Abstract
Abstract. Genetic trends based on 30 395 records with parities 1 to 8 were estimated using average information restricted maximum likelihood algorithm (AIREML), which had overall mean, herd, month of calving, year of birth, parity and linear and quadratic effects of days in milk. Due to changes in the Zimbabwean milk recording system data set was split into records of cows born from 1973 to 1989 and the other from 1987 to 1994. The first data set was further split into even numbered herds (EVNH73) and odd numbered herds (ODNH73), due to computational limitations. The highest milk production was from May to August. Peak milk production was observed in parity 4. The heritabilities for FCM (0.22 to 0.26), fat percent (0.24 to 0.28) and protein percent (0.21) were comparable to literature values. The annual genetic trends of FCM ranged from 8.36 kg to 13.2 kg, fat per cent ranged from 0.0 % to −0.008 % and protein per cent was −0.003 % in the three data sets. Genetic trends for milk yield, fat and protein yield were also positive but overally lower than in most other countries. The positive genetic trends obtained for milk, fat and protein yields imply that some progress has been made in increasing the milk production despite the relatively small population and significant restrictions. There is need to develop a selection index to improve milk yield and milk composition.
Highlights
The effectiveness of any animal-breeding programme is measured by the genetic progress obtained
Both genetic and environmental improvements offer opportunity to KUNAKA et al.: Genelic trends for milk, fat and protein in the zimbabwean holslein-friesian population increase the production of dairy cattle
By analyzing the genetic trends, the need to take corrective measures or improve or even change the criteria used for choosing the genetic material for import to Zimbabwe may be highlighted
Summary
The effectiveness of any animal-breeding programme is measured by the genetic progress obtained. The mixed model methodology (MMM) initially described by HENDERSON (1975) has recently been used to determine what proportion of the phenotypic value is genetic It has been favored because MMM has smaller sampling variance and it accounts for competition by adjusting for differences in herd-mates ofthe daughters of the sires being evaluated. Restricted Maximum Likelihood (REML) estimates the unknown parameters from the data One such REML programmes is AIREML, which has the large sample properties of Maximum Likelihood (ML) and accounts for the loss of the degrees of freedom in the simultaneous estimation of fixed effects. It is robust against selection and nonrandom mating (MUSANI and MAYER, 1997)
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