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Humboldt-Universität zu Berlin (1999)

Effects of seasonality on the productivity of pastoral goat herds in northern Kenya

Hary, Ingo

Titre : Effects of seasonality on the productivity of pastoral goat herds in northern Kenya

Auteur : Hary, Ingo

Université de soutenance : Humboldt-Universität zu Berlin

Grade : Doctor rerum agriculturarum (Dr. rer. agr.) 1999

Résumé
Under semi-arid rangeland conditions in northern Kenya, the main factor influencing the productivity of small ruminant flocks is climatic seasonality. In pastoral production systems, alternatives to herd mobility as an efficient adaptive management strategy to overcome nutritional deficits are few. One possible intervention is to manipulate the total seasonal nutrient requirements of the herd through controlled seasonal breeding, which commonly is not practised by pastoralists.
The restriction of breeding as a management strategy to match periods of critical nutrient demands with seasonal feed supply in pastoral goat flocks has so far received little attention in research. In order to gain a clearer understanding of the merits and demerits of controlled seasonal breeding, a systematic breeding programme in a herd of Small East African goats was initiated for a period of four years (1984-1988) in Isiolo District, northern Kenya. The study was undertaken to (1) assess the effect of seasonal forage supply on various parameters determining pastoral goat flock performance, and (2), using these baseline data, to test the hypothesis that a seasonally restricted breeding regime can increase flock productivity.
A total of 145 does of the Small East African type were maintained under simulated pastoral management conditions and used for a total of 381 exposures which were distributed among 18 consecutive breeding groups consisting of approximately 18 does each. The experimental design resulted in six different, consecutive mating periods or seasons per year, which were replicated three times during the course of the experiment. A total of 8547 recordings were obtained on survival, liveweight, and milk production of does ; 9837 observations were available on survival and liveweight development of youngstock. Detailed statistical analyses were performed on all traits relevant to assessing overall biological herd productivity, including : survival of kids and does ; reproductive performance of does ; growth performance of kids and body weight development of does ; and milk production. A steady-state herd model was developed and used to assess overall flock productivity for each of the six consecutive two-month breeding seasons. The procedure is based on a stage-specific description of population dynamics and uses non-linear programming to derive the steady-state herd structure and culling policy that maximizes overall energetic efficiency of the herding enterprise.
Mating season had no statistically significant effect on reproduction traits, most likely due to the large variability in within-season environmental conditions among the three production cycles. Differences in kid survival among mating seasons were marked. The results demonstrated that restricted breeding can be an effective means to control kid mortality. Similar conclusions apply with respect to milk yield, which was an important risk factor affecting kid survival until weaning. Although growth performance of kids until weaning differed substantially among mating seasons, these had largely disappeared by one year of age. Therefore, seasonal breeding does not seem to confer any major advantage in terms of growth performance of youngstock per se.
Steady-state herd productivity assessments revealed that under the current production conditions reproductive performance traits are far less important as contributors to biological productivity than is often assumed. Sensitivity analyses showed that juvenile survival rate is the most important factor determining overall energetic efficiency. Restricted breeding can be used as a management control to manipulate overall biological herd productivity primarily because of its positive effect on youngstock mortality rates. In contrast, yield levels, i.e., growth and milk performance, are less important as determinants of biological herd productivity, once their effect on youngstock mortality has been accounted for. Joining does at the peak of the long dry season (July and August) proved to be the optimal management strategy in terms of energetic efficiency at the herd level. Whether restricted breeding is biologically superior to an aseasonal breeding management, as is often practised by pastoral producers, remains ambiguous. The results for a simulated aseasonal breeding regime indicated that the potential improvements in biological productivity are probably much smaller than is usually presumed.
With respect to steady-state herd productivity assessment, results of the present work emphasized the importance of utilizing an optimality approach for obtaining a common basis on which management alternatives can be compared in terms of their effect on energetic efficiency at the herd level. The assessment procedure is essentially a device with which standardized comparisons of biological or economic productivity in livestock herds can be carried out, but it can also be a valuable aid in understanding or optimising the production system looked at.

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