Figure1. Birth weights of calves born across years of birth at RAB Songa Research Station.

Figure 2. Weaning weights of calves born at RAB Songa Research Station: Breed group season interactions

Animals that were calved in 1998 were significantly heavier than those born in 1999, 2000, 2001, 2003, 2006 and 2007. Calving of 1999, 2003 and 2004 registered higher mature weights than those of 2001 and 2007. The mature weights of calving from 2000 were higher than those from 2003, 2006 and 2007. Animals born in 2002 also weighed more than those calved in 2003, 2004, 2006 and 2007 (Figure 4).

Due to limited data, partitioning between pre weaning and post weaning growth rates could not be done. Therefore, growth rates from birth to 18 months of age were computed as one data set. Growth rates of calves from birth to maturity differed by breed group (P=0.002), and year of birth (P=0.02). Season and its interaction with breed group did not significantly influence as well as sex did not affect growth rates.

Figure 3. Variations in weaning weight of calves across years of birth at Songa Research Station

 

Figure 4. Variation of weight at 18 months age of cattle breed groups by year of birth at Songa Research Station.

 Table 3 shows that breed group AB grew significantly faster than all other breed groups (P<0.01). The lowest growth rate was registered in breed group AS (P<0.01). However, the rate of growth of AJ, AJS, ASJ were similar (P>0.05).

On the contribution of the environment to variance components, this study adopted year and season of birth as proxy indicators for environmental influence on progeny performance (birth weight, weaning, weight at 18 months, and growth rates). Nested random effect analysis of variance was executed to assess the contribution of genetic and environmental source to total variance. The random effects associated with error variance increased with age; while the aggregate proportional contribution of environment (YoB and SoB) declined (Table 4). The most consistent environmental factor has been the year of birth on all performance indicators.

Discussion

Causes of variation in birth weight 

The influence of breed group on birth weight was not significant despite the observed difference in mean birth weights (Table 2). No previous studies on the breed group effects on birth weight under Rwanda conditions have been documented. However, from work done with other comparable breed groups, the results of this study are in agreement with those from crossbreeding experiments carried out at Naivasha NAHRS (Annual report 1974); but contradicts those of Trail et al (1971) who reported that the progeny of Boran and Ankole dams were significantly heavier (14.0%) than those of zebu dams (P>0.05). Several researchers have reported comparable, lighter or heavier birth weight values obtained from different crossbreeding programs. Trail et al (1985) reported 30, 30, 28.2, 32.6, 31.9, 27.8 kg weights of Angus x Ankole, Angus x Boran, Red Poll x Ankole, Red Poll x Boran, Red Poll x Zebu calves respectively raised in Uganda. Later on, Rege et al (1994b) reported birth weights of 20.4 kg for Jersey x Ghana Short horn while Danbaro (1990) reported 21.2 kg for the same breed.  Higher estimate of birth weight (39.1 kg) for Friesian calves have been reported by Shin et al (1986). Some workers reported lower mean Friesian birth ranging from 25.8 to 33.5 kg (Frietas et al 1987; Bilal 1996; Singh et al 1997, Vassilev 1998, probably working in more difficult environments.    

Even though not statistically significant, the birth weight of male calves was approximately 4% higher than that of female calves. The non-significant effect of sex on birth weight in this study is in agreement with observations by Diack et al (2004) and Bosso (2006). On the contrary, Tadesse and Dessie (2003) reported higher birth weight in male calves (30.8 kg) than in female calves (29.9 kg) of Holstein calves kept in Ethiopia. Nevertheless, many researchers have reported significant effect of sex on birth weight, including Freitas et al (1987) and Guaragna et al (1990) who reported that birth weight was significantly affected by sex of calves in Holstein Friesian. The difference in birth weight between sexes was lower than most values reported in literature. This is attributed to longer gestation period of male calves or higher androgen concentration in foetuses. The heaviest calves were born in 2001, followed by 2005, 2002, 2000 respectively, while the lightest birth weights were registered in 1998, and 2008. Despite the observed variation (Figure 1), the effect of the year on birth weight was not significant; yet, a significant effect of year on birth weight has been reported by various researchers (e.g. Sang et al 1986, Kayigisiz et al 1995). Year effect doubtlessly reflects climatic, feeding, sanitary, and management conditions of the dam during gestation; and because of changes that occur in these conditions from year to year, differences in calf birth weights among years are expected. Therefore, the years 2001, followed by 2005, 2002, 2000 were respectively more favourable than 1998 and 2008.

The non-significant effect of season on birth weight observed in this study is in agreement with results from other studies.  Oddoye et al (1999) found a non-significant effect of season on birth weight and pre-weaning average daily gain of Sanga cattle. Similarly Orneas (1982), Shin et al (1986) and Guaragna et al (1990) reported non-significant effect of season of calving on birth weight of calves in Holstein Friesian calves. In contrast, other workers have reported significant effect of season on birth weight Okantah (1990) in Sanga calves and Chaudhri et al (1995) in Jersey calves. Seasonal fluctuations in temperature, precipitation, relative humidity and other climatic factors are the most important causes of the influence of season of birth since these variables determine the abundance or scarcity of forage available, which is directly related to the general physical condition of the dam during gestation, and thus perhaps reflect on birth weights of the calves. Thus the non-significance on influence of season on birth weight suggests that the variation in temperature, precipitation and other environment factors among seasons had little effect on dams during gestation period.

Weaning weight 

The significant effect of breed group on weaning weights observed in this study is in agreement with those of Trail et al (1971) on crossbreeding Ankole, Boran and Zebu in Western Uganda; and Sottie et al  (2009) who found significant breed group effect on weaning weight of Sanga and Friesian-Sanga crossbred calves in Ghana. The effect of season on calf body weight and growth is related to the availability of feed (Okantah and Curran 1982). Season of birth as expected had significant influence on weaning (Table 2), supporting earlier studies (e.g. Okantah and Curran 1982).   The significant effect of breed group by season interaction on weaning traits in this study indicates breed groups are responding in different ways to effects of season within the same ecological zone and breed groups cannot be discussed in isolation without reference to the respective seasons of birth. The results from analysis indicate that some seasons of birth had greater effect on some breed groups than others. The effects of season of birth and breed group have also been found to be significant under tropical conditions elsewhere (Magaña and Segura-Correa, 2006). In our study, there was a change of merit when animals were compared across seasons. For instance, the weaners of Ankole × Friesian crosses were significantly heavier than weaners of other breed groups in season 1 and 2, while Ankole × Brown Swiss crosses were the heaviest in season 3 and 4. This suggests that the variation in temperature, precipitation and other environmental factors among seasons had significant effect on the calves or on their dams at weaning.   

The best season of birth for nearly all breed groups was season 4 (February to Mid- June). On this basis, it appears that there is advantage in mating cows at a certain time of the year, so that the birth – weaning intervals includes the largest number of days with the highest availability of forage, which in turn would determine plentiful feeding for dams and thus more efficient nursing conditions. The results suggest that the best breeding season for Songa is that of May to the end of August. The mean values of weight at weaning reported in this study are lower than mean values reported by Joao et al (1982) of 217±1 kg for Canchim cattle when similarity in weaning age is considered, but are higher than those reported by Danbaro (1990) for Jersey x Sokoto Gudali, Ngere (1971) for Friesian x Ghana short horn and Ahunu et al (1997) for Red Poll x Ndama. However, the apparent variations in weaning weight in the present study when compared to data reported elsewhere should be interpreted with caution in view of the great variability in weaning age. In addition, the different conditions prevailing at experimental stations or farms and management with or without supplementation could be among the probable sources of error when herds of different breeds and in different environments are compared. The wide range of weaning weight for AB and AF breed groups (Figure 2) compared with the range of mean weights for AJ, AJS, AS, and ASJ suggests breed groups containing Jersey and Sahiwal blood were more resilient to environment pressure and therefore these breed groups are considered the best for Songa environment.

The effect of sex was not significant, however, the least square constants showed a 15 kg difference in favour of males. The sexual dimorphism observed in weaning weight could be partially explained by the action of sex hormones on secondary sex characteristics (e.g. skeletal conformation and muscle size). In addition different figures mostly found in literature are due to difference in breed groups used, management practice of supplementation or time of castration (Pahnish et al 1961; Sacker et al 1971b). Also owing to the longer gestational period, males are heavier at birth and carry this advantage throughout the nursing period, with higher weaning weights. 

The year of birth effect significantly influenced weaning weight with increased weights occurring in 2008 (Figure 3). The significant effect of year on weaning weight has been reported by several other studies on different breeds of cattle (Badran and El-Barbary 1986; Kabuga and Agyemang 1984; Oudah and Mehrez, 2000; and Lengyel et al 2002). This significant effect seems to depend largely on favourable climatic conditions with increased weaning weights being observed during the years with good rainfall and consequently abundant forage. Favourable environments also ensure low incidence of diseases. The variation in weaning weights caused by year effect in this study agrees with those reported by Kennedy and Chirchir (1971), Tonn (1974) and Mortojo (1973). In a situation where calves and their dams are not supplemented as is the case for Songa farm, the weaning weights of calves seems to vary with rainfall amounts and distribution in each year. This determines the amount and quality of forage available to the animals. 

Weight at 18 month of age 

The significant effect of breed group on weight at 18 month of age is confirmed in this study and agrees with Trail et al (1971) who worked with crossbred beef cattle in Western Uganda. The results of this study were highly significant (P<0.001) (Table 5), however what is surprising is that the weight difference of 41 kg between means of the female and male cattle obtained in this study was significant, and is contrary to several investigations where the male performance is superior, because the poorer performance of females compared to males is expected from their sexual hormonal imbalance. The possible reason could be due to sale of fast growing male animals right after weaning. This is the cattle management practice at the station, hence possibility of missing the records of fast growing animals. The performance of breed groups varied from year to year. For instance animals born in 1998 were significantly heavier than those born in 1999-2003 and 2006-2007 (Figure 4); while animals born in 2006 and 2007 registered lightest weight. In situations where cattle are not supplemented as is the case for Songa farm, the body weight of animals seem to vary with amount and distribution of rainfall in each year, which directly determines the quantity and quality of pastures in each particular year and this is also different from place to place. On the other hand, season of birth had no significant influence on weight at 18 month of age. This suggests that the variation in temperature, precipitation and other environmental factors among seasons had little effect on heifers at 18 month of age.

Growth rates

The breed group effect was significant for growth rates. It should be noted however, that breed effect was partially confounded with period as some breed group data was missing in some periods. Therefore, only general trends can be inferred from the results for breed group effect. Breed group AB grew faster than all other breed groups; but its growth rate was not significantly higher than breed group AF. The lowest growth rate was registered in breed group AS. However, the speed of growth was similar to those in breed groups AJ, AJS, AS and AJS (P>0.05) (Table 8). This is perhaps due the higher growth potential of Holstein Friesian and Brown Swiss, thus the crossbreds with Ankole excelled due heterotic effect. Effect of season was significant (P<0.001) for weaning weight, however, what was surprising was that season of birth did not significantly influence weaning to 18 month growth rate and had no contribution to variation of growth rate after weaning period. This could be due to the post weaning stress which may have obscured the effect of season of birth. It was clear that no particular season of birth was significantly associated with superior performance of any breed group studied. 

Variance components

From Table 4, only the year of birth contributed consistently to the variance components. The breed group had no contribution for birth weight, while the variance ratios for effect of sex (7.8%), Year of birth (44.4%), Season of birth (5.9%), and the composite (Yob x Sob x Mob) effect accounted for 73.8% of the variability.  The present composite effect reported agrees with Pribyl et al (2003) who reported that the composite (herd × year × month) effect accounted for 73% of variability. However, when herd, year and month effect were separately included in the analysis, reported proportions of variability were 48% herd, 20% year, and 5% month. Krupa et al. (2005) reported the variance ratios for effect of sex (6%) breed (2.2%) and dams age (2.2%) as well as herd-year-season effect (72%) and these findings were similar to those reported by Jakubec et al (2003). When compared to the findings of the present study, those earlier reports show that environmental factors contributed a higher proportion to the total variance. The higher contribution of the environmental effect may be due to high herd variability in both management practices and climatic conditions. The random effects associated with error variance increased with age; while the aggregate proportional contribution of environment declined (Table 4). Except the year of birth that consistently contributed to all age groups, only breed group contributed to the growth rate. 

Conclusion

• In this study, aimed at assessing the  non-genetic (environmental factors) influencing growth and performance of cattle at Songa Research Station, Rwanda, results show that environmental factors had a significant influence on weaning weight, and weight at 18 month of age, but not earlier, on birth weight. 

• The interaction of Breed group × Season had a significant effect on the weaning weight, hence emphasizing the importance of matching cattle breed groups with particular environments.

• No phenotypic parameter estimates were found in literature for the cattle breed groups studied, whether in different or similar production system/conditions to Rwanda. However, the results obtained in the current study for birth, weaning and mature weights are comparable with those reported for other pure and crossbred populations.

• The breed groups, Ankole × Brown Swiss (AB), and Ankole × Friesian (AF) grew faster than those containing Sahiwal and Jersey blood; however, they showed wide variation in growth indicating their susceptibility/sensitivity to tropical environmental stress.

• Breed groups: AS, ASJ, AJ and AJS proved to be less sensitive to seasonal variation and hence, are the best breed groups for the Songa and related environments.

• It can be concluded that Ankole × Jersey and Ankole × Sahiwal crossbred cattle have the potential of surviving and producing under minimal management, typical under resource poor farmer’s conditions. However, the development of Friesian × Ankole and Brown Swiss × Ankole crosses under a management system of improved nutrition can hold a promise for the cattle industry in Rwanda.

Acknowledgment

The authors thank the many people who have assisted in the conduct of this research, in particular, the staff of livestock section at Songa research station. We are also grateful to the anonymous reviewer for valuable comments which have greatly enriched the manuscript.

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Received 3 March 2012; Accepted 21 March 2012; Published 2 April 2012

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