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Reproductive performance of Pelibuey and Blackbelly sheep under tropical management systems in Mexico
ELSEVIER
Small Ruminant
Research
22 (1996) 3 l-37
Reproductive performance of Pelibuey and Blackbelly sheep under tropical management systems in Mexico M.A. Galina a*bV *, R. Morales b, E. Silva b, B. L6pez a a Facultnd b Posgrudo
Interinstitucional
de Estudios Superiores en Ciencias Pecuurias,
de Cuuutitlin
UNAM, Mexico City, Mgxicico
CUIDA Universidad
Accepted
22 December
de Coha,
AP 22, Colima, 28000,
Mbico
1995
Abstract An analysis of reproductive performance records from 1988 to 1994 of seven hair sheep herds in the dry tropics of Mexico was conducted on 785 Black Belly (BB) and 1810 Pelibuey ewes (P). Reproductive management on four farms included the continuous presence of the male (1471 lambing& and the three other herds had biannual breeding exposure to rams (1057 lambings). In 7 years, the combined herds produced 2528 parturitions from 2585 ewes bred (0.97 births per year per ewe). Number of parturitions were 1.lO year-’ for BB (863 births in 785 ewes) and 0.92 for P (1665 births in 18 10 ewes). Prolificacy was 1.46 lambs per birth with a total of 3690 lambs in 7 years; per breed it was 1.37 for BB and 1.55 for P lambs per parturition (P > 0.05); except for first lambing in both breeds, averaging 1.05 lambs per ewe (P < 0.01). There was an average of 530 (k299) lambs per year with a mortality of 15%; still births, malnutrition, diarrhea and pneumonia accounted for most of the losses. Mortality increased to 23% when more than two lambs were nursed by one ewe and reduced to 8% when only one lamb was born. Average age at first lambing was 465 days (15.25 months) for the combined herds. Yearly average (1988-1994) for lambing interval (LI) was 265, 227, 238, 225, 252, 230 and 258 days, with an average of 242 days (+ 16). Quarterly seasonal birth distribution was 50% from January to March (n = 1264); 25% from April to June (n = 632); 15% from July to September (n = 379) and 10% from October to December (n = 252). No statistical differences were observed among farms in reproductive performance. Seasonality of reproduction was demonstrated with minimum (7%) of lambs born in the fall and 50% in the winter. Rainfall had a significant correlation with fertility and estrous production (P < 0.051, but temperature and photoperiod did not. Grass growth seemed to be the key factor influencing sexual activity in ewes. Interval per lambing was statistically different (P < 0.01) with extended intervals for those ewes lambing during the autumn, winter and spring, compared with the summer lambing ewes. Rearing management had a statistical effect on interval per lambing (P < 0.05) increasing when lactation length augmented from 60 to 120 days. Fertility was higher (P < 0.05) for BB as compared with P, but the latter had a better prolificacy (P < 0.05) during the study. Parturition showed seasonal distribution but 1.5 lambings per year could be achieved throughout with good reproductive management. Keywords:
Reproduction;
Sheep; Seasonality;
Photoperiod;
Pelibuey;
* Corresponding author. Tel: (331) 41 l-33, fax: (331) 275.81; e-mail: [email protected]. 00921-4488/96/$15.00 Published SSDI 0014-2999(95)00878-O
by Elsevier Science B.V.
Blackbelly
32
M.A. Galim
et ul./Small
Ruminmt
1. Introduction Tropical hair sheep have been an important source of meat in many countries in Latin America (Fitzhugh and Bradford, 1983). The hair sheep of Mexico are mainly Pelibuey (P) and Blackbelly (BB). The P breed predominates and makes up 90-95% of the total hair sheep population in Mexico, which amounts to approximately 500000 (Gonzalez-Reyna et al., 1991). Seasonality of reproduction is a common feature in sheep and goat breeds in temperate latitudes but less marked in the tropics, where all year around breeding is practiced (Gonzalez-Reyna et al., 1991; Valencia and Gonzalez, 1983; Chemineau et al., 1992; Galina et al., 1995). Puberty has been recorded from as early as 5 months, with an average age from 210 to 273 days, and as in wool sheep, is related to body weight (Gonzalez-Reyna et al., 199 1). Reproductive activity in the literature shows seasonality, represented by reduced periods of estrus from February to April (Gonzalez-Reyna et al., 199 1; Gonzalez-Reyna and Murphy, 1987b), while in other studies, estrus cycles seemed to be present all year; fertility ranged from 70 to 97% and prolificacy varied from 113 to 144 lambs born from 100 ewes exposed (Valencia and Gonzalez, 1983; Valencia et al., 198 1). Photoperiod does not seem to be the main factor responsible for seasonal breeding in the P ewe. Instead, other factors such as nutrition and possibly relative humidity, precipitation and temperature may have a more direct effect on periods of reduced cyclicity during the months of February to April (Gonzalez-Reyna et al., 1991). Gestation length was comparatively similar to wool sheep. Postpartum period as related to season and lactation length seemed to be the key factor that determined appearance of es&us which varied from 33 to 98 days. Finally perinatal lamb mortality has been documented from 11 to 75% (Gonzalez-Reyna et al., 1991; Valencia et al., 1981). Mexico has a population of 4 X lo6 million sheep and only a small portion are hair sheep (500000). The hair sheep are not usually kept under good nutritional management and it is rare to find concentrates as part of the diet (Galina and Guerrero, 1993). In the dry tropics, reproduction seems to be all year round and reproductive control mechanisms have not been discussed. The objective of this study was to
Research 22 (1996) 31-37
measure reproductive performances under various management systems to improve production to 1.5 parturitions per year as a goal.
2. Materials
and methods
Seven farms in Colima, Mexico were studied from 1988 until 1994. Flock compositions are described yearly in Table 1. Colima is located at 19” 13’ latitude N and 104’48’ longitude W; at 550 m above sea level. The climate is classified as dry tropical, with rain from June to October, average temperature 25°C with a minimum of 17°C and maximum of 30°C. There is an average rain precipitation of 850 mm, minimum relative humidity of 56% and maximum of 80% (Garcia, 1973). Each farm provided monthly information. Data included sheep identification, number of lactations, date of birth, date at first lambing, interval between parturition, age and breed. Fertility and litter size were measured yearly and overall. All farms used grazing as a management system on tropical grasses with low supplementation of 100 to 200 g per ewe of a commercial concentrate of 16% CP during the dry season. Four herds kept males year around. The other three farms introduced the male only 9 months of the year. Both managements included 45 days reproduction rest after lambing. Ewes were bred day or night. Young ewes were exposed to breeding when they attained from 20 to 25 kg body weight (BW). Weaning was performed
Table 1 Flock composition Farm
No. ewes 1 2 3 4 5 6 7 Total
by farms, total number of parturitions
Black belly
18 25 27 45 35 57 27 234
Tabasco pelibuey
a
No. parturitions b
No. parhrritions b
No. ewes ’
95 105 80 145 88 180 92 785
120 157 45 125 47 28 25 547
a No. of ewes present in the flock parturation per farm and breed.
as a yearly
375 460 120 450 185 145 75 1810 average.b Total
M.A. Galim Table 2 Multiple regression
et al./Small
Ruminant Reseurch 22 (1996) 31-37
The equation ships was:
values for fertility Participation estimated
Variables studied
‘ijklm
t Intercept Month Year Number of parturitions Lambing interval
0.8872 3.2 0.8482 0.3345 4.1
value
Significance level
4.65 1.21 12.83 1.17 6.3
0.3201 0.622 I 0.0005 0.6100 0.5430
when animals were from 10 to 15 weeks old (70 to 105 days). Once settled, ewes were kept with other pregnant and non pregnant sheep. Lambs stayed with their mother for 70 to 110 days of lactation until weaned. Statistical analysis included multiple linear regression to study the relationship between month and year of lambing, number of parturition and lambing interval to explain fertility, (defined as the number of parturitions divided by the number of ewes exposed).
Table 3 Fertility per breed and year for Blackbelly Year I988 1989 1990 1991 1992 1993 1994 Total Overall BB + P
Fertility Prolificacy Lambing interval * P < 0.05.
=bo+b,Xi+b,Xj+b,Xk+b,X,+E,
Where YijkIm was the fertility in the ith month from the jth year in the kth parturition with Itb lambing interval after birth and the mth random error; b,, b, , b,, b, and b, were the parameters of the model. Student’s unpaired r-test compared means among breeds. Square root transformation was made on data for intervals prior to analysis. Finally a correlation study of fertility, prolificacy, lambing interval with average temperature, rain fall and photoperiod monthly was performed (CSS, 1988)
3. Results Age at first lambing averaged 15.5 months (+ 3); 15.8 months for BB and 14.6 for P. Lamb weight at birth varied from 1.2 to 3.1 kg being larger for single birth. Seventy-four percent of the lambs weighed were P and 26% BB.
(BB) and Pelibuey (P)
BB (n)
Fertility (%)
P(n)
Parturitions
Fertility (%/o)
27 57 150 228 101 loo 122 785 2595
26 75 126 255 85 107 189 863 2528
96 132a 84 112a 84 107 a 155 c 1 IO a 97
71 114 152 230 445 417 381 1810
64 85 138 178 430 360 410 1665 b
90 75 b 91 77 b 97 86 b 108 d 92 b
between reproductive
Variable
to estimate overall fertility relation-
Parturition
Different letters (a,b) showed differences divided by the number of ewes exposed.
Table 4 Correlation
33
parameters
among
columns
and temperature,
(P < 0.05); (c,dXP
< 0.01). Fertility
defined
as the number
of parturitions
rainfall and photoperiod
With male all year round
Male present biannually
Temperature
Rainfall
Photo period
Temperature
Rainfall
Photo period
0.18 -0.12 0.04
0.75 * 0.09 0.63 *
0.07 - 0.04 -0.18
0.21 - 0.09 0.07
0.63 * - 0.08 0.65 *
0.27 0.14 - 0.09
34 Table 5 Reproductive
M.A. Galina
performance
et al./
Small Ruminant
Research 22 (1996) 3 I-37
of the T and BB hair sheep from the expermental
Trait
Age first lambing (days) Lambing per ewe per year ’ Fertility lambing rate (o/o) ’ Birth weight (kg) Lamb mortality (o/o) Annual weaning rate 3 Interval between lambing (days)
flocks
Black belly
Pelibuey
n
Mean
SD
n
Mean
SD
728 785 785 863 863 848 785
483 1.37 a 1.10 2.850 17 140 230 a
26 0.52 0.12 0.55 3 18 14
916 1810 1810 1665 1665 1580 1810
447 1.55 b 0.92 2.450 14 150 254 b
74 0.35 0.18 0.35 7 16 25
Different letters (a,b) show differences among columns (P < 0.051.’ Total number of lambs per ewe in one or two parturitions.2 Fertility calculated as the number of parturitions divided by the number of ewes exposed.3 Annual weaning rate as the number of lambs that reach weaning divided by the number of ewes lambing.
There was an average of 530 (+299) lambs per year with a mortality of 15%; still births, malnutrition, diarrhea and pneumonia were the major diseases recorded. Mortality increased to 23% when more than two lambs were nursed by one ewe and diminished to 8% when only one lamb was born in both breeds. Fig. 1 summarizes the parturition distribution by months. Distribution differed (P < O.Ol>, being highest in January (n = 490); February (n = 425) and March (n = 3531, decreasing from April (n = 270) to October (n = 35). Fig. 2 shows the interval between lambing distributed by years. Variability existing from year to year during the whole study, as related to rainfall was significant (P < 0.05). However hair sheep showed the possibility of at least one parturition every year. To determine the statistical significance of fertility in relation to month of lambing, year of parturition, number of birth in the productive life of the ewe and interval of lambing, a multiple linear regression was performed. Results are shown in Table 2. Year was the major cause of variation on fertility. Table 3 summarizes the yearly number of observations of total births and fertility per breed. A good reproduction year seemed to be followed by a bad one, with a difference (P < 0.05) for 1989, 1991, 1993, with the exception of 1994 (best year> which had the most significant difference for fertility of the study followed by 1993, which also showed good fertility (P < 0.01). Table 4 shows the correlation coefficients separately for those farms that kept males year round and
those that had a reduced reproduction period for rainfall, length of daylight, temperature and reproductive parameters. Fertility and lambing intervals were significantly correlated with rainfall. All reproductive values, including prolificacy, were not correlated either with temperature or photoperiod. This observation between reproductive performance and environmental factors demonstrated the significance of rainfall to reproduction. Table 5 shows the reproductive performance of BB and P accumulating all values from the seven farms and the seven years studied. Fertility was better for BB (1 .lO> than P (0.92). On the other hand, prolificacy was slightly better for P (1.55) than BB (1.37).
4. Discussion Seasonal breeding in tropical hair sheep is considered to be different from that of wool breeds in the tropics. Environmental factors other than light seem to be more important in influencing seasonal patterns in reproduction. The most important of these is probably nutrition, which is influenced by forage availability and digestibility (Gonzalez-Reyna et al., 1991; Valencia et al., 1981). In the present study, seasonal parturition was demonstrated. Temperature, rainfall and photoperiod were studied. From the above, only rainfall showed an effect in controlling reproduction, probably by nutritional influence as suggested before. On the other hand, these findings did not agree with earlier studies in Mexico and Venezuela (Castillo et al., 1972; Gonzalez-Reyna,
M.A. Gulina et al. / Small Ruminant Research 22 (1996) 31-37
1977; Navarro, 1972) but did agree with the more recently introduced concept of periods of reduced reproduction activity by Gonzalez-Reyna and Murphy (1987a) and Valencia et al. (198 1). Ewes showed reduced estrus activity from February to April. The present work suggested extended postpartum intervals for those ewes lambing during the autumn, winter and spring, as compared with the summer lambings as shown by Valencia et al. (1981) and for those ewes that had an increased lactation length due to lambs nursing for more than 60 days. Positive correlation has been demonstrated among lactation and lambing interval, with the present findings in agreement with the literature (Alvarez et al., 1984; Castillo et al., 1974; Leyva et al., 1983). An interval per lambing of 210 days is needed to have two lambings per year. Nutritional status of the ewe during the last third of gestation and early lactation plays an important role in the return to activity post partum. A reduction in the interval to first estrus of 43 days to 33 days has been found when ewes were supplemented with concentrates (Salinas et al., 1975; Mazzari et al., 1984). A 25% decrease in body weight, at the time of parturition, increased the interval to first estrus from 51 to 91 days (Gonzalez-Reyna et al., 1987). In the present study, LI of 230 to 250 days showed that concentrate supplementation played a key role in the reproductive management accompanied by a reduced lactation of 70 days. Under similar conditions, BB ewes seemed to perform better with LI of 230 days as compared with 254 days for the P ewes. Larger size and better physical condition could explain these results. Age at first lambing was higher in BB (483 days) than in P (447 days) ewes. The latter seemed to be able to reproduce at a lighter weight. In previous literature, age at first lambing was shown to be affected by several genetic and environmental factors. A significant correlation has been demonstrated before, between birth weight and age at puberty (Fuentes et al., 1983). Single birth and larger ewes obtained first lambing faster than those from multiple birth or smaller birth weight (P < 0.05). Nutrition also correlated with age at puberty. Lambs with higher weight gains reached puberty and lambed at a younger age (420 days) than lambs with low gains, results that agreed with previous work (Valencia and Gonzalez, 1983). Season of birth affected age at
35
puberty and age at first lambing. Ewes born in May-June reached puberty at a younger age than those born in November-April (Cruz et al., 1983). Previous studies showed that ewes born in winter have their first lambing at a younger age (398 days) than those born in either spring (454 days), summer (488 days) or autumn (435 days) as demonstrated by Ortega et al. (1981). BB and P ewes that give birth in the winter did have a smaller LI than those that lambed later in the year in the present study. Fertility, defined as the number of ewes lambing divided by the number of ewes exposed to the ram was 97% in the present study, a value which is greater than those from previous studies (70% as compared with 97%) (Valencia et al., 1975; Valencia and Gonzalez, 1983). Black belly ewes were significantly superior (P < 0.05) with 110% to 92% from P but prolificacy, defined as the number of lambs born per 100 ewes exposed to males, was smaller for BB (137) than for P (155) which varies from 113 to 144 in the literature; both comparable with the average in Pelibuey studies (Gonzalez-Reyna, 1983; Valencia and Gonzalez, 1983) Lamb mortality in tropical hair sheep has been reported to vary from 11.2 to 75% (Gonzalez-Reyna, 1983; Valencia and Gonzalez, 19831, with up to 65% of deaths taking place during the first week of life. Perinatal mortality is largely due to lack of attention to the newborn lamb by the dam and/or weak lambs due to lack of nursing after birth. In the present study, lamb mortality was 15% increasing when more than two lambs were born and diminishing when dam nutrition was improved with supplementation. A possible management practice to reduce lamb mortality was to improve care and attention at the time of lambing and support ewes that gave birth to three or more lambs with artificial rearing and better nutrition during gestation. In summary, smaller size compared with wool ewes and adaptation in the tropics suggested that BB or P ewes could be used competitively for economical sheep meat production and that reproduction could be improved through management. 5. Resumen Se realize caracten’sticas
un analisis de 10s registros de las reproductivas de 785 ovejas Black
36
M.A. Galina et al./ Smull Ruminanr Research 22 (1996) 31-37
belly (BB) y 1,810 ovejas Pelibuey (P) de siete rebaiios ovinos en el tropic0 seco mexicano de 1988 a 1994. El analisis fue dividido en dos partes la primera de cuatro rebai?os cuyas ovejas tenian presencia de1 macho todo el afio (1471 partos) y la segunda con tres rebahos con empadre bianual (10.57 partos). Los rebaiios combinados en 10s 7 aiios produjeron un total de 2528 partos de 2585 ovejas con una fertilidad global de 0.97 nacimientos por aiio por oveja. La fertilidad fue el ntimero de partos at-males por el numero de ovejas expuestas al macho. Divididos por tipo racial, la fertilidad fue de 1.10 partos por tie para las BB (863 partos para 785 ovejas) y 0.92 para P (1665 nacimientos en 18 10 hembras). La prolificidad fue de 1.46 corderos por nacimiento con un gran total de 3690 corderos en 7 adios. La prolificidad por tipo racial fue de 1.37 para las BB y 1.55 para las ovejas P por parto no encontrandose diferencias significativas entre razas o numero de parto con la exception de1 primer parto que dio solamente 1.05 corderos por oveja (P < 0.01). Se observe un promedio de 530 ( + 299) corderos por aho con una mortalidad de1 15%, las principales causas de las pe’rdidas fueron corderos nacidos debiles, ma1 nutrition, diarrea y neumonia. La mortalidad aumento a un 23% cuando nacieron mas de dos corderos por oveja y disminuyeron a un 8% cuando solo parieron un cordero. La edad promedio al primer parto fue de 465 dias o 1.27 adios (15.5 meses) para todos 10s rebailos estudiados. El interval0 entre parto fue de 265, 227, 238, 225, 230 y 285 dias, el IEP promedio fue de 242 dias ( f 16). La fertilidad total incluye 10s partos de las primalas y de las ovejas en su Gltima exposition al macho de todas las hembras expuestas. La distribucidn por cuatrimestres de 10s partos fueron un 50% entre enero y marzo (n = 1,264); 25% de abril a junio (n = 632); 15% de julio a septiembre (n = 379) y 10% de octubre a diciembre (n = 252). No se encontro diferencia estadistica entre rebaaos en su comportamiento reproductive. Se demostrd una estacionalidad reproductiva con un minim0 de un 7% de corderos nacidos en el otoiio y un 50% en el inviemo. Las lluvias tuvieron una correlaci6n significativa con la fertilidad y la presentation de1 estro (P < 0.05) pero la temperatura y el foto perfodo no la tuvieron. La disponibilidad de1 forraje aparenta ser el factor clave que determina el inicio de la actividad
reproductiva en las ovejas. El interval0 entre parto fue significativamente diferente (P < 0.01) con una mayor extension cuando las hembras parieron en el otofio, inviemo o primavera, comparada con 10s partos de1 verano. El manejo de1 corder0 two una significancia estadistica con el IEP (P < 0.05) aumentando cuando 10s dfas de lactation subieron de 60 a 120 dias. La fertilidad fue significativamente diferente para la BB comparada con la P (P < 0.05) mientras que la prolificidad fue mayor en la P sobre la BB (P < 0.05). Durante el estudio las ovejas mostraron una distribution estacional de 10s partos sin embargo 1.5 partos por oveja se pueden lograr con un buen manejo.
Acknowledgements Many thanks are due to Janet Hummel for technical assistance to elaborate this manuscript and to DGAPA, UNAM, Grant IN 300192 and to CONACYT Grant 3108-N for support of the study.
References Alvarez, A., Valencia, M., and Rodriguez, O.L., 1984. Efecto de1 destete en el comportamiento reproductive de la oveja Pelibuey. (Effect of weaning on reproductive behavior in the Pelibuey ewe). Memorias X Congreso National de Buiatria, Acapulco, Mexico: 17% 181. Castillo, H., Valencia, M. and Berruecos, J.M., 1972.. Comportamiento reproductive de1 borrego Tabasco mantenido en clima tropical y subtropical. 1. Indices de fertilidad (Reproductive behaviour of Pelibuey sheep maintained under tropical and subtropical climates. 1. Fertility index). Tecnica Pecuaria Mexico, 20: 52-56. Castillo, H., Roman-Ponce, H. and Berruecos, M., 1974. Caracterfsticas de crecimiento de1 borrego Tabasco 1. Efecto de la edad y peso al destete y su influencia sobre la fertilidad de la madre. (Growth characterisics of Pelibuey Sheep 1. Effect of age and weight at weaning and their influence on fertility of the dam). Tecnica Pecuaria Mexico, 20: 52-56. Chemineau, P., Deveau, A., Maurice, F. and Delgadillo, J., 1992. Seasonality of estrus and ovulation is not modified by subjecting female Alpine goats to a tropical photoperiod. Small Ruminant Res., 8: 299-3 12. Cruz, L.C., Femandez-Baca, S., Escobar, M.F. and Quintana. F., 1983. Edad al primer parto e interval0 entre partos en ovejas Tabasco en el tr6pico humedo. (Age at first parturation and parturation interval in Tabasco ewes in the humid tropics). Veterinaria Mexico, 14: 1-5.
M.A. Galina et al./ Small Ruminant Research 22 (1996) 31-37 CSS, 1988. Complete Statistical System. Version B640. Statsoft Inc., Tulsa, OK, 1273 pp. Fitzhugh, H.A. and Bradford, G.E., 1983. Hair sheep of Western Africa and the Americas: A genetic resource for the tropics. West View Press, Boulder, CO, pp. 235. Fuentes, J.L., Lima, T., Pulentes, N.M., Albuemes, R., Sanz, V., Pavon, M. and Per&t, N., 1983. Efecto de1 tipo de parto y la edad al destete en la edad y peso a la pubertad en corderas Pelibuey. (Effect of type of parturition and age at weaning on the age and weight at puberty in Pelibuey ewe lambs). Colloque ‘La reproduction des ruminants en zone tropicale’. INRA-CRAAAG, Guadelopue, Pinte-a Pitre, 500 pp. Galina, M.A. and Guerrero, M., 1993. La Ganaderia Mexicana. Caracteristicas y Perspectivas de1 sector. (Mexican livestock. Characteristics and perspectives of the sector). Avant. Invest. Agropcc., 1 (2): 13-40. Galina, M.A., Silva, E., Morales, R. and Lopez, B., 199.5. Reproductive performance of Mexican dairy goats under various management systems. Small Ruminant Res., 18: 149-253. Garcia, E., 1973. Modificaci6n al sistema de clasificaci6n climatica de Kiipen. (Modifications to the climate classification system of Kdpenl. lnstimto de Geografia. National Autonomus University of Mexico, Mexico City, pp. 145. Gonzalez-Reyna, A., 1977. Reproduction in Pelibuey sheep in the Mexican tropics. M.Sc. Thesis. Utah State University, Logan, UT, 93 pp. Gonzalez-Reyna, A., 1983. The postpartum period in the Pelibuey ewe. Ph.D. Thesis. University of Saskatchewan, Saskatoon, Saskatchewan, Canada, 3 16 pp. Gonzalez-Reyna, A., Murphy, B.D., De Alba, J. and Maims, J.G., 1987. Endocrinology of the postpartum period in the Pelibuey ewe. J. Anim. Sci., 64: 1717-1723. Gonzalez-Reyna, A. and Murphy, B.D., 1987a. Effects of GnRH on luteinizing hormone release and onset of cyclic ovarian activity postpartum in Pelibuey ewes. Can. J. Anim. Sci., 68: 359-366. Gonzalez-Reyna. A. and Murphy, B.D., 1987b. Circannual ovulation rate and oestrous cyclicity in Pelibuey ewes. J. Anim. Sci., 65 (Suppl. 1): 221. Gonzalez-Reyna, A., Valencia, J., Foote, W. and Murphy, B.D., 1991. Hair sheep in Mexico: reproduction in the pelibuey sheep. Anim. Breed. Abstr., 59 (6): 509-524.
37
Leyva, R.G., Septilveda, S.R., Flares, L.R. and Valencia, M., 1983. Efecto de la lactancia controlada y el destete prec6z en la duraci6n de1 posparto en borrego Pelibuey. (Effect of controlled suckling and early weaning on the duration of the postpartum period in Pelibuey ewes). Memotias de la Reunion Anual de Investigaci6n Pecuaria, Mexico, pp. IS- 157. Mazzari, G., Fuenmayor, C.E., and Duque, C.M.. 1984. Efecto de diferentes niveles alimenticios sobre el comportamiento reproductivo de ovejas tropicales (Effect of different feeding levels on the reproductive behaviour of tropical ewes). Agron. Trop., 26: 205. Navarro, L., 1972. Observaciones sobre estros y Cpocas de apriciones en diferentes razas de ovejas en la Mesa de Guanipa. El Tigre, Venezuela. (Observations on estrous and season of lambing in different breeds of sheep on the Guanipa Plain. El Tigre, Venezuela) Ministerio de Agricultura y Cria de Venezuela, 75 pp. Ortega, E., Acosta, C., Gonzalez, A. and De Alba, J., 198 1. Edad al primer parto y frecuencia reproductiva en ovinos de pelo. (Age at first parturation and reproductive frequency in hair sheep). Memorias de la Asociacidn Latinoamericana de Producci6n Animal, 16: 135. Salinas, T.E., Martinez, R.L., Peha, T.F. and Gonzalez, P.E., 1975. Efecto de la suplementaci6n en gestaci6n y lactacidn de borregas Tabasco o Pelibuey sobre la aparici6n de1 primer celo y el peso al destete de 10s corderos (Effect of flushing during gestation and lactation of Tabasco or Pelibuey ewes on the appearance of first oestrus and weight at weaning of lambs). Memorias XII Reuni6n Anual de1 Institute National de Investigaciones Pecuarias, Mexico D.F.: 48. Valencia, J., Bar&, C. and Fem&ndez-Baca, S., 1975. Pubertad en corderos Tabasco-Dorset (Puberty in Tabasco-Dorset lambs). Veterinaria Mexico, 8: 127- 130. Valencia, M., Heredia, M. and Gonzalez, E., 1981. Estacionalidad reproductiva en la oveja Pelibuey. (Seasonal reproduction on the Pelibuey sheep). XV ReuniQ Anual de Investigaciones Pecuatias, Mexico City, Mexico: 34-38. Valencia, M., and Gonzalez, E., 1983. Pelibuey sheep in Mexico. In: H.A. Fitzhugh and G.E. Bradford (Editors). Hair Sheep of West Africa and the Americas. A Genetic Resource for the Tropics. West View Press, Boulder, CO, pp. 235.
Small Ruminant
Research
22 (1996) 3 l-37
Reproductive performance of Pelibuey and Blackbelly sheep under tropical management systems in Mexico M.A. Galina a*bV *, R. Morales b, E. Silva b, B. L6pez a a Facultnd b Posgrudo
Interinstitucional
de Estudios Superiores en Ciencias Pecuurias,
de Cuuutitlin
UNAM, Mexico City, Mgxicico
CUIDA Universidad
Accepted
22 December
de Coha,
AP 22, Colima, 28000,
Mbico
1995
Abstract An analysis of reproductive performance records from 1988 to 1994 of seven hair sheep herds in the dry tropics of Mexico was conducted on 785 Black Belly (BB) and 1810 Pelibuey ewes (P). Reproductive management on four farms included the continuous presence of the male (1471 lambing& and the three other herds had biannual breeding exposure to rams (1057 lambings). In 7 years, the combined herds produced 2528 parturitions from 2585 ewes bred (0.97 births per year per ewe). Number of parturitions were 1.lO year-’ for BB (863 births in 785 ewes) and 0.92 for P (1665 births in 18 10 ewes). Prolificacy was 1.46 lambs per birth with a total of 3690 lambs in 7 years; per breed it was 1.37 for BB and 1.55 for P lambs per parturition (P > 0.05); except for first lambing in both breeds, averaging 1.05 lambs per ewe (P < 0.01). There was an average of 530 (k299) lambs per year with a mortality of 15%; still births, malnutrition, diarrhea and pneumonia accounted for most of the losses. Mortality increased to 23% when more than two lambs were nursed by one ewe and reduced to 8% when only one lamb was born. Average age at first lambing was 465 days (15.25 months) for the combined herds. Yearly average (1988-1994) for lambing interval (LI) was 265, 227, 238, 225, 252, 230 and 258 days, with an average of 242 days (+ 16). Quarterly seasonal birth distribution was 50% from January to March (n = 1264); 25% from April to June (n = 632); 15% from July to September (n = 379) and 10% from October to December (n = 252). No statistical differences were observed among farms in reproductive performance. Seasonality of reproduction was demonstrated with minimum (7%) of lambs born in the fall and 50% in the winter. Rainfall had a significant correlation with fertility and estrous production (P < 0.051, but temperature and photoperiod did not. Grass growth seemed to be the key factor influencing sexual activity in ewes. Interval per lambing was statistically different (P < 0.01) with extended intervals for those ewes lambing during the autumn, winter and spring, compared with the summer lambing ewes. Rearing management had a statistical effect on interval per lambing (P < 0.05) increasing when lactation length augmented from 60 to 120 days. Fertility was higher (P < 0.05) for BB as compared with P, but the latter had a better prolificacy (P < 0.05) during the study. Parturition showed seasonal distribution but 1.5 lambings per year could be achieved throughout with good reproductive management. Keywords:
Reproduction;
Sheep; Seasonality;
Photoperiod;
Pelibuey;
* Corresponding author. Tel: (331) 41 l-33, fax: (331) 275.81; e-mail: [email protected]. 00921-4488/96/$15.00 Published SSDI 0014-2999(95)00878-O
by Elsevier Science B.V.
Blackbelly
32
M.A. Galim
et ul./Small
Ruminmt
1. Introduction Tropical hair sheep have been an important source of meat in many countries in Latin America (Fitzhugh and Bradford, 1983). The hair sheep of Mexico are mainly Pelibuey (P) and Blackbelly (BB). The P breed predominates and makes up 90-95% of the total hair sheep population in Mexico, which amounts to approximately 500000 (Gonzalez-Reyna et al., 1991). Seasonality of reproduction is a common feature in sheep and goat breeds in temperate latitudes but less marked in the tropics, where all year around breeding is practiced (Gonzalez-Reyna et al., 1991; Valencia and Gonzalez, 1983; Chemineau et al., 1992; Galina et al., 1995). Puberty has been recorded from as early as 5 months, with an average age from 210 to 273 days, and as in wool sheep, is related to body weight (Gonzalez-Reyna et al., 199 1). Reproductive activity in the literature shows seasonality, represented by reduced periods of estrus from February to April (Gonzalez-Reyna et al., 199 1; Gonzalez-Reyna and Murphy, 1987b), while in other studies, estrus cycles seemed to be present all year; fertility ranged from 70 to 97% and prolificacy varied from 113 to 144 lambs born from 100 ewes exposed (Valencia and Gonzalez, 1983; Valencia et al., 198 1). Photoperiod does not seem to be the main factor responsible for seasonal breeding in the P ewe. Instead, other factors such as nutrition and possibly relative humidity, precipitation and temperature may have a more direct effect on periods of reduced cyclicity during the months of February to April (Gonzalez-Reyna et al., 1991). Gestation length was comparatively similar to wool sheep. Postpartum period as related to season and lactation length seemed to be the key factor that determined appearance of es&us which varied from 33 to 98 days. Finally perinatal lamb mortality has been documented from 11 to 75% (Gonzalez-Reyna et al., 1991; Valencia et al., 1981). Mexico has a population of 4 X lo6 million sheep and only a small portion are hair sheep (500000). The hair sheep are not usually kept under good nutritional management and it is rare to find concentrates as part of the diet (Galina and Guerrero, 1993). In the dry tropics, reproduction seems to be all year round and reproductive control mechanisms have not been discussed. The objective of this study was to
Research 22 (1996) 31-37
measure reproductive performances under various management systems to improve production to 1.5 parturitions per year as a goal.
2. Materials
and methods
Seven farms in Colima, Mexico were studied from 1988 until 1994. Flock compositions are described yearly in Table 1. Colima is located at 19” 13’ latitude N and 104’48’ longitude W; at 550 m above sea level. The climate is classified as dry tropical, with rain from June to October, average temperature 25°C with a minimum of 17°C and maximum of 30°C. There is an average rain precipitation of 850 mm, minimum relative humidity of 56% and maximum of 80% (Garcia, 1973). Each farm provided monthly information. Data included sheep identification, number of lactations, date of birth, date at first lambing, interval between parturition, age and breed. Fertility and litter size were measured yearly and overall. All farms used grazing as a management system on tropical grasses with low supplementation of 100 to 200 g per ewe of a commercial concentrate of 16% CP during the dry season. Four herds kept males year around. The other three farms introduced the male only 9 months of the year. Both managements included 45 days reproduction rest after lambing. Ewes were bred day or night. Young ewes were exposed to breeding when they attained from 20 to 25 kg body weight (BW). Weaning was performed
Table 1 Flock composition Farm
No. ewes 1 2 3 4 5 6 7 Total
by farms, total number of parturitions
Black belly
18 25 27 45 35 57 27 234
Tabasco pelibuey
a
No. parturitions b
No. parhrritions b
No. ewes ’
95 105 80 145 88 180 92 785
120 157 45 125 47 28 25 547
a No. of ewes present in the flock parturation per farm and breed.
as a yearly
375 460 120 450 185 145 75 1810 average.b Total
M.A. Galim Table 2 Multiple regression
et al./Small
Ruminant Reseurch 22 (1996) 31-37
The equation ships was:
values for fertility Participation estimated
Variables studied
‘ijklm
t Intercept Month Year Number of parturitions Lambing interval
0.8872 3.2 0.8482 0.3345 4.1
value
Significance level
4.65 1.21 12.83 1.17 6.3
0.3201 0.622 I 0.0005 0.6100 0.5430
when animals were from 10 to 15 weeks old (70 to 105 days). Once settled, ewes were kept with other pregnant and non pregnant sheep. Lambs stayed with their mother for 70 to 110 days of lactation until weaned. Statistical analysis included multiple linear regression to study the relationship between month and year of lambing, number of parturition and lambing interval to explain fertility, (defined as the number of parturitions divided by the number of ewes exposed).
Table 3 Fertility per breed and year for Blackbelly Year I988 1989 1990 1991 1992 1993 1994 Total Overall BB + P
Fertility Prolificacy Lambing interval * P < 0.05.
=bo+b,Xi+b,Xj+b,Xk+b,X,+E,
Where YijkIm was the fertility in the ith month from the jth year in the kth parturition with Itb lambing interval after birth and the mth random error; b,, b, , b,, b, and b, were the parameters of the model. Student’s unpaired r-test compared means among breeds. Square root transformation was made on data for intervals prior to analysis. Finally a correlation study of fertility, prolificacy, lambing interval with average temperature, rain fall and photoperiod monthly was performed (CSS, 1988)
3. Results Age at first lambing averaged 15.5 months (+ 3); 15.8 months for BB and 14.6 for P. Lamb weight at birth varied from 1.2 to 3.1 kg being larger for single birth. Seventy-four percent of the lambs weighed were P and 26% BB.
(BB) and Pelibuey (P)
BB (n)
Fertility (%)
P(n)
Parturitions
Fertility (%/o)
27 57 150 228 101 loo 122 785 2595
26 75 126 255 85 107 189 863 2528
96 132a 84 112a 84 107 a 155 c 1 IO a 97
71 114 152 230 445 417 381 1810
64 85 138 178 430 360 410 1665 b
90 75 b 91 77 b 97 86 b 108 d 92 b
between reproductive
Variable
to estimate overall fertility relation-
Parturition
Different letters (a,b) showed differences divided by the number of ewes exposed.
Table 4 Correlation
33
parameters
among
columns
and temperature,
(P < 0.05); (c,dXP
< 0.01). Fertility
defined
as the number
of parturitions
rainfall and photoperiod
With male all year round
Male present biannually
Temperature
Rainfall
Photo period
Temperature
Rainfall
Photo period
0.18 -0.12 0.04
0.75 * 0.09 0.63 *
0.07 - 0.04 -0.18
0.21 - 0.09 0.07
0.63 * - 0.08 0.65 *
0.27 0.14 - 0.09
34 Table 5 Reproductive
M.A. Galina
performance
et al./
Small Ruminant
Research 22 (1996) 3 I-37
of the T and BB hair sheep from the expermental
Trait
Age first lambing (days) Lambing per ewe per year ’ Fertility lambing rate (o/o) ’ Birth weight (kg) Lamb mortality (o/o) Annual weaning rate 3 Interval between lambing (days)
flocks
Black belly
Pelibuey
n
Mean
SD
n
Mean
SD
728 785 785 863 863 848 785
483 1.37 a 1.10 2.850 17 140 230 a
26 0.52 0.12 0.55 3 18 14
916 1810 1810 1665 1665 1580 1810
447 1.55 b 0.92 2.450 14 150 254 b
74 0.35 0.18 0.35 7 16 25
Different letters (a,b) show differences among columns (P < 0.051.’ Total number of lambs per ewe in one or two parturitions.2 Fertility calculated as the number of parturitions divided by the number of ewes exposed.3 Annual weaning rate as the number of lambs that reach weaning divided by the number of ewes lambing.
There was an average of 530 (+299) lambs per year with a mortality of 15%; still births, malnutrition, diarrhea and pneumonia were the major diseases recorded. Mortality increased to 23% when more than two lambs were nursed by one ewe and diminished to 8% when only one lamb was born in both breeds. Fig. 1 summarizes the parturition distribution by months. Distribution differed (P < O.Ol>, being highest in January (n = 490); February (n = 425) and March (n = 3531, decreasing from April (n = 270) to October (n = 35). Fig. 2 shows the interval between lambing distributed by years. Variability existing from year to year during the whole study, as related to rainfall was significant (P < 0.05). However hair sheep showed the possibility of at least one parturition every year. To determine the statistical significance of fertility in relation to month of lambing, year of parturition, number of birth in the productive life of the ewe and interval of lambing, a multiple linear regression was performed. Results are shown in Table 2. Year was the major cause of variation on fertility. Table 3 summarizes the yearly number of observations of total births and fertility per breed. A good reproduction year seemed to be followed by a bad one, with a difference (P < 0.05) for 1989, 1991, 1993, with the exception of 1994 (best year> which had the most significant difference for fertility of the study followed by 1993, which also showed good fertility (P < 0.01). Table 4 shows the correlation coefficients separately for those farms that kept males year round and
those that had a reduced reproduction period for rainfall, length of daylight, temperature and reproductive parameters. Fertility and lambing intervals were significantly correlated with rainfall. All reproductive values, including prolificacy, were not correlated either with temperature or photoperiod. This observation between reproductive performance and environmental factors demonstrated the significance of rainfall to reproduction. Table 5 shows the reproductive performance of BB and P accumulating all values from the seven farms and the seven years studied. Fertility was better for BB (1 .lO> than P (0.92). On the other hand, prolificacy was slightly better for P (1.55) than BB (1.37).
4. Discussion Seasonal breeding in tropical hair sheep is considered to be different from that of wool breeds in the tropics. Environmental factors other than light seem to be more important in influencing seasonal patterns in reproduction. The most important of these is probably nutrition, which is influenced by forage availability and digestibility (Gonzalez-Reyna et al., 1991; Valencia et al., 1981). In the present study, seasonal parturition was demonstrated. Temperature, rainfall and photoperiod were studied. From the above, only rainfall showed an effect in controlling reproduction, probably by nutritional influence as suggested before. On the other hand, these findings did not agree with earlier studies in Mexico and Venezuela (Castillo et al., 1972; Gonzalez-Reyna,
M.A. Gulina et al. / Small Ruminant Research 22 (1996) 31-37
1977; Navarro, 1972) but did agree with the more recently introduced concept of periods of reduced reproduction activity by Gonzalez-Reyna and Murphy (1987a) and Valencia et al. (198 1). Ewes showed reduced estrus activity from February to April. The present work suggested extended postpartum intervals for those ewes lambing during the autumn, winter and spring, as compared with the summer lambings as shown by Valencia et al. (1981) and for those ewes that had an increased lactation length due to lambs nursing for more than 60 days. Positive correlation has been demonstrated among lactation and lambing interval, with the present findings in agreement with the literature (Alvarez et al., 1984; Castillo et al., 1974; Leyva et al., 1983). An interval per lambing of 210 days is needed to have two lambings per year. Nutritional status of the ewe during the last third of gestation and early lactation plays an important role in the return to activity post partum. A reduction in the interval to first estrus of 43 days to 33 days has been found when ewes were supplemented with concentrates (Salinas et al., 1975; Mazzari et al., 1984). A 25% decrease in body weight, at the time of parturition, increased the interval to first estrus from 51 to 91 days (Gonzalez-Reyna et al., 1987). In the present study, LI of 230 to 250 days showed that concentrate supplementation played a key role in the reproductive management accompanied by a reduced lactation of 70 days. Under similar conditions, BB ewes seemed to perform better with LI of 230 days as compared with 254 days for the P ewes. Larger size and better physical condition could explain these results. Age at first lambing was higher in BB (483 days) than in P (447 days) ewes. The latter seemed to be able to reproduce at a lighter weight. In previous literature, age at first lambing was shown to be affected by several genetic and environmental factors. A significant correlation has been demonstrated before, between birth weight and age at puberty (Fuentes et al., 1983). Single birth and larger ewes obtained first lambing faster than those from multiple birth or smaller birth weight (P < 0.05). Nutrition also correlated with age at puberty. Lambs with higher weight gains reached puberty and lambed at a younger age (420 days) than lambs with low gains, results that agreed with previous work (Valencia and Gonzalez, 1983). Season of birth affected age at
35
puberty and age at first lambing. Ewes born in May-June reached puberty at a younger age than those born in November-April (Cruz et al., 1983). Previous studies showed that ewes born in winter have their first lambing at a younger age (398 days) than those born in either spring (454 days), summer (488 days) or autumn (435 days) as demonstrated by Ortega et al. (1981). BB and P ewes that give birth in the winter did have a smaller LI than those that lambed later in the year in the present study. Fertility, defined as the number of ewes lambing divided by the number of ewes exposed to the ram was 97% in the present study, a value which is greater than those from previous studies (70% as compared with 97%) (Valencia et al., 1975; Valencia and Gonzalez, 1983). Black belly ewes were significantly superior (P < 0.05) with 110% to 92% from P but prolificacy, defined as the number of lambs born per 100 ewes exposed to males, was smaller for BB (137) than for P (155) which varies from 113 to 144 in the literature; both comparable with the average in Pelibuey studies (Gonzalez-Reyna, 1983; Valencia and Gonzalez, 1983) Lamb mortality in tropical hair sheep has been reported to vary from 11.2 to 75% (Gonzalez-Reyna, 1983; Valencia and Gonzalez, 19831, with up to 65% of deaths taking place during the first week of life. Perinatal mortality is largely due to lack of attention to the newborn lamb by the dam and/or weak lambs due to lack of nursing after birth. In the present study, lamb mortality was 15% increasing when more than two lambs were born and diminishing when dam nutrition was improved with supplementation. A possible management practice to reduce lamb mortality was to improve care and attention at the time of lambing and support ewes that gave birth to three or more lambs with artificial rearing and better nutrition during gestation. In summary, smaller size compared with wool ewes and adaptation in the tropics suggested that BB or P ewes could be used competitively for economical sheep meat production and that reproduction could be improved through management. 5. Resumen Se realize caracten’sticas
un analisis de 10s registros de las reproductivas de 785 ovejas Black
36
M.A. Galina et al./ Smull Ruminanr Research 22 (1996) 31-37
belly (BB) y 1,810 ovejas Pelibuey (P) de siete rebaiios ovinos en el tropic0 seco mexicano de 1988 a 1994. El analisis fue dividido en dos partes la primera de cuatro rebai?os cuyas ovejas tenian presencia de1 macho todo el afio (1471 partos) y la segunda con tres rebahos con empadre bianual (10.57 partos). Los rebaiios combinados en 10s 7 aiios produjeron un total de 2528 partos de 2585 ovejas con una fertilidad global de 0.97 nacimientos por aiio por oveja. La fertilidad fue el ntimero de partos at-males por el numero de ovejas expuestas al macho. Divididos por tipo racial, la fertilidad fue de 1.10 partos por tie para las BB (863 partos para 785 ovejas) y 0.92 para P (1665 nacimientos en 18 10 hembras). La prolificidad fue de 1.46 corderos por nacimiento con un gran total de 3690 corderos en 7 adios. La prolificidad por tipo racial fue de 1.37 para las BB y 1.55 para las ovejas P por parto no encontrandose diferencias significativas entre razas o numero de parto con la exception de1 primer parto que dio solamente 1.05 corderos por oveja (P < 0.01). Se observe un promedio de 530 ( + 299) corderos por aho con una mortalidad de1 15%, las principales causas de las pe’rdidas fueron corderos nacidos debiles, ma1 nutrition, diarrea y neumonia. La mortalidad aumento a un 23% cuando nacieron mas de dos corderos por oveja y disminuyeron a un 8% cuando solo parieron un cordero. La edad promedio al primer parto fue de 465 dias o 1.27 adios (15.5 meses) para todos 10s rebailos estudiados. El interval0 entre parto fue de 265, 227, 238, 225, 230 y 285 dias, el IEP promedio fue de 242 dias ( f 16). La fertilidad total incluye 10s partos de las primalas y de las ovejas en su Gltima exposition al macho de todas las hembras expuestas. La distribucidn por cuatrimestres de 10s partos fueron un 50% entre enero y marzo (n = 1,264); 25% de abril a junio (n = 632); 15% de julio a septiembre (n = 379) y 10% de octubre a diciembre (n = 252). No se encontro diferencia estadistica entre rebaaos en su comportamiento reproductive. Se demostrd una estacionalidad reproductiva con un minim0 de un 7% de corderos nacidos en el otoiio y un 50% en el inviemo. Las lluvias tuvieron una correlaci6n significativa con la fertilidad y la presentation de1 estro (P < 0.05) pero la temperatura y el foto perfodo no la tuvieron. La disponibilidad de1 forraje aparenta ser el factor clave que determina el inicio de la actividad
reproductiva en las ovejas. El interval0 entre parto fue significativamente diferente (P < 0.01) con una mayor extension cuando las hembras parieron en el otofio, inviemo o primavera, comparada con 10s partos de1 verano. El manejo de1 corder0 two una significancia estadistica con el IEP (P < 0.05) aumentando cuando 10s dfas de lactation subieron de 60 a 120 dias. La fertilidad fue significativamente diferente para la BB comparada con la P (P < 0.05) mientras que la prolificidad fue mayor en la P sobre la BB (P < 0.05). Durante el estudio las ovejas mostraron una distribution estacional de 10s partos sin embargo 1.5 partos por oveja se pueden lograr con un buen manejo.
Acknowledgements Many thanks are due to Janet Hummel for technical assistance to elaborate this manuscript and to DGAPA, UNAM, Grant IN 300192 and to CONACYT Grant 3108-N for support of the study.
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M.A. Galina et al./ Small Ruminant Research 22 (1996) 31-37 CSS, 1988. Complete Statistical System. Version B640. Statsoft Inc., Tulsa, OK, 1273 pp. Fitzhugh, H.A. and Bradford, G.E., 1983. Hair sheep of Western Africa and the Americas: A genetic resource for the tropics. West View Press, Boulder, CO, pp. 235. Fuentes, J.L., Lima, T., Pulentes, N.M., Albuemes, R., Sanz, V., Pavon, M. and Per&t, N., 1983. Efecto de1 tipo de parto y la edad al destete en la edad y peso a la pubertad en corderas Pelibuey. (Effect of type of parturition and age at weaning on the age and weight at puberty in Pelibuey ewe lambs). Colloque ‘La reproduction des ruminants en zone tropicale’. INRA-CRAAAG, Guadelopue, Pinte-a Pitre, 500 pp. Galina, M.A. and Guerrero, M., 1993. La Ganaderia Mexicana. Caracteristicas y Perspectivas de1 sector. (Mexican livestock. Characteristics and perspectives of the sector). Avant. Invest. Agropcc., 1 (2): 13-40. Galina, M.A., Silva, E., Morales, R. and Lopez, B., 199.5. Reproductive performance of Mexican dairy goats under various management systems. Small Ruminant Res., 18: 149-253. Garcia, E., 1973. Modificaci6n al sistema de clasificaci6n climatica de Kiipen. (Modifications to the climate classification system of Kdpenl. lnstimto de Geografia. National Autonomus University of Mexico, Mexico City, pp. 145. Gonzalez-Reyna, A., 1977. Reproduction in Pelibuey sheep in the Mexican tropics. M.Sc. Thesis. Utah State University, Logan, UT, 93 pp. Gonzalez-Reyna, A., 1983. The postpartum period in the Pelibuey ewe. Ph.D. Thesis. University of Saskatchewan, Saskatoon, Saskatchewan, Canada, 3 16 pp. Gonzalez-Reyna, A., Murphy, B.D., De Alba, J. and Maims, J.G., 1987. Endocrinology of the postpartum period in the Pelibuey ewe. J. Anim. Sci., 64: 1717-1723. Gonzalez-Reyna, A. and Murphy, B.D., 1987a. Effects of GnRH on luteinizing hormone release and onset of cyclic ovarian activity postpartum in Pelibuey ewes. Can. J. Anim. Sci., 68: 359-366. Gonzalez-Reyna. A. and Murphy, B.D., 1987b. Circannual ovulation rate and oestrous cyclicity in Pelibuey ewes. J. Anim. Sci., 65 (Suppl. 1): 221. Gonzalez-Reyna, A., Valencia, J., Foote, W. and Murphy, B.D., 1991. Hair sheep in Mexico: reproduction in the pelibuey sheep. Anim. Breed. Abstr., 59 (6): 509-524.
37
Leyva, R.G., Septilveda, S.R., Flares, L.R. and Valencia, M., 1983. Efecto de la lactancia controlada y el destete prec6z en la duraci6n de1 posparto en borrego Pelibuey. (Effect of controlled suckling and early weaning on the duration of the postpartum period in Pelibuey ewes). Memotias de la Reunion Anual de Investigaci6n Pecuaria, Mexico, pp. IS- 157. Mazzari, G., Fuenmayor, C.E., and Duque, C.M.. 1984. Efecto de diferentes niveles alimenticios sobre el comportamiento reproductivo de ovejas tropicales (Effect of different feeding levels on the reproductive behaviour of tropical ewes). Agron. Trop., 26: 205. Navarro, L., 1972. Observaciones sobre estros y Cpocas de apriciones en diferentes razas de ovejas en la Mesa de Guanipa. El Tigre, Venezuela. (Observations on estrous and season of lambing in different breeds of sheep on the Guanipa Plain. El Tigre, Venezuela) Ministerio de Agricultura y Cria de Venezuela, 75 pp. Ortega, E., Acosta, C., Gonzalez, A. and De Alba, J., 198 1. Edad al primer parto y frecuencia reproductiva en ovinos de pelo. (Age at first parturation and reproductive frequency in hair sheep). Memorias de la Asociacidn Latinoamericana de Producci6n Animal, 16: 135. Salinas, T.E., Martinez, R.L., Peha, T.F. and Gonzalez, P.E., 1975. Efecto de la suplementaci6n en gestaci6n y lactacidn de borregas Tabasco o Pelibuey sobre la aparici6n de1 primer celo y el peso al destete de 10s corderos (Effect of flushing during gestation and lactation of Tabasco or Pelibuey ewes on the appearance of first oestrus and weight at weaning of lambs). Memorias XII Reuni6n Anual de1 Institute National de Investigaciones Pecuarias, Mexico D.F.: 48. Valencia, J., Bar&, C. and Fem&ndez-Baca, S., 1975. Pubertad en corderos Tabasco-Dorset (Puberty in Tabasco-Dorset lambs). Veterinaria Mexico, 8: 127- 130. Valencia, M., Heredia, M. and Gonzalez, E., 1981. Estacionalidad reproductiva en la oveja Pelibuey. (Seasonal reproduction on the Pelibuey sheep). XV ReuniQ Anual de Investigaciones Pecuatias, Mexico City, Mexico: 34-38. Valencia, M., and Gonzalez, E., 1983. Pelibuey sheep in Mexico. In: H.A. Fitzhugh and G.E. Bradford (Editors). Hair Sheep of West Africa and the Americas. A Genetic Resource for the Tropics. West View Press, Boulder, CO, pp. 235.