An investigation into the consequences of selection for growth, size and efficiency^*

Abstract

The effect of selecting for growth, size and efficiency on fitness (fertility and survival rate) and body composition was investigated by surveying a number of selection  experiments which have appeared in Animal Breeding Abstracts since 1938. In the case of rodents and poultry, selection for body mass, growth rate and feed efficiency was studied in 26, 21 and 13 experiments respectively. Much less information on the relationship between growth, size or efficiency and fitness is available for beef cattle and mutton sheep. When selecting for increased body mass or growth rate in rodents and poultry, almost 90% of the experiments yielded data which  demonstrated an increase in body fat, or a decrease in fitness, or both in the offspring. When selecting for increased feed efficiency, only 15% of the experiments surveyed reported un favourable results. In general, the relationship between growth rate or body mass and fertility in beef cattle tends to be negative. Animal breeding for meat should therefore not be based on selection for growth or size alone. Other selection criteria should be investigated and the principle of utilizing sire and dam lines in beef cattle should receive attention.

'n Ondersoek na die invloed van seleksie vir groei, grootte en voerdoeltreffendheid op fiksheid (vrugbaarheid en oorlewing) en liggaamsamestelling is uitgevoer deur seleksie-eksperimente te bestudeer wat sedert 1938 in Animal Breeding Abstracts verskyn het. By knaagdiere en pluimvee is 26, 21 en 13 seleksie-eksperimente vir onderskeidelik massa, groeitempo en voerdoeltreffendheid bestudeer. Baie minder inligting oor die verwantskap tussen groei, grootte of voerdoeltreffendheid en fiksheid is vir vleisbeeste en -skape beskikbaar. Met seleksie vir 'n hoer liggaamsmassa of groeitempo by knaagdiere en pluimvee het ongeveer 90% van die eksperimente 'n toename in liggaamsvet of 'n afname in fiksheid of beide getoon, in vergelyking met 15% wanneer vir 'n beter voerdoeltreffendheid geselekteer is.
Dor die algemeen neig die verwantskap tussen groeitempo of liggaamsmassa en vrugbaarheid by vleisbeeste om negatief te wees. Vleisdierteelt kan dus nie net op seleksie vir groei of grootte gefundeer word nie. Ander seleksiekriteria behoort ondersoek te word en die beginsel van die gebruik van vader- en moederlyne by vleisbeeste behoort oorweeg te word .