Rambouillet and Romanov reciprocal breed effects on survival and growth traits of F1 lambs and on reproductive traits of F1 ewes

Abstract

Objectives were to estimate reciprocal effects of Romanov and Rambouillet breeds on survival, growth, and reproductive traits of F1 progeny and direct breed effects (Suffolk and Composite – ½ Columbia, ¼ Hampshire, and ¼ Suffolk) on survival and growth traits of the subsequent terminally sired lambs. Mature Rambouillet ewes (n = 243) were exposed to 20 Romanov rams over two seasons producing 621 lambs for evaluation of growth and survival traits with 274 F1 ewes being evaluated for reproduction traits through 4 yr of age. Similarly, mature Romanov ewes (n = 116) were exposed to 20 Rambouillet rams producing 601 lambs for evaluation of growth and survival traits with 176 F1 ewes being evaluated for reproduction traits through 4 yr of age. A total of 433 of those F1 ewes produced 3,431 lambs (1,552 litters) from 1,634 exposures to terminal sires over 4 yr. Terminal sires consisted of 38 Suffolk and 44 Composite rams. Reciprocal crossbred ewe lambs were produced from dramatically different uterine and neonatal environments, with litter size at birth from Romanov dams exceeding those from Rambouillet dams by 1.52 lambs (P < 0.001) and birth weight of lambs from Romanov dams averaged 3.41 kg compared with 4.26 kg from Rambouillet dams. Differences in BW were still evident at 140 d (P < 0.001) for dam-reared lambs. However, reciprocal ewe first breeding BW of both types were similar (P = 0.38). Minimal differences were observed in performance of reciprocal cross ewes through 4 yr for productivity, longevity, or progeny growth and survival. One exception was BW at 140 d where an interaction of dam breed with terminal sire breed reached significance for both dam-reared (P = 0.05) and nursery-reared (P = 0.02) lambs. This interaction was due to the lower weight of Composite-sired lambs out of reciprocal cross ewes born from Rambouillet dams. Composite rams increased number born (P < 0.01) and number weaned (P < 0.05) of the reciprocal cross ewes. Suffolk rams increased (P < 0.001) BW and growth rates from birth to 140 d of terminal progeny. Thus, there were little cumulative differences accrued over the 4 yr and no differences were detected for cumulative kilogram of lamb generated at 140 d per ewe exposed. The practical outcome of this evaluation was that performance levels of both types of Romanov crossbred ewes was similar allowing the industry to produce the desired crossbred ewes without needing large purebred ewe flocks of the less numerous Romanov breed.

INTRODUCTION

Improved ewe productivity has been identified as a critical priority to allow sustained competitiveness in the global market for U.S. lamb production (ASI, 2016). Within the common annual production system of fall breeding and spring lambing, biological efficiency was shown to be most influenced by survival, fertility, and prolificacy (Wang and Dickerson, 1991a,b,c). Romanov sheep have documented superior levels of performance for survival, fertility, prolificacy, and length of seasonal fertility (Casas et al., 2004, 2005; Freking et al., 2004).

Efficiency of commercial sheep production could be improved markedly by greater industry use of specialized superior dam lines as maternal contributors in terminal crossbreeding systems. An important issue for commercial producers is the relative performance of F1 replacement ewes sired by Romanov rams compared to F1 ewes produced by Romanov dams. Potential genetic causes of reciprocal breed effects include maternal and paternal breed effects, maternal and paternal imprinting effects, and mitochondrial effects. While little is known about imprinting and mitochondrial effects in sheep, maternal breed effects are well documented (Bradford, 1972). Significant maternal effects on prenatal survival, birth weight, postnatal survival, and postnatal growth are often detected and associated with differences in litter size. Maternal effects on reproductive traits have been less studied.

Rambouillet and Romanov represent an extreme contrast in average litter size of roughly 2.0 lambs and thus are excellent resources to investigate reciprocal breed effects. Our objectives in this experiment were first to estimate Rambouillet and Romanov reciprocal breed effects on survival and growth traits of F1 lambs and on reproductive traits of F1 ewes. Secondly, our objective was to estimate direct breed effects on survival and growth traits of crossbred lambs sired by Suffolk and Composite rams.

MATERIALS AND METHODS

General Experimental Design and Traits Recorded

The U.S. Meat Animal Research Center (USMARC) Institutional Animal Care and Use Committee approved the experiment following recommendations by FASS (1999). The primary experimental objective is to estimate reciprocal breed (Romanov vs. Rambouillet) effects on reproductive traits of F1 ewes (Figure 1). The intent was to produce at least 200 breeding ewes of each reciprocal cross over 2 yr (2005 and 2006) for subsequent evaluation of reproductive traits under annual production systems at 1, 2, 3, and 4 yr of age (2006 to 2010). Given the expected reproductive rate of the breeds, twice as many Rambouillet ewes were needed to generate roughly equivalent numbers of crossbred females. Actual numbers of ewes exposed to rams for each year and mating type are represented in Table 1. The experiment consisted of two phases that represented distinct generations: production of reciprocal crosses of F1 lambs; and production of terminally-sired lambs by reciprocal crosses of F1 ewes. Romanov ewes and rams were sampled from the USMARC flock which represented most of the available diversity of the Romanov breed in the U.S. Purebred Rambouillet ewes and rams were purchased from seven different producers in Texas. Breeding flocks of Romanov and Rambouillet ewes were similar in age distribution and all ewes at least 3 yr of age at breeding. Twenty rams (10 per year) from each breed contributed progeny for the production and evaluation of F1 crosses in this experiment.

Figure 1.

Picture depicting the design contrast of this reciprocal cross experiment. Purebred populations of these two breeds of ewes have an extreme difference in reproductive rate. The top two panels represent Rambouillet rams mated to Romanov ewes where the average litter size of mature ewes is about 3.7 lambs born per ewe lambing. The bottom two panels represent Romanov rams mated to Rambouillet ewes where the average litter size of mature ewes is about 1.7 lambs born per ewe lambing.

Table 1.

Number of Romanov, Rambouillet, and reciprocal F1 cross ewes joined by mating types and year of lambing subclasses

		Year of lambing
Breed of Ewe	Breed of Ram	2005	2006	2007	2008	2009	2010
Romanov	Rambouillet	116	94
Rambouillet	Romanov	243	178
F1 Paternal Romanov	Suffolk		74	132	124	120	52
F1 Paternal Romanov	Composite		75	133	121	117	54
F1 Paternal Rambouillet	Suffolk		50	83	75	70	33
F1 Paternal Rambouillet	Composite		50	85	81	72	33

Traits evaluated were divided into those measured from the start of the breeding season to weaning of the lambs, those measured postweaning to 140 d of age of the lambs, and those measured cumulatively after ewes were 4 yr of age. Traits measured from the start of the breeding season to weaning were weight of the ewe at the beginning of the first breeding season, conception rate (ewe exposed but did not lamb = 0; ewe lambed = 1), number born, litter birth weight, number of dam and nursery-reared weaned lambs, and dam and nursery-reared litter weaning weight. Traits measured at 140 d of age were the number of dam and nursery-reared lambs, dam and nursery-reared litter weight per ewe lambing, and dam and nursery-reared litter weight per ewe exposed. Total productivity through 4 yr of age for each ewe entering the breeding flock was calculated as the sum of 140-d weights for dam or nursery-reared lambs. Traits measured at weaning and 140 d of age were defined separately for dam-reared and nursery-reared lambs to evaluate both aspects of reproduction of crossbred ewes.

Phase I: Production of Reciprocal Crosses of F1 Lambs

Breeding flocks of Romanov and Rambouillet ewes were similar in age distribution and ewes were at least 3 yr of age at breeding. A total of 116 (28 3 yr old; 50 4 yr old; 27 5 yr old; and 11 6 yr old) mature Romanov ewes were single-sire mated to 10 Rambouillet rams (ranged 4 to 7 yr age) during a 35-d breeding season beginning September of 2004. A total of 94 of those remaining Romanov ewes were again exposed to 10 new Rambouillet rams (ranged 1 to 4 yr of age) the following September of 2005. Likewise, 243 (51 3 yr old; 41 4 yr old; 61 5 yr old; 54 6 yr old; 22 7 yr old; and 14 8 yr old) mature Rambouillet ewes were single-sire mated to 10 Romanov rams (ranged 1 to 5 yr of age) in September of 2004, with 178 of the remaining ewes exposed to the second sample of 10 Romanov rams (ranged 1 to 3 yr of age) in September of 2005.

Purebred ewes were shorn about 30 d before the start of lambing and separated by breed in drop pens. Ewes were limited to rearing two lambs, with additional lambs reared in the nursery without regard to sex of the lamb, although smaller lambs were typically selected. This approach was intended to help standardize early growth and development of the two types of crossbred ewe lambs available for subsequent evaluation. Ram lambs were castrated and tails docked on all lambs. Lambs were weighed at 0 (birth), 56 (weaning), 70, and 140 d of age. Nursery-reared lambs were weaned at ~35 d from milk replacer as they transitioned to creep feed rations and later re-joined contemporaries at the time of weaning for dam-reared lambs (56 d of age). Lambs were provided ad libitum access to a total mixed diet (18% CP) from creep to about 27 kg BW and then fed ad libitum a total mixed diet (2.96 Mcal of ME/kg of DM with 14.5% CP) during the finishing period. To account for variation in range of ages for a common weight date, individual lamb BW was adjusted to the intended target ages using the individuals own ADG for the period involved. After 140 d weights were recorded, replacement ewe lambs were identified and moved to pasture. Selection of F1 ewe lambs was based on routine culling for structural abnormalities and health, with additional culling of the bottom 10% to 15% based on adjusted 140 d BW, ignoring rearing status. Culling based on this BW was conducted separately within each line to reduce bias against potential growth differences between lines. Using this criterion, 132 dam-reared and 44 nursery-reared F1 ewes out of Romanov dams were selected for evaluation. Likewise, 249 dam-reared and 25 nursery-reared F1 ewes out of Rambouillet dams were initially selected. BW of all F1 ewes evaluated for reproduction was also recorded at the time of first breeding.

Phase II: Production of Terminally Sired Lambs by Reciprocal Crosses of F1 Ewes

F1 ewes were maintained as a single contemporary group except during breeding and lambing. Ewes were primarily managed on grass pastures with supplementation only if forage was limiting. Multi-sire breeding groups on pasture exposed half of the ewes to Suffolk rams the other half to Composite rams (Leymaster, 1991) during 35 d breeding seasons beginning each September. This composite was developed from ½ Columbia, ¼ Suffolk, and ¼ Hampshire germplasm. Each ewe was randomly assigned to either Suffolk or Composite rams for the duration of the experiment to facilitate investigation of potential effects of ram mating breed (Suffolk and Composite) on ewe longevity. To sample the ram breeds adequately, roughly one ram was used for each 15 ewes. These rams consisted of roughly equal numbers of 1 and 2 yr old rams at the beginning of the experiment. Once a ram was used in multi-sire mating, he continued to be used in subsequent years unless health or fertility problems occur. A total of 38 Suffolk and 44 Composite rams were utilized over the 4 yr of the experiment. The F1 ewes were evaluated for reproductive traits over 4 yr of production. Ewes born in 2005 were exposed to lamb in 2006 to 2009. Ewes born in 2006 were exposed to lamb in 2007 to 2010.

Ewes were shorn about 30 d before the start of lambing and separated by reciprocal type in drop pens to avoid incorrect line assignment data due to mis-mothering. Ewes were limited to rearing two lambs, with additional lambs reared in the nursery without regard to sex of the lamb. Management and data recorded on Phase II lambs was the same as previously described for Phase I. Ewe longevity was recorded as presence or absence in the herd at about 50 mo age.

Statistical Analysis

Data were analyzed with the mixed-model analysis of variance procedure of SAS (SAS Inst., Inc., Cary, NC). In Phase I of the experiment where the reciprocal cross progeny were generated, the models included fixed effects of year (2005, 2006), and ewe line (Rambouillet, Romanov) for traits recorded on ewes (Table 3). Additional fixed effects for sex of lamb (male, female), type of birth (1, 2, 3, or 4+) for birth weight and survival traits to weaning, or rearing (1, 2) for subsequent BW traits, were added for analysis of traits recorded on lambs (Tables 2 and 4). Nursery-reared lambs were analyzed separately for traits after birth. Interactions among fixed effects that included line of ewe were also fitted. The random effect of individual rams nested within ewe line was included. Levels of significance associated with the effects of ewe line were tested with this individual ram within ewe line mean square and are considered approximations due to unbalanced data. Remaining fixed effects and interactions were tested against the residual mean square.

Table 3.

Levels of significance, least squares means, and standard errors for the effect of ewe breed producing F1 reciprocal cross lambs for traits recorded on purebred ewes

	Ewe breed
Item1	Romanov	Rambouillet	Level of significance
Conception rate, %	85.4 ± 2.7	81.6 ± 2.1	0.28
Number born	3.36 ± 0.06	1.84 ± 0.04	<0.0001
Litter birth weight, kg	10.27 ± 0.23	8.62 ± 0.18	<0.0001
Number weaned2	2.37 ± 0.07	1.60 ± 0.05	<0.0001
Litter 56 d wt2, kg	32.54 ± 0.93	24.76 ± 0.68	<0.0001
Litter 70 d wt2, kg	38.82 ± 1.11	29.07 ± 0.81	<0.0001
Litter 140 d wt2, kg	101.57 ± 3.27	75.08 ± 2.51	<0.0001

¹All traits are on a per ewe lambing basis except for conception rate.

²Calculated for genetic birth dam, combining both dam- and nursery-reared lambs.

Table 2.

Levels of significance, least squares means, standard errors, and number of lambs for the effect of ewe breed producing F1 reciprocal cross lambs for growth and survival traits recorded on F1 lambs

Item	Least squares means (N for F1 lambs) for breed of ewe
	Romanov	Rambouillet	Level of significance
Birth weight, kg	3.41 ± 0.08 (601)	4.26 ± 0.10 (621)	<0.0001
56d weight on dam, kg	14.10 ± 0.24 (237)	16.36 ± 0.17 (488)	<0.0001
56d weight nursery, kg	13.71 ± 0.21 (184)	14.57 ± 0.38 (54)	0.05
70d weight on dam, kg	16.78 ± 0.30 (235)	19.22 ± 0.21 (485)	<0.0001
70d weight nursery, kg	16.41 ± 0.27 (182)	17.24 ± 0.47 (51)	0.13
140d weight on dam, kg	47.53 ± 0.60 (234)	50.04 ± 0.49 (477)	0.002
140d weight nursery, kg	40.35 ± 0.74 (171)	39.65 ± 1.10 (48)	0.61
Survival to wean on dam, %	63.2 ± 2.8 (411)	82.3 ± 2.5 (568)	<0.0001
Survival to wean nursery, %	78.0 ± 6.1 (190)	88.0 ± 7.2 (53)	0.17
Survival to 140 d on dam, %	60.5 ± 2.8 (411)	80.6 ± 2.6 (568)	<0.0001
Survival to 140 d nursery, %	76.0 ± 6.5 (190)	85.3 ± 7.6 (53)	0.22
Ewe lamb breeding weight, kg	41.0 ± 0.46 (176)	41.6 ± 0.45 (274)	0.38

Table 4.

Levels of significance, number of lambs, least squares means and average standard errors of growth traits for the interaction effect of breed of ewe by type of birth on birth weight or rearing type on dam-reared only wean weight of phase I lambs

	Least squares means for breed of ewe				Level of significance
Trait	Romanov	(N)	Rambouillet	(N)
Birth weight, kg					<0.0001
Single	4.26 ± 0.27	7	5.38 ± 0.08	87
Twin	3.36 ± 0.09	68	4.70 ± 0.05	446
Triplet	3.08 ± 0.07	156	4.04 ± 0.08	84
Quad and above	2.95 ± 0.05	370	2.90 ± 0.37	4
Adjusted 56 d wt on dam, kg					0.006
Single	14.91 ± 0.40	62	16.74 ± 0.24	134
Twin	13.28 ± 0.26	175	15.98 ± 0.26	354

Phase II of the experiment evaluated the reciprocal types of ewes over 4 yr of production when mated to either Suffolk or Composite rams under an annual production system. Models for traits recorded on the terminal-sired lambs included fixed effects of year (2006, 2007, 2008, 2009, and 2010), age of ewe (1, 2, 3, and 4), reciprocal ewe line (Romanov or Rambouillet as paternal parents), terminal sire line (Suffolk or Composite), sex of lamb (male, female), and type of birth (1, 2, or 3+) or rearing (1, 2). Two-way interactions among fixed effects that included reciprocal ewe line were also fitted (Table 5). The random effect of individual sire of each ewe within reciprocal ewe line was fitted to test effects associated with reciprocally produced ewe line. Models for traits recorded as traits of the ewe (Table 6) included fixed effects of contemporary mating groups (five groups over 4 yr), ewe age (1, 2, 3, and 4), reciprocal ewe line (Romanov or Rambouillet as paternal parents), terminal sire line (Suffolk or Composite), and the two-way interactions among fixed effects that included reciprocal ewe line. Models for cumulative traits calculated per ewe were similar but did not include the effects of contemporary groups or ewe age.

Table 5.

Levels of significance, least squares means, and standard errors for the interaction effect of ewe reciprocal line and terminal sire ram breed for growth and survival traits recorded on Phase II lambs.

Trait	Dam breed of F1 terminal breed	Least squares means for parental breeds interaction1				Avg. SEM	Level of significance
		Rom × Suff	Rom × Comp	Ramb × Suff	Ramb × Comp		Dam line	Sire line	Interaction
Birth weight, kg		4.60	4.47	4.61	4.44	0.06	0.84	<0.0001	0.53
56 d weight on dam, kg		17.29	16.20	17.24	15.92	0.21	0.53	<0.0001	0.38
56 d weight nursery, kg		14.97	15.14	14.86	14.51	0.31	0.28	0.73	0.85
70 d weight on dam, kg		20.41	19.07	20.33	18.73	0.26	0.54	<0.0001	0.39
70 d weight nursery, kg		17.71	17.98	17.60	17.21	0.38	0.29	0.85	0.86
140 d weight on dam, kg		42.87	40.49	42.20	38.83	0.38	0.02	<0.0001	0.05
140 d weight nursery, kg		34.98	34.21	34.17	30.83	0.64	0.01	<0.0001	0.02
Survival to wean on dam, %		84.3	85.2	86.2	86.3	1.40	0.28	0.74	0.78
Survival to 140 d on dam, %2		97.8	96.7	96.7	97.0	0.80	0.68	0.61	0.34

¹Two reciprocal F1 ewe types crossed with two terminal sire breeds. Rom = Romanov dams producing F1 ewes. Ramb = Rambouillet dams producing F1 ewes. Suff = Suffolk ram breed. Comp = Composite ram breed.

²Postweaning survival of those that survived to weaning.

Table 6.

Levels of significance, least squares means, and standard errors for the interaction effect of ewe reciprocal line and terminal sire ram breed for reproductive traits recorded on reciprocal F1 ewes

Trait	Dam breed of F1 terminal breed	Least squares means for parental breeds interaction 1				Avg. SEM	Level of significance
		Rom × Suff	Rom × Comp	Ramb × Suff	Ramb × Comp		Dam line	Sire line	Interaction
Conception rate3, %		95.3	94.9	93.9	95.8	1.40	0.89	0.49	0.30
Number born2		2.19	2.27	2.15	2.24	0.04	0.40	0.01	0.98
Litter birth wt2, kg		10.09	10.05	9.88	9.78	0.16	0.21	0.60	0.80
Number weaned2		1.90	1.99	1.91	1.99	0.04	0.98	0.02	0.82
Litter 56 d wt2, kg		31.62	30.88	31.64	29.96	0.65	0.54	0.03	0.38
Litter 70 d wt2, kg		35.90	35.60	36.08	34.64	0.84	0.69	0.20	0.41
Litter 140 d wt2, kg		74.73	73.95	72.71	69.71	1.70	0.12	0.19	0.44
Ewe longevity to 50 mo3, %		72.8	78.4	80.2	82.8	4.40	0.24	0.29	0.69
Cumulative number born3		7.32	7.83	7.35	7.83	0.33	0.97	0.07	0.95
Cumulative number weaned3		6.35	6.89	6.51	6.95	0.30	0.76	0.06	0.85
Cumulative litter birth wt3, kg		33.61	34.39	33.62	34.18	1.44	0.90	0.52	0.86
Cumulative litter 140 d wt3, kg		247.2	251.9	246.0	241.2	11.5	0.65	0.99	0.63

¹Two reciprocal F1 ewe types crossed with two terminal sire breeds. Rom = Romanov dams producing F1 ewes. Ramb = Rambouillet dams producing F1 ewes. Suff = Suffolk ram breed. Comp = Composite ram breed.

²Trait is on a per ewe lambing basis.

³Trait is on a per ewe exposed basis

RESULTS AND DISCUSSION

General Results

Levels of significance and least squares means are reported for effects of ewe breed producing reciprocal cross lambs, and both ewe and sire breed for the production of terminal-sired progeny from the evaluation of reproduction in the reciprocal cross ewes. Estimated differences between these ewe breeds can be the result of direct additive genetic effects as well as any differences due to maternal effects, imprinted and transgenerational effects, and differences in mitochondrial origin. In particular, it was expected that we could observe maternal genetic effects associated with differences in litter size.

Effects of year and its interaction with ewe breed and sire breed were fit in the model but are not discussed because conditions contributing to these effects could not be identified, the effects cannot be predicted to recur in the future, and it is likely that decisions on genetic choices by breeders will do so with average year effects in mind. Genotype (breeds) and environmental interactions (year) did not exhibit consistent trends and were outside of the primary objectives of the study. Age of ewe effects was consistently detected as highly significant for most ewe traits analyzed (the exception being conception rate) and results were consistent with the well-known influences on ewe productivity traits where ewe lamb performance is typically less than in the subsequent two to three parities and did not interact with the main effects of ewe breed or sire breed. Therefore, results associated with ewe age were not tabulated or further discussed; however, it is notable that differences between ewe breeds were not detected even at the first parity where one might anticipate the impact of this contrast to be most pronounced.

Phase I: Production of Reciprocal Crosses of F1 Lambs

Effects of ewe breed producing F1 reciprocal cross lambs are presented for traits recorded on a per lamb basis (Table 2) and for traits recorded on a per ewe basis (Table 3). The Romanov breed originated in northwestern Russia and excels in adaptability, length of breeding season, age at puberty, prenatal and postnatal survival, maternal behavior, and ewe productivity (Fahmy, 1996; Freking et al., 2000). Although the reproductive rate advantages seem to be associated with many genes each with small effects rather than genetic differences at few loci with larger effects, recent evidence of a mutation in the Romanov breed (Heaton et al., 2017) for the same gene (BMPR-1B) containing the Booroola allele is worthy of future investigation. The Rambouillet breed is one of the most predominant wool breeds used throughout the extensively managed areas such as Texas and western range of the United States with a lower reproductive rate, although they can responded to selection applied for reproduction traits (Burfening et al., 1993).

Over the 2 yr period of matings, mature Romanov ewes (n = 116) produced 601 lambs and mature Rambouillet ewes (n = 243) produced 621 lambs for evaluation of growth and survival traits. This result highlights the contrast of these breeds that would be informative for producers trying to make breeding system decisions. Despite no detectable difference in conception rate (P = 0.28; Table 3), twice as many Rambouillet ewes compared to Romanov ewes were required at the start of the experiment to produce similar numbers of contemporary reciprocal crossbred progeny. These higher litter sizes were associated with differences in survival rate of reciprocal crossbred lambs. Differences in ewe breeds influenced (P < 0.001) survival of dam-reared progeny by showing an increase of 20% from Rambouillet dams at weaning and 140 d compared to progeny from Romanov dams. Survival rates of nursery-reared progeny were similar between the two ewe breeds (P > 0.17). Subsequent BW measures on dam-reared lambs were significantly higher (P < 0.001) at 56, 70, and 140 d for Rambouillet. Litter weight measured out to d 140 indicated an advantage of >25 kg for Romanov ewes on a per ewe lambing basis. However, on an individual lamb basis no differences were detected for ewe breed between reciprocal F1 ewe types from the recorded first breeding weight at ~7 mo. The impact of ewe breed was much less for nursery-reared lambs and was not significant (P = 0.61) by the time lambs reached 140 d.

The dramatically different maternal environments resulted in detectable interactions of breed of ewe with type of birth for birth weight and with type of rearing for weaning weight (Table 4). The higher litter sizes in the Romanov litters restricted individual fetal size compared to similar genetic fetuses gestated in smaller litter sizes in Rambouillet uterine environments. As indicated from the larger standard errors, very few singles were born to Romanov ewes and very few triplets, and a single set of quads were born to Rambouillet ewes. Single, twin, and triplet F1 lambs were lighter when gestated in Romanov ewes compared to Rambouillet. There was a 1.6 kg difference between singles and twins reared by Romanov dams as opposed to only a 0.76 kg difference between singles and twins reared by Rambouillet ewes.

Phase II: Production of Terminally Sired Lambs by Reciprocal Crosses of F1 Ewes

A total of 433 reciprocal cross F1 ewes produced 3,431 lambs (1,552 L) from 1,634 exposures to terminal sires over 4 yr. Terminal sires consisted of 38 Suffolk and 44 Composite rams. Effects of reciprocal cross ewe breed and terminal sire breed are presented for traits recorded on a per lamb basis (Table 5) and for traits recorded on a per ewe basis (Table 6).

Considering only the main effect of dam line, birth weight of terminal sired lambs was similar (P = 0.84) from the two reciprocal cross ewe breeds and BW after birth revealed few detectable differences from weaning through 140 d for either dam-reared or nursery-reared progeny. The presence of an interaction of dam line with sire line for 140 d weight is discussed later in the manuscript. Reciprocal cross ewe breed also did not influence (P = 0.28) survival rate of terminal-sired progeny.

Terminal sire breed effects were evident (P < 0.0001) at birth with lambs born from Suffolk rams exceeding those from Composite rams by 0.15 kg. Suffolk-sired lambs continued to grow at a faster rate than Composite-sired lambs and were heavier by 1.2, 1.5, and 2.8 kg at 56-, 70-, and 140-d, respectively. The two terminal sire breeds were similar for survival rates to weaning (P = 0.74) and 140 d (P = 0.61). Survival rates were high in this phase of the experiment highlighting the importance of maximizing both maternal and individual lamb heterosis. These higher survival rates in this phase of the experiment can also be attributed in part to less variable litter sizes in the F1 ewes, especially compared to the variability in the purebred Romanov. Leeds et al. (2012) reported results from a terminal sire study that included the USMARC Composite and Suffolk as sire breeds mated to Rambouillet ewes under extensive rangeland systems and did not detect sire breed differences in ewe fertility or productivity but approached significance for crossbred lamb survival to weaning. In that study, an interaction was detected between sire breeds and birth weight, indicating that lightweight Suffolk-sired lambs had a greater risk of death than lightweight Composite-sired lambs. Similar to the current study, Suffolk-sired progeny in the rangeland study had greater pre- and postweaning growth than Composite-sired progeny (Notter et al., 2012).

Minimal differences were observed in performance of reciprocal cross ewes through their age 4 yr seasons for productivity, longevity, or progeny growth and survival. One exception was BW at 140 d where an interaction of dam breed with terminal sire breed reached significance for both dam-reared (P = 0.05) and nursery-reared (P = 0.02) lambs. In both cases, the interaction was due to the lower than average weight of Composite-sired lambs out of reciprocal cross ewes born from Romanov sires and Rambouillet dams. Composite terminals increased number born (P = 0.01) and number weaned (P = 0.02) leading to greater litter weight at 56 d (P < 0.05) from the reciprocal cross ewes. Although the interaction of dam breed with terminal sire breed did not reach significance for litter birth weight (P = 0.80), one could speculate that the apparent improved embryonic survival rates could be due to less extreme embryonic growth from the Composite sires. Suffolk rams increased (P < 0.001) BW and growth rates from birth to 140 d of terminal progeny. Comparing the relative differences between the two terminal-sire breeds, Composite sires increased numbers of live terminal progeny while Suffolk sires increased the growth rates. Thus, there were very little cumulative differences accrued over the 4 yr and no differences were detected for cumulative kilogram of lamb generated at 140 d per ewe exposed. Production systems utilizing terminal sire breeds can affect ewe productivity of maternal lines by increasing fitness traits of crossbred progeny as well as growth and carcass traits. These results were consistent with previous research comparing these two breeds as terminal sires, where similar growth rates but improved fitness traits were observed for Composite-sired lambs (Leymaster, 1991).

We did not detect any large differences in reproductive traits between reciprocal crosses of these two maternal breeds, despite the dramatic differences in litter sizes at birth they were derived from. Limited amounts of evidence in the literature from other experiments conducted in sheep would also suggest that these differences are limited. Shrestha et al. (1983) were unable to detect any reciprocal effects on reproductive traits from five breeds. Contributions to reciprocal differences theoretically could be due to impacts of genes with specific maternally imprinted expression patterns. However, experimental evidence to document the effect of maternally imprinted genes is difficult to distinguish from maternal environment effects (Goddard and Whitelaw, 2014). The practical outcome of this specific reciprocal evaluation is that performance levels of both types of Romanov crossbred ewes will be similar allowing the industry to produce the desired crossbred ewes without needing large purebred ewe flocks of the less numerous Romanov breed.

Conflict of interest statement. None declared.