P < 0.001). In conclusion, variations in birthweight and glucose metabolism are associated with breed differences in thermogenesis of neonatal lambs.

Lamb survival in Australian flocks: a review

Lamb survival is known to be a key component of reproductive efficiency in sheep flocks. It has been the subject of a large number of studies over the decades and this review provides an update and insights into the latest understanding of the extent and variability of lamb losses across Australia. The pattern of causes of lamb losses, including dystocia, starvation/exposure, mismothering and predation, are examined. Likewise the importance of birthweight in contributing to such losses is discussed as are strategies for improving lamb survival. The strategies examined include improved ewe nutrition, shelter provision and selection for cold resistance and improved maternal behaviour.

A review of the genetic and epigenetic factors affecting lamb survival

Poor lamb survival pre-weaning is a major source of reproductive inefficiency in Australian sheep flocks. While nutrition and management options have been extensively researched and promoted to improve lamb survival, the present review focuses on the prospects for obtaining genetic gain and helps identify selection strategies for boosting such gains to improve overall reproductive efficiency in the Australian sheep industry. Estimated heritability for lamb survival using linear model analysis is low, although use of threshold models suggests that heritability could be higher, which, if true, could help explain the substantial genetic gains obtained in long-term selection experiments. Epigenetic mechanisms may hinder selection and quantitative trait-loci identification through confounding and/or masking genetic variances and co-variances. With sufficient information, these effects could be considered in genetic evaluations by identifying those components that are amenable to selection. Regarding indirect selection, finding effective criteria for improving lamb survival has proved elusive. Most measures of maternal behaviour, temperament and lambing difficulty researched are poorly correlated genetically with lamb survival. Of lamb behaviours and thermo-genic indicators studied, latency to bleat following handling by humans is moderately genetically correlated with lamb survival, as is neonatal rectal temperature. Industry application remains to be adequately explored for the more promising of these measures. Finally, in lieu of direct selection for lamb survival, there is merit in selecting for multiple-rearing ability or its equivalent, possibly with additional selection criteria for lamb survival and reproductive efficiency.

Sire effects on neonatal lamb vigour and following-behaviour

The ability of the lamb to maintain contact with the ewe as she moves away from the birth site is critical to the lamb’s subsequent survival. If this contact is compromised then lamb loss is likely to occur due to starvation/mismothering. This study uses sires from the Sheep CRC Information Nucleus Flock to compare the effect of selection of sires within the Merino breed for high or low losses due to starvation/mismothering on neonatal lamb vigour. Lamb vigour was measured using conventional measures of time to perform early neonatal behaviours, early physiological measures (rectal temperatures and blood glucose), and performance in a modified barrier test while under physiological stress from cold exposure. Lambs were exposed to cold at a time (4–6 h after birth) when the ewe is likely to be moving away from the birth site and therefore when losses due to starvation/mismothering are likely to begin. Progeny from the high-loss sires were significantly (P < 0.05) slower to attempt to stand, and to stand, than progeny from the no-loss sires and tended to be slower to reach the udder and suckle (P = 0.07). Lambs from the no-loss group also had a significantly (P < 0.01) higher vigour score than the high-loss group. There was no effect of sire group on the performance of lambs in the modified barrier test; however, cold-treated lambs performed poorly in the test compared with control lambs. It was concluded that sire can have an effect on lamb vigour, but it does not necessarily translate into effects on later following-behaviour while under stressful conditions.

Importance of birthcoat for lamb survival and growth in the Romane sheep breed extensively managed on rangelands

The Romane sheep breed proved to be adapted to harsh conditions with high prolificacy and lamb survival in outdoor farming, even under bad climate conditions. This breed shows large variability in its fleece type at birth and it has been suggested that lamb survival could be related to birthcoat type. The aim of the present study was to: i) characterize the coat of the lamb at birth and quantify lamb survival in relation to the birthcoat type and its protective properties concerning heat loss in the Romane breed raised under permanent exposure outdoors from birth, and ii) estimate genetic parameters of birthcoat type in relation to lamb survival and live body weight. A total of 7,880 lambs from 104 sires and 1,664 dams were used in a 14-yr experiment. The pedigree file included 9,625 individuals over 15 generations. Birthcoat type, coat surface temperature, coat depth, lamb survival, and growth were measured from birth to weaning. Weather data (temperature, wind speed, relative humidity, and precipitation) were recorded daily during lambing time. Two types of coats were observed at birth: hairy coat (62.9% of lambs) with a long coat depth (average 23.3 mm) or woolly 1 (37.1% of lambs) with a short coat depth (average 8.3 mm). Birthcoat type was an important factor affecting lamb survival and growth from birth in the Romane breed. Total mortality rate was significantly less in hairy-bearing coat lambs than in short-woolly coat ones: 7.0% vs. 9.6%, 11.6 % vs. 14.8%, and 15.7 % vs. 20.1 % at 2, 10, and 50 d, respectively, and the relative risks of death increased by 37%, 67%, and 46 % at 2, 10, and 50 d of age, respectively, in short-woolly lambs. At birth, a significant lower coat surface temperature, indicating less heat loss, was observed in long-hairy coat lambs compared with others (21.1°C vs. 26.1°C). Heavier body weights and better growth performances up to the age of 50 d were observed in long-hairy-bearing coat lambs. Lamb survival was positively correlated to weather conditions at lambing time in short-woolly lambs, whereas no relation was observed in hairy-coat lambs. Heritability estimate of birthcoat type (hairy or woolly) is high (0.88). Genetic correlations between birthcoat type, lamb survival, and live body weight at birth were positive and moderate, suggesting that it would be beneficial to take birthcoat into account as an adaptive trait in any genetic strategies for sheep production in harsh conditions on rangelands.

ESTIMATIVA DE PERDA DE CALOR SENSÍVEL SOB ESTRESSE POR FRIO EM CORDEIROS CONFINADOS

Com o objetivo de estimar a perda de calor sensível em cordeiros em confinamento e quantificar a energia metabolizável utilizada para manutenção da temperatura corporal, sob estresse térmico pelo frio, foram utilizados 24 cordeiros cruzados e não castrados, (média de 120 dias de idade, 26,3 kg de peso vivo e sob dieta com 2,94 kcal animal-1 dia de energia metabolizável). Destes, selecionou-se aleatoriamente seis animais, que foram referência para coleta dos seguintes parâmetros do ambiente de alojamento: temperatura do ar mínima e máxima, velocidade do ar, umidade relativa do ar e luminosidade. Para estimar o gasto energético foram consideradas as temperaturas superficiais médias das regiões de cabeça, orelhas e pernas, obtidas com o auxilio da câmara termográfica infravermelho e respectivo software. Durante o período foi verificado que a temperatura ambiente ficou abaixo da zona de conforto térmico e que a temperatura superficial corporal média dos cordeiros foi de 11,48 ± 0,92 °C, indicando que houve uma interação entre os parâmetros climáticos avaliados, promovendo o resfriamento do ambiente e influenciando na redução da temperatura superficial. A dissipação total de calor sensível por animal nas partes analisadas foi de 39,18 W, o que corresponde ao consumo de 0,388 kcal 12 h-1. Constatou-se que, 26,4% da energia metabolizável diária ingerida pelos cordeiros, nos dias de temperatura abaixo da zona de conforto térmico, foi utilizada para termogênese. Contudo, foi verificado que, por estarem abaixo da zona de conforto térmico, os cordeiros dissiparam o calor sensível por convecção e radiação, utilizando parte da energia metabolizável da dieta para manter a temperatura corporal, situação que pode acarretar perdas econômicas para a produção.

Selection for easier managed sheep

Current alterations in the farm environment, such as a reduced number of farm workers, may mean that sheep genotypes that are highly dependent on man for nutritional and reproductive success will experience poorer welfare within that environment. In the past 30 years, average flock size has doubled, and flocks of over 1,000 ewes managed by one stockperson are common. The reduction in the ratio of stockpeople to sheep affects animal welfare, with less time for tasks such as healthcare and inspection. It has also led to increased interest in the development of new genotypes that are better able to look after themselves. Selection and management of sheep to promote behaviours associated with survival, and selection of robust animals that require less human intervention for good welfare, are important breeding goals. As these animals will receive less inspection at close quarters, selection for resistance to disease will have significant animal welfare benefits. In addition, the development of sheep lines that require little or no intervention at lambing will be important. In areas where wool is not valuable, the use of wool-shedding breeds to avoid the stress associated with shearing, and to reduce the incidence of flystrike are already proving to be beneficial. Importantly, this selection should not be interpreted as providing no care to these animals, and careful management during the production of these genotypes is needed to avoid at least transient welfare problems where genotypes and environment (eg lower shepherding) are mismatched.

Opportunities for genetic improvement of lamb survival

Data on lamb survival and associated traits involving records from 15 192 lambs, 6308 dams and 284 sires from the Sheep CRC’s Information Nucleus were studied. Lamb survival to 3 days of age and to weaning was 85 and 80%, respectively, and heritability (±s.e.) was 0.014 ± 0.010 and 0.010 ± 0.010, respectively. Of the 14 traits recorded at birth, time taken for the lamb to bleat, rectal temperature and crown–rump length had the highest genetic correlations with lamb survival to weaning (–0.43 ± 0.32, 0.56 ± 0.33 and –0.38 ± 0.36, respectively). Under selection for a multi-trait objective including net reproduction rate (but not lamb survival), survival was predicted to decline genetically by 0.25 lambs weaned per 100 lambs born.year, although this was reversed to a gain of 0.20 lambs weaned per 100 lambs born.year by including the trait in the breeding objective and using 50 half-sib and 50 progeny records per selection candidate. Accuracy of selection for lamb survival was improved to 0.735 with a selection index of lamb survival to weaning, lamb ease, birth coat score, time taken to bleat, rectal temperature and crown–rump length, with the addition of 50 half-sibs and 50 progeny records per candidate. Our results suggest that unless actively incorporated into breeding objectives, lamb survival may genetically decline; however, gains are possible with direct selection using half-sib and progeny records. The addition of indirect selection criteria for lamb survival can further improve accuracy, up to 93.4%, but requires further investigation.

Maintenance of body temperature in the neonatal lamb: effects of breed, birth weight, and litter size

To survive, the newborn lamb must be able to maintain body temperature and to stand and move to the udder to suck colostrum to fuel heat production. The objective of this study was to investigate whether neonatal lambs showing slow behavioral progress to standing and sucking also have an impaired ability to maintain body temperature. The time taken to stand and suck after birth, rectal temperatures, and blood samples were collected from 115 newborn single, twin, and triplet lambs of 2 breeds, Scottish Blackface and Suffolk, which are known to show variations in their neonatal behavioral progress. Blood samples were assayed for thyroid hormones, known to be involved in heat production, and cortisol, which plays a role in tissue maturation before birth. In addition, colostrum samples were collected from the 56 ewes that gave birth to the lambs, and assayed for protein, fat, and vitamin contents. Heavy lambs (more than 1 SD above the breed mean), Blackface lambs, and singleton or twin lambs were quicker to stand and suck from their mothers than lightweight (more than 1 SD below the breed mean), Suffolk, or triplet lambs. Low birth weight lambs also had lower rectal temperatures than heavier lambs (P < 0.01), as did Suffolk compared with Blackface lambs (P < 0.001). Lambs that were slow to suck after birth had lower rectal temperatures than quick lambs, and this difference persisted for at least 3 d after birth. Within breed, heavy lambs had greater plasma triiodothyronine and thyroxine immediately after birth than light lambs. Blackface lambs had greater plasma triiodothyronine and thyroxine than Suffolk lambs but tended to have less cortisol. Colostrum produced by Blackface ewes had a greater fat content than that of Suffolk ewes (P < 0.001). Thus, lambs that are behaviorally slow at birth are also less able to maintain their body temperature after birth. Although part of their lower body temperature might be attributable to behavioral influences on thermoregulation, the data also suggest that physiological differences exist between these animals. These differences may be related to different degrees of maturity at birth.

Differences in survival and neonatal metabolism in lambs from flocks selected for or against staple strength

In the field, we found that lamb mortality was lower (P < 0.01) in a flock selected for wool staple strength (SS+) than in a flock selected against staple strength (SS–), or a random-bred control flock (R); mortality rates calculated from 4200 records were 17.0, 22.8, and 24.1%, respectively. The proportion of twins was similar in all flocks, but birth weights differed (P < 0.01), being 4.40, 4.25, and 4.14 kg, respectively. Biochemical profiles were measured in lambs from 20 ewes from each flock around the time of birth in an animal-house study, to define potential causes of the genetic difference in survival. In the intensive study, birth weight was similar for all groups; however, gestation length was slightly shorter in the SS+ lambs (P < 0.05 for SS+ v. R). All lambs had low plasma concentrations of the essential amino acids and glucose at birth, compared with after suckling. Plasma concentrations of glucose, non-esterified fatty acids (NEFA), and blood urea nitrogen (BUN) at birth differed between lambs from the flocks with the greatest difference in mortality (i.e. SS+ v. R); that is, SS+ lambs had higher concentrations of glucose (1.84 v. 1.23 mm; P < 0.01) and NEFA (1.52 v. 1.08 mm; P < 0.05), and lower concentrations of BUN (7.7 v. 9.7 mm; P < 0.01) at birth, and for the next 9 h. The semimembranosus and semitendinosus muscle of the SS+ lambs also had lower concentrations of ATP (7.4 v. 5.0 μmol/g, respectively) compared with lambs from the other flocks, immediately post-partum. Muscle phosphocreatinine was also slightly lower in the SS+ lambs. No differences between the genotypes were observed in reserves of glycogen, carcass fat, brown adipose tissue, or in neonatal thyroid hormones. In summary, sheep selected for high wool staple strength had lower lamb mortality than the other 2 flocks. This was accompanied by metabolic differences at birth that can be associated with greater metabolic maturity, rather than greater energy reserves.

Competing risks analysis of lamb mortality in a terminal sire composite population1

Mortality records from birth to weaning of 8,301 lambs from a composite population at the U.S. Meat Animal Research Center were analyzed using a competing risks model. The advantage of the competing risks model over traditional survival analyses is that different hazards of mortality can be assigned to different causes, such as disease, dystocia, and starvation. In this study, specific causes of mortality were grouped into dam-related (DAMR; e.g., dystocia and starvation), pneumonia (PNEU), disease (DIS; excluding pneumonia), and other (OTHER) categories. The hazard of mortality was analyzed using a competing risk approach, where each mortality category was assumed to be independent. Continuous- and discrete-time survival analyses were implemented using sire, animal, and maternal effects mixed models. The continuous-time survival analysis used the Weibull model to describe the hazard of mortality for each category of mortality. Under the discrete-time survival analysis, a complementary log-log link function was used to analyze animal-time data sets using weekly intervals for each category of mortality. Explanatory variables were sex, type of birth, contemporary group, and age of dam. The significant influences of type of birth and age of dam effects were consistent across category of mortality, and the sex effect was significant for all categories except the OTHER category. Estimates of variance components indicated strong maternal effects for all categories except for PNEU. Estimates of additive genetic heritabilities from the discrete maternal effects models were 0.08+/-0.04, 0.09+/-0.18, 0.16+/-0.12, 0.19+/-0.09, and 0.14+/-0.10 for OVERALL (all causes combined), DIS, DAMR, PNEU, and OTHER categories, respectively. Ignoring the cause of the defining event in mortality and longevity studies may hide important genetic differences. Therefore, the effectiveness of breeding programs relying on models that ignore multiple causes of an event in time-to-event data, such as mortality and longevity, could be affected.