Genetic parameters were estimated for birth weight and weaning weight from three year (1991)(1992)(1993) data totalling 1100 records of 25 rams to 205 ewes of Indigenous Sabi flock maintained at Grasslands Research Station in Zimbabwe. AIREML procedures were used fitting an Animal Model. The statistical model included the fixed effects of year of lambing, sex of lamb, birth type and the random effect of ewe. Weight of ewe when first joined with ram was included as a covariate. Direct heritability estimates of 0.27 and 0.38, and maternal heritability estimates of 0.24 and 0.09, were obtained for birth weight and weaning weight, respectively. The total heritability estimates were 0.69 and 0.77 for birth weight and weaning weight, respectively. Direct-maternal genetic correlations were high and positive. The corresponding genetic covariance estimates between direct and maternal effects were positive and low, 0.25 and 0.18 for birth weight and weaning weight, respectively. Responses to selection were 0.8 kg and 0.14 kg for birth weight and weaning weight, respectively. The estimated expected correlated response to selection for birth weight by directly selecting for weaning weight was 0.26. Direct heritabilities were moderate; as a result selection for any of these traits should be successful. Maternal heritabilities were low for weaning weight and should have less effect on selection response. Indirect selection can give lower response than direct selection.

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A C o m p ara tive study of the perfo rm an ce of the Chitungwiza, M arondera a n d Crow borough sew ag e treatm en t plants in Zim babw e. M. F. Zaranyika, C. Mahamadi and A. S. MathuthuSludge deposition a n d caustic em brittlem ent in the p re-m atu re failure of a waste h ea t boiler steel p late. D.J. Simbi and O.S. ChinyamakobvuEvaluation of D u ck w ee d (Lem na minor) as a fee d ingredient in the finisher diets of broiler chickens J. Kusina, C. Mutisi, W. Govere, R. Mhona, K. Murenga, J. Ndamba, and P. Taylor Vesicular arbuscular mycorrhizal fungi p re v a le n c e a n d diversity in Z im b a b w ea n soils. As evidence has shown that dairy cattle from more variable herds may be over-evaluated, the effect of heterogeneous variances across herd production levels on genetic evaluation was investigated using lactation milk production records of Zimbabwean Holstein cows. A total of 12420 first lactation milk production records of Zimbabwean Holstein cows were split into three groups (Low, Medium and High) according to herd production level. Preliminary analysis to determine the fixed effects of herd average group, herd, class of cow, year of calving, month of calving and age at calving on milk yield was done. An animal model containing additive genetic effects pertaining to cows, dams and sires was implemented in AI-REML package to estimate additive genetic, residual and phenotypic variances and heritability estimates. The F-max test was used to test for homogeneity of variances. All fixed effects significantly affected milk yield (P < 0.05). Estimates of phenotypic, genetic and environmental variances increased with herd production level. Additive genetic variances were 200 275kg2, 320 400kg2, 376 900kg2 for Low, Medium and High herd production levels, respectively. Heritability (h2) also increased with herd production level. Estimates of heritability were 0.25,0.29,0.30 for Low, Medium and High herd production levels, respectively. Breeding values for the top ten sires varied with herd production levels suggesting a re-ranking of sires due to herd production level. This study showed that intra-herd variances are heterogeneous. Methods for data transformation to account for heterogeneous variances should be developed. F. T . Makonese, S. Mpepereki and P. Mafongoya

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