Estimates of breed and heterosis effects for some reproductive traits of Brown Swiss and Zebu-related breeds in South-eastern Mexico

J G Magaña  and J C Segura-Correa

Facultad de Medicina Veterinaria y Zootecnia, UADY, 
Km 15.5 Carretera Mérida-Xmatkuil, AP 4-116, Mérida, Yucatán, México
jmagana@tunku.uady.mx

Abstract

Data from 992 cows from a commercial herd in Southeastern Mexico were used to estimate breed, general heterosis and some environmental effects on traits related to cow reproductive performance. The herd was comprised of the following genetic groups: commercial Zebu (CZ), Brahman (BR), Indo-Brazil (IB) and Brown Swiss (BS), and several crosses amongst these breeds. The reproductive traits studied were age at first calving (AFC), calving interval (CI), total days in herd from first to last calving (TDH) and total number of parities (TNP).

Statistical analyses, using least squares procedures, showed significant effects of genetic group (GG) and year of birth on AFC, TDH and TNP. Season of calving and parity number were significant for CI (P<0.05). Overall means for AFC, CI, TDH and TNP were 1027, 448, 1307 days and 3.92 parities. There were significant differences among Zebu breeds (CZ, BR and IB). Brown Swiss additive effects (expressed as a deviation from Zebu) was important only on AFC, favoring the BS breed, –60 for BR, and –90 days for both CZ and IB. General heterosis estimations were important and highly significant (P<0.001) for AFC, TDH and TNP. Heterosis reduced AFC by more than 90 days, increased TDH for more than 1.5 years and TNP for almost one parity. Reciprocal backcrosses to either BS or Zebu for the traits evaluated were not significantly different. The F1’s had the best reproductive performance under this study.

It is likely that a two-breed rotational crossbreeding program between BS and Zebu could increase and maintain good reproductive performance in farms where the level of management is similar to this study.

 Key words: Zebu, Brown Swiss, additive, heterosis, reproductive performance

Introduction

Cattle production is one of the main animal production activities in southeastern Mexico (Anderson et al 1995). In this area, cow reproductive performance is low which causes low overall production, expressed as more than 36 months of age at first calving, 18 months of calving interval, less than 150 kg per calf weaned at eight months of age, 350 g of average daily gain from weaning up to 15 months of age (Duarte–Ortuño et al 1988; Magaña and Segura 1997; Magaña and Delgado 2000). Breed differences exist and positive effects of heterosis have been reported for several traits. These genetic resources should be utilized to increase production levels, but the environment in which they will be applied must also be considered (Kock et al 1989; Tewolde and Nuñez 1998). However, in the tropics there is a lack of information related to comparisons of breeds and crosses (Benyshek 1998).  

Among animal genetic resources available, Zebu and Brown Swiss breeds have great importance to animal production in tropical environments, especially in Mexico. The first for its adaptation attributes such as heat and parasite tolerance and ability to survive with limited feed resources. The second, due to its ability to produce milk and beef, its rusticity and pigmentation suitable to high solar radiation (Olson et al 1985). In southeastern Mexico, there does not exist an organized database for breed evaluation, although there are commercial farms which have information on cattle performance, which could contribute data for breed comparisons to improve cattle production systems. 

The objective of this breeding experiment was to compare several component traits of reproductive efficiency between Zebu breeds and their crosses with Brown Swiss. Another objective was to estimate additive and general heterosis effects for these traits from data obtained from a commercial farm in southeastern Mexico.

Material and Methods 

Environment, animals and management

 Data were collected from a farm located in the state of Yucatan, Mexico, located between 19º40’ y 21º37’ north latitude parallels and 87º32’ y 90º25’ west longitude meridian (Duch 1988). Average annual rainfall in the region is 1100 mm (range 621 to 1669 mm), of which about 70% falls between May and October. Average annual temperature is 26°C with a range from 24°C in December to 28°C in May. The soil varies from slight to medium depth, overlays limestone and is classified as a luvisol (COTECOCA 1977).

The data collected involved cows born over a 15-year period (1975 to 1989). The base population was Creole cattle crossed with Guzerat bulls imported from Brazil in 1935, and from 1940 to 1945. In 1950, European genes were introduced to the population through use of crossbred bulls, mainly Brown Swiss*Zebu and Charolais*Zebu. Since 1960, purebred Brown Swiss (BS) bulls were introduced. Purebred Brahman (BR) bulls from the United States and Indo-Brazil (IB) bulls were also introduced since 1965. Other European breeds used were: Holstein, Charolais, Simmental and Limousin; however, because of few numbers of observations these data were not analyzed. Data analyzed belong to Zebu and BS as purebreds, and F1 and reciprocal crosses among them (Table 1). The number of cows for each genetic group distributed among years is shown in Table 2.

The cattle grazed rotationally on paddocks of Guinea grass (Panicum maximum) during the night and were corralled during the day for watering and general management. Cows and calves were maintained together up to weaning around eight months of age. Mating of cows and bulls occurred throughout the year. The stocking rate was about one animal unit per hectare. Cows were managed as follow: between the day 30 to 120 post-partum, all cows were on a standard artificial insemination (AI) program. Heat detection was done twice during the day in a corral, with the help of some teaser bulls, not equipped with chin-balls. Thereafter, the cows were maintained in two groups, Zebu cows ran with Zebu bulls and crossbred cows mated to BS, Charolais or Zebu bulls. Since 1982, each bred female received daily 1 kg supplement based on a mixture of poultry litter and molasses, which was fed during the AI period. 

Heifers were vaccinated against Brucellosis at six months of age and were introduced to the breeding herd between 18 to 20 months, weighing at least 320 kg of body weight. The herd was vaccinated against rabies once a year. Ticks were controlled monthly by use of dipping baths.

Cows included in this study were daughters from more than 80 bulls. However, some sires of cows were unknown. In this case the farmer classified the cow’s genotype according to its appearance.  

Preparation and analyses of data

Data on 992 cows born from 1975 to 1989 and culled up to 1996 were grouped into seven genetic groups (GG). Age at first calving (AFC), calving intervals (CI), total days in herd (TDH) from first to last calving and total number of parities (TNP) were calculated from the record cards kept at the ranch. TDH were calculated for each cow as the number of days from first to last calving.  

Four seasons were defined based on monthly precipitation means: season 1 (May to July) when rains and pasture growth began; season 2 (August to October) when heavy rain occurs and an excess of pasture exists; season 3 (November to January) when rain is sporadic, pastures start to dry and their quality declines; and season 4 (February to April) when a severe dry period occurs and the quantity and quality of forage available is poor. 

Data were analyzed using the GLM procedure (SAS 1989). The statistical model for AFC included the fixed effects of breed group, year and season of birth, month of conception, sex of the calf and first order interactions. For CI, the model included the fixed effects of breed group, year and season of calving, parity number of the cow (1 to 9), sex of the calf weaned and first order interactions between breed group and year of calving. Finally, for TDH and TNP the model included the effects of cow breed group, year and season of birth, and AFC as a covariate. Due to the large coefficient of variation (> 40%) TDH data were transformed to log 10 and TNP data to square root prior to analysis (Snedecor and Cochran 1982).

Orthogonal contrast comparisons were made between Zebu genotypes; CZ and BS, and F1 against ¼ and ¾ BS. These orthogonal contrasts were performed by the Contrast option of the GLM procedure of SAS (1989).

Additive (breed differences) and non-additive (general heterosis) were estimated using the Greek Temple Model proposed by Cunningham (1987). Adding all Zebu breeds as a single group, the relative performance of various breeding groups and(or) strategies were predicted using an additive-dominant model.

Results

 Age at first calving (AFC)

The overall mean (± SD) for AFC was 1027 ± 113 days. Year of birth and GG effects were significant sources of variation (P<0.01). Neither season of birth nor conception month had significant effect on AFC (P>0.05). AFC decreased as year of birth increased. Mean squares according to GG groups are presented in Table 3. There were differences among the Zebu breeds. Indo-Brazil cows were the oldest at first calving. BS cows had better performance than Zebu cows. Crossbred cows performed according to the proportion of Zebu and BS genes; heifers with 25% or 75% BS genes had similar AFC. Heifers with 25% BS genes performed similar to Brahman and CZ heifers. The F1 heifers calved between three and five months earlier than Zebu heifers and between two and three months earlier than heifers with 25 or 75% BS genes (Table 3). 

Additive-breed effects, with respect to the commercial Zebu (CZ) group, were +10 and –53 days for BR and IB, respectively. The additive breed difference between BR and IB was -63 days. Additive breed differences estimated between BS and Zebu breeds were favorable for the BS breed (Table 4). Percentage of general heterosis estimated for BS with Zebu breeds were between –71 to –102 days (Table 4). 

 Calving interval (CI)

 The overall mean (± SD) for CI was 448 ± 90 days. Calving season (P<0.05) and parity number (P<0.01) were important sources of variation for CI. Breed effects for CI were not significant (P>0.05). Cows which calved from November to January had CI which was 55 days shorter than those cows that calved from May to October. Cows that calved during the rainy season had CI about 35 days greater than those that calved between November to January. First and second parity cows had longer CI than multiparous cows. In this study, CI decreased steadily as parity number increased (from 556±130 days for the first calving interval to 388±63 days for cows with more than four intervals).

Additive breed differences for CI between BS and Zebu breeds were unimportant (P>0.10). Also general heterosis estimated for BS with CZ, BR and IB breeds were close to zero for CI (Table4).

Total number of days in the herd (TDH)

The overall mean (± SD) for TDH was 1307 ± 105 days. Only year of birth and GG were important sources of variations for this trait (P< 0.01). Cows born before 1977 spent more time in the herd than those born later on. Neither the effect of season of birth nor AFC were important (P>0.10). However, cows younger at first calving tended to stay longer in the herd. The F1 cows spent more time in the herd (1772 days) than any other cow genetic group (Table 3).  

There were additive differences for TDH among the Zebu breeds (P<0.05). The I cows stayed in herd longer than either CZ or B. There were also differences for TDH between BS and CZ and B. The F1 had greater longevity than Brahman cows in the herd. Heterosis estimates for TDH for BS with Zebu crosses were between 42 to 60% (Table 4). 

Number of parities (TNP)

The overall mean (±SD) for TNP was 3.92 ± 0.63 parities. Year of birth and GG effects were significant (P< 0.05). Cows with 50% of BS genes had the greatest number of parities per cow (4.96 parities), followed by 75% BS cows. The poorest performance corresponded to the Zebu breeds (Table 3). Brahman cows had fewer parities than the other two Zebu breeds. Additive breed differences for TNP between BS with Zebu were unimportant (P>0.10), except for the Brahman (Table 4). The amount of heterosis estimated for BS with Zebu breeds was greater than one calving (Table 4).

Discussion

Age at first calving

 The mean AFC (1027 days) is within the range of reported values for the tropics (Haile-Mariam and Kassa-Mersha 1994); however, in other studies with more harsh environments. AFC was almost two years greater (Lôbo 1998). Highly significant differences among year of birth of cows may be expected due to prevailing extensive farming conditions, as well as differences in herd management practices, which tended to be improved through the period studied.

 The BS females were younger at first calving than Zebu females, which confirms the precocity of Bos taurus breeds and its advantage over Zebu breeds (Frankee 1980; Turner 1980; Cunningham and Syrstad 1987; Martínez et al 1988). Except for the Indo-Brasil breed, Zebu females had similar AFC as has been shown in other reports using the same breeds in the same area of study (Duarte-Ortuño et al 1988; Magaña and Segura 1997).

 Cunningham and Syrstad (1987) discussed the relevance of crossbreeding to improve reproductive performance in the tropics. Plasse et al (1968), Escamilla et al (1982) and  Teodoro et al (1984) reported that F1 (Bos Taurus*Bos indicus) females reach puberty at a younger age than Zebu or European breeds. Also, due to the heterosis present for age at first conception, crossbreeding is a good way to improve age at first calving. The advantage of F1 over Zebu ranged from two to five months of age, whereas the advantage of the other genetic groups was from one to three months. In Brazil, Teodoro et al (1984) and Martinez et al (1988) reported similar differences between Zebu and Holstein. However, Cunningham and Syrstad (1987), reanalyzing crossbreeding data in tropical countries, showed larger differences between Bos taurus and Bos indicus crosses, around 12 months of age.

General heterosis estimated at first calving was approximately 10%, which is higher than values reported by Duarte-Ortuño et al (1988) between Bos indicus populations, but it is within the range of values reported by Cunningham and Syrstad (1987) and Martinez et al (1988).

Calving Interval

 The overall mean CI (448 days) is within the range of reported values for the tropics of Latin America (Cunningham and Syrstad 1987; Duarte-Ortuño et al 1988; Magaña and Segura 1998), but shorter than 577 days reported in more harsh environments (Lôbo 1998). Cows which calved from November up to May had lower CI than those that calved in other season as found by Magaña  and  Segura  (1998). First and second calvers had longer CI compared to older cows, this is a well known effect in cattle which is consistent with reports from other authors (Duarte-Ortuño et al 1988; Magaña and Segura 1998). The lactation stress effect on CI may be confounded with postpartum management (heat detection, artificial insemination) and probably calf mortality.

Cow breed differences were not significant (P>0.10) for CI and were different compared to other reports in the tropics (Cunningham and Syrstad 1987; Duarte-Ortuño et al 1988; Martinez et al 1988). Perhaps, one of the reasons was that all cows during 120 postpartum days were mated by artificial insemination. In this respect, Vaccaro (1973) concluded that in the tropics, heat detection was one of the main causes of low cow reproductive performance under artificial insemination programs and this activity limited the advantages of crossbreed cows over both Zebu and BS cows.

Another possible reason that could explain the lack of differences in CI among the cow genetic groups evaluated was that the nutritional management based on pasture was not adequate enough to maintain good cow nutritional status. It has been shown that nutritional deficiencies can compromise endocrine functions mainly those related to reproductive events such as resumption of ovarian activity (Randel et al 1990). 

Total days in herd and number of parities

In the tropics few studies have considered the importance of these reproductive variables. Cows which calved earlier tended to stay longer and had more parities than those which calved later. Morris (1980) showed the advantage of first calving cows at two years compared to three years; this advantage reflected more parities and more kg of weaned calves, although the study involved Bos taurus cattle managed under a seasonal short-mating period.

Cow genetic effects were important for both traits (TDH and TNP), which agree with the results of Nuñez-Dominguez et al (1985), Sacco et al (1987) and Tanida et al (1988). These reports, except for Sacco et al (1987), were based on Bos taurus cattle data. There appear to be no comparable estimates available in the literature for the tropics and for crosses between Zebu and European breeds for TDH. However, again the advantages of crossbreed cows, mainly F1, over either Zebu or BS breeds were more evident.

Zebu and BS cows had similar overall performance. Sacco et al (1987) in the sub-tropics showed advantages of Bos taurus breeds compared to Brahman in both total number and kg of calves weaned per cowadvantages that were due to higher fertility of Bos taurus breeds compared to Brahman. However, the expected higher reproductive performance of BS cows could not be expressed in this study due to poor feeding and different reproductive management. In the same region, Magaña et al (1996) and Magaña and Delgado (1998), working with Holstein and BS cows, respectively, reported that less than 60% of cows were serviced during the first 120 days postpartum and all cows had lost body weights since calving.

Values for general heterosis for THD and TNP were equivalent to almost two years and more than one calving, which represents a great opportunity to improve reproductive performance of cattle populations in the tropics. Nuñez-Dominguez et al (1985) reported higher heterosis, equivalent to more than one calving for crossbreed cows compared to purebred Bos taurus breeds. Also, Sacco et al (1987) showed that heterosis for both total number and kg of weaned calves per cow tended to increase as cow age increased. These results confirm the relevance of maternal heterosis and the advantages in efficiency of crossbred performance between Bos taurus and Bos indicus (Grenn et al 1991).
 

Conclusions

• Except for the poor reproductive performance of Indo-Brazil cows, there were no additive breed differences detected between commercial Zebu and Brahman breeds. 

• Additive breed differences among BS and Zebu were only relevant for AFC. Crossbreeds cows had better performance than purebreeds.

• The advantages observed with F1 cows, due to higher levels of general heterosis for AFC, TDH and TNP, tended to decrease as the proportion of either Zebu or BS genes increased.

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Received 21 August 2001 

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