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The production of tropical sheep is becoming increasingly important in Mexico, because, due to its adaptability to tropical conditions and efficient utilization of forage resources, this species allows its production in small, medium and large scale (González et al. 2002). For several years, it has been identified that in lamb production programs, body weight at birth of lambs depends on the genetic type, sheep age, sex, type of birth (simple vs. multiple), season and year of birth are factors that exert higher influence on sheep growth. These variables, in turn, significantly affect profitability of production systems, which purpose is to obtain the highest economic benefit in the shortest possible time (Forero et al. 2017).
Cienfuegos-Rivas et al. (2010) mentioned that to achieve the maximum productive performance of an animal, it is necessary to know its genetic capacity and the environmental conditions in which it develops.
Results of several authors (González et al. 2002, Arias 2006, Macedo and Arredondo 2008, Macias-Cruz et al. 2012, Hinojosa-Cuéllar et al. 2013 and Forero et al. 2017) agree that the non- genetic factors that impact on birth weight (BW) were birth year, birth season, parturition number, type of parturition and sex of the offspring. These authors reported that 30 to 40 % of total variation in birth weight was due to environmental factors, which reduced the precision of genetic studies.
In the state of Guerrero, Mexico, there is a sheep population of 140,663 heads, and in Tierra Caliente region, there are 38,825 heads (SAGARPA 2016), among which there is a mixture of breeds, including animals from other areas of the country that are not adapted to the conditions of that region. Temperatures in Guerrero are high, low relative humidity and deficient feeding, which influence sheep rearing and production. Therefore, the Faculty of Veterinary Medicine and Zootechnics carried out a research to create a breed of sheep from the region, which was named MEVEZUG. However, there are no studies related to the factors that influence on BW of this breed. Hence, the objective of this study was to know the main non-genetic factors that affect the BW in MEVEZUG creole sheep in the dry tropic of Guerrero, Mexico.
Materials and Methods
The information of 1,157 birth weight (BW) records, from 324 MEVEZUG crossbred sheep, with births during the years 2004 to 2017, located in the Faculty of Veterinary Medicine and Zootechnics N.1 in the Municipality of Pungarabato, from Tierra Caliente region from the state of Guerrero, Mexico was used.
This region is located at 18°20'30'' North and 100°39'18'' West in an altitudinal range of 250 mosl. The average temperature is 25°C with average precipitation of 1,200 mm per year. Four stations are defined. Spring that corresponds to the months from March to May, summer from June to August, autumn from September to November and winter from December to February. There are two very marked seasons, one with rains (from June to November), with mean temperatures of 18 °C and rainfall of 1,027 mm, and another dry (from December to May), with average temperatures of 32 °C and 750 mm of precipitation (INEGI 2017 y UNAM 2017).
For the creation of the breed began with a base herd of 50 crossbred females, Dorper, Pelibuey and Black Belly breeds, in 2004. From there, several crossings were carried out with studs of different breeds (Dorper, Black Belly, Pelibuey and Kathadin), with four registration periods, during the years 2004-2005, 2006-2007, 2008-2009 and 2009-2010 respectively. Animals with light brown or tobacco brown hair were selected, with birth weights averaged at least 2.5 kg, which came from multiple births. Afterwards, inter se crosses were carried out.
Feeding consisted basically on rotating grazing in five meadows, with native pastures of the region such as bermuda (Cynodon dactylom), Venezuelan grass (Panicum fasciculatum), spider three-awn (Aristida ternipes), smooth mesquite (Prosopis laevigata), cueramo (Cordia elaeagnoides), cubata (Acacia cochlicantha), pinzan (Pithecellobium dulce), huizache (Acacia farneasiana), cheeseweed (Malva parviflora), morning-glory (Ipomoea purpure), zapotillo (Cuphea aequipetala), cassia (Cassia didymobotrya) and railroad-creeper (Ipomoea cairica). These pastures were only irrigated during the dry season.
The reproductive management was carried out by natural mating with controlled breeding, which consisted in separating the sheep from the stud, without having any visual, auditory and olfactory contact for a minimum period of one month. The stud stayed an average of 35 d, with the females suitable for reproduction (minimum weight of 25 kg at the first service). The sudden introduction of the males into the herd induces hormonal changes in the females, which lead to ovulation and estrus (male effect). This allowed us to concentrate parturition season in the same month, which guarantees a better care of the newborn and of mothers during and after parturition, as well as having lots of more lambs.
Lambs were left with the mothers in a tillering pen (sheep-lamb link), during the first three days of birth, to verify the consumption of colostrum, disinfection of the navel and place the identification earring. Later, and during the lactation period, they stayed with their mothers during the day (from 9 a.m. to 6 p.m.) in grazing, where they were given water ad libitum. In the afternoons, sheep, lambs and replacement females were locked in the same pen, where they were offered 100g/animal/d of maize in grain, only in the critical period of forage availability (in the months from February to June).
BW distribution was analyzed by SAS/INSIGHT. Non-normality was found through the results of the goodness of fit test for the normal distribution (P <0.001) according to Shapiro-Wilk, Kolmogorov- Smirnov, Cramer-von Mises and Anderson-Darling. According to the type of distribution followed by the BW (Gamma), the corresponding link function (η = log (λ)) was used.
A mixed generalized linear model was applied using the GLIMMIX SAS procedure, version 9.3 (2013). This model allows to obtain the retransformed values, according to the inverse function to the link function (λ = exp (xβ)):
Where:
Yklmopj |
- f(µ) phenotypical value expected from birth weight, according to the specific link function μ = mean or intercept |
Ak |
- fixed effect of the k-th year of birth (k=2004,…, 2017) El = fixed effect of the l-th birth season (l=rain, dry) |
Lm |
- fixed effect of the m-th parturition number (m=1,…,6 or more) |
Ro |
- fixed effect of the o-th type of parturition (o=single, double, triple, quadruple) Sp = fixed effect of the p-th sex of the offspring (o=female, male) |
Cj |
- random effect of the j-th sheep |
eklmopj |
- random error due to each observation NID~ (0, σ2) |
Tukey |
-Kramer test was applied for multiple comparison of means of minimum squares, according to Kramer (1956). |
Results and Discussion
Table 1 presents the descriptive statistics for birth weight. The general mean obtained in this study (2.71 kg) was similar to that reported by other authors in research conducted in Mexico. García (2000), in Nuevo León, in a study with Saint-Croix sheep, obtained estimates of 2.99 kg. González et al. (2002), in Tabasco, with the Black Belly race, reported 2.7 kg. Macías-Cruz et al. (2012), in Baja California, working with lambs from Pelibuey female terminal crossing with Dorper, Kathadin or Pelibuey studs, reported 2.8 kg of BW. In Tabasco, Hinojosa-Cuéllar et al. (2013), in F1 Pelibuey x Black Belly crossings, and in other undefined ones of Dorper x Kathadin, under conditions of humid tropic, referred a BW of 3.2 kg in both crosses.
Higher means for BW were reported in different regions of Mexico. González-Anaya et al. (2013) obtained 4.7 kg in Rambouillet lambs, in a study conducted in San Luis Potosí and Ramírez-Tello et al. (2013) in Hidalgo, they referred 5.1 kg in Hampshire lambs. These means were superior because they used meat breeds and an intensive management system.
Birth weight in sheep defines, to a great extent, survival and subsequent growth, until reaching the final weight. This is a determining indicator in the development and productivity of sheep (Perón 2008). In this sense, Nowak and Poindron (2006) and Macedo et al. (2010) noted that low values of birth weight in sheep predispose to death by various reasons like starvation, weakness, low reserve energy, hypothermia and immaturity.
The significant effect of birth year, birth season, birth number, type of parturition and sex in BW was evidenced (table 2). These results correspond to those of Hinojosa-Cuéllar et al. (2009), Hinojosa-Cuéllar et al. (2013) and Ríos-Utrera et al. (2014), under the conditions of the humid tropic of Mexico.
Table 3 shows the means of least squares, obtained for the effect of year and birth season, where differences in BW were observed. In the herd under study, the best means for BW, according to birth year, were in the period from 2004 to 2008 and in 2014. These results are mainly due to the fact that the period from 2004 to 2008 is in agreement with the F1 crossings, carried out with males of breeds specialized in meat production (Dorper, Kathadin and Pelibuey), in which higher heterosis was manifested, according to Falconer and Mackay (1996). The high value of BW in 2014 could be due to the low number of observations available in this year (only 42) with respect to the rest. From 2009 to 2017, the BW was affected because, during this period, the first inter se crossings were performed, from which there are losses of heterosis by recombination, which decreases the performane of certain traits, such as weight at birth. This performance agrees with the results of Cienfuegos-Rivas et al. (2010) and with those reported in different sheep breeds.
Table 3 Effect of year and season of birth on weight at birth of MEVEZUG creole sheep
a,b,c Different superscripts in the same column differ significantly (P<0.05), according to Kramer (1956)
The best average according to birth season was during rains. The seasonal effect on birth weight is associated with the performance of rains, due to its important effect on production, availability and quality of forage, according to the criteria of Rodríguez et al. 2006 and Hinojosa-Cuéllar et al. (2009).
Steinheim et al. (2007) reported that different climatic aspects, such as the rainfall regime, can favor growth of lambs at different times of the year. Steinheim et al. (2007) and Martínez (2008) reported that there are breeds that are more sensitive to environmental changes, and that often the best environment for some breeds is not the best for others. These authors considered that to understand some effects between the herd and the year, it is necessary to consider environmental characteristics such as climate and topography, which can affect the production of different breeds. Forero et al. (2017) noted that high temperatures during pregnancy result in a reduction in the weight of the lamb at birth, due to the possible effect of caloric stress on feed intake and on the reduction of tissue weight of cotyledons. All this reduced, consequently, the carrying of nutrients to the fetus.
Lambs born from sheep of the first calving were less heavy (2.18 ± 0.05 kg) than those born from the second to the sixth (table 4). This is a common result, in which the highest weights at birth are usually found in lambs of mothers of intermediate ages, while the smaller weights correspond to lambs born to first-time mothers. This is attributed to the fact that young ewes, which have not reached their adult size, continue to grow during pregnancy, which establishes a competition for the nutrients available for fetus growth and that of the sheep itself (Osorio and Montalvo 2007, Sulaiman et al. 2009, Bermejo et al. 2010, González-Anaya et al. 2013 and López-Leyva et al. 2017). Other possible causes include the effect of weight of the sheep at the time of breeding and feeding in the last third of gestation (De Lucas et al. 2003, Arias 2006 andDíaz et al. 2012).
Table 4 Effect of parturition number on weight at birth of MEVEZUG creole sheep
a, bDifferent superscripts in the same column differ significantly (P<0.05), according to Kramer (1956)
Several authors found that lamb weights at birth increase for each year of increase in sheep age, then decrease after six years of age (Dickson et al. 2004, Arias 2006, Ríos-Utrera et al. 2014 and López-Leyva et al. 2017).
The effect of the type of birth on BW of MEVEZUG creole sheep (table 5) showed that sheep with a single parturition, lambs were born with higher average weight (3.19 ± 0.04 kg) with respect to double, triple and quadruple births. Several authors (Bentancor 2013, González-Anaya et al. 2013, Hinojosa- Cuéllar et al. 2013, Ramírez-Tello et al. 2013, and Moyano et al. 2017) reported a significant effect on birth weight due to the type of parturition. As they refer, the explanation for this better growth is due to the fact that lambs of single birth have no competence at uterus level for the nutrients of the mother. In addition, birth weight of lamb seems to be positively correlated to the number and weight of the cotyledons of the uterus. In multiple gestations, the number of cotyledons per fetus decreases, the exchange of nutrients per fetus is reduced, fetal growth decreases and, consequently, birth weight. At the beginning of gestation, single and double fetuses have a similar weight, but, after the third month, growth differences between both types begin to be marked.
Table 5 Effect of type of parturition on birth weight in MEVEZUG creole sheep
a, bDifferent superscripts in the same column differ significantly (P<0.05), according to Kramer (1956)
The effect of sex on BW (table 6) in MEVEZUG creole sheep showed better performance in males with respect to females (P ˂ 0.0001). The weight of females was 6% lower than that of males. This performance is consistent with the results reported in different sheep breeds (Macedo and Arredondo 2008, Ramírez-Tello et al., 2013, Ríos-Utrera et al., 2013 and Forero et al. 2017).
Regarding the influence of the sex of the offspring on birth weight, in general terms, it is estimated that, in male lambs, it is 5 to 22 % higher than females (Rodríguez et al. 2006 and Macedo and Arredondo 2008). This is explained because the number of placental cotyledons per fetus is little variable between males and females, although the weight of cotyledons associated with males is 10.5% higher than that associated with females, which would mean better passage of nutrients to the fetus. This difference in weight is attributed to caruncular competition among fetuses of different sex. A twin-calving female, born with another female, weighs more at birth than one that shares the maternal womb with a male.
Martínez (2008) and Ramírez-Tello et al. (2013) noted that the rate of skeletal growth in the uterus is faster in males than in females, which is attributed to several physiological functions, which are primarily of a hormonal nature (abundant production of testosterone), by male fetuses. This causes greater weight at birth and then, faster growth until weaning.
It is concluded that the effects of the year and birth season, number and type of parturition and sex of the offspring, should be considered for purposes of general management of the sheep herd, as well as for the design of a program for genetic improvement of birth weight of lambs of MEVEZUG breed.
