Growth and feed conversion of Boran (Bos indicus) and Holstein×Boran heifers during three physiological states receiving different levels of a tropical diet
Introduction
The average growth rate for milk production in the developing world has been 3.1% per annum from 1984 to 1993 and is forecasted to be further growing until 2020 at a rate of 3.3% per annum, this even more pronounced for Sub-Saharan Africa with expected rates of 3.8% (Delgado et al., 1999). Important trends, which reflect this change, are intensification and more production-oriented animal keeping systems mainly in peri-urban regions (Staal et al., 1997). However, smallholder livestock farming systems in Sub-Saharan Africa, particularly those in remote areas, are mainly based on wide-spread livestock activities, where livestock serves, beside production aspects, also as a source for draught, capital assets, insurance and cultural affiliations. So far, mainly oxen are employed for work (e.g., ploughing) and this on average for 51 days per annum with the cost of maintaining the animals exceeding the economic benefits of work, especially in a situation of increased industrialisation (Cole, 1997). Dairy heifers might be a better alternative although only in areas where there are no cultural barriers against the use of female animals for draught. The multipurpose function of livestock in remote areas is guaranteed best by indigenous, mostly Bos indicus, breeds which are well adapted to the specific environmental conditions Pearson and Dijkman, 1994, Lawrence and Pearson, 2002. Market-oriented smallholder dairy farmers, by contrast, increasingly employ crossbred animals since their major purpose is to produce goods that can be sold (Bebe et al., 2003). In both, rural or peri-urban areas, feed scarcity is still a major problem. Efficient means to increase feed utilisation are demanded to be able to maintain or enhance livestock productivity, thus increasing the supply of valuable food to the population and alleviating poverty of the livestock keepers. Only few and contradictory information is available on growth performance and feed utilisation of dairy heifers of indigenous or crossbred origin under tropical conditions (Agnew and Yan, 2000). There is evidence that the efficiency of conversion of feed energy into weight gain is more variable than it is accounted for in current net energy (NE) or metabolisable energy (ME) based systems Nsahlai et al., 1997, Yan et al., 1997, NRC (National Research Council), 2001. Growth performance and metabolic energy utilisation may also change in different physiological states. For instance, after exposure to work, pregnant Holstein×Boran cows were found to restore proportionately 0.95 of their previous body weight (W) loss in a recovery period of 6 months and even 1.77 of the loss when receiving a ration of grass hay and 3 to 5 kg/day of a supplement; by contrast non-pregnant lactating crossbreds restored proportionately only 0.12 (no supplement) and 0.59 (3 kg supplement) of the losses (Zerbini et al., 1996).
The basic hypothesis to be tested in the present study was that there may exist significant differences between purebred indigenous B. indicus heifers and crossbreds with Bos taurus in their response in growth, feed conversion and reproduction to long-term exposure with low, medium or high level of energy supply. Particular emphasis should also be put on the effect of working exercise as most previous controlled studies have investigated the influence of work with oxen and lactating cattle (e.g., Pearson and Lawrence, 1992, Zerbini et al., 1993). The approach chosen comprised about 300 days of experiment where genotypes were compared in three different physiological states including exposure to work and subsequent recovery.
Section snippets
Animals and research site
Twenty-four East African Boran (B. indicus; Tegegne et al., (1992) heifers and 24 Holstein Friesian×Boran (B. taurus×B. indicus) heifers were used in an experiment carried out at the ILRI Debre Zeit experimental farm located in the Rift Valley at 1920 m o.d. At the time when the experiment was conducted, the minimum and maximum ambient temperatures were 10 (±2) and 27 (±2) °C, respectively, and relative humidity was 59 (±12)% during the rainy season and 41 (±9)% during the dry season. Animals
Intake, digestibility and performance
Intakes of dry matter (DMi) and organic matter (OMi) differed (P<0.001) among the three feeding treatments in both genotypes as expected (Table 1). On average, heifers receiving medium and high feeding levels, respectively, consumed 22.0% and 43.6% more DM, and 21.8% and 43.7% more OM than heifers with low feed supply across all physiological states. Nevertheless, differences among feeding levels were slightly smaller than expected since grass hay refusals by the Boran genotype increased with
Feed intake and digestibility
Intake differences between feeding levels were always significant. When receiving the high feeding level, the intended level of increase in intake was smaller in Boran heifers in the two first physiological states because of refusals of 12% to 13% of the feed offered. Obviously, this amount of feed exceeded either their forage intake capacity or the threshold for physiological satiety. The first explanation seems more likely in the second experimental period when requirements were enhanced by
Conclusions
The type and the low and medium amount of feed provided in the present study were corresponding to those available to smallholder farming systems in Sub-saharan Africa. Results showed that crossbred heifers responded to higher feed intake with higher growth rates at any of the physiological states. Responses of Boran heifers, assumed to have a genetically lower growth potential, showed the largest increment in growth rates between the low and medium feeding level. This would suggest that extra
Acknowledgements
The authors are grateful to Olga Karenfil, Bekele Andarge, Worku Getachew, Wana Abaguchi and Arage Bekele for handling the experimental animals. Assistance in the construction of a walking path and harness for single cows by the technical staff of the Animal Feed and Nutrition Unit and the workshop of Debre Zeit is appreciated. Further thanks are portrayed to the biometric team of Mamadou Diedhiou, and Aklilu Bogale, Amare Atale for data handling. This work has been partially supported by the
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