Twinning in Dairy Cattle and Its Relation to Production

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Abstract
Twinning and its effects on fat production were investigated in the Iowa State College dairy herd and in 9 herds of the Iowa Board of Control. Twin births were 2.9% of the total of 10,885 births. From the sex ratio, the frequency of identical twins was estimated as 8.6 [plus or minus] 1.1%. In lifetime average production per 10-month lactation, the dams of twins averaged 19 [plus or minus] 9 pounds of butter-fat higher than their herd mates, but this was almost wholly due to confounding the effects of selection with the effects of age on twinning frequency. The difference fell to 3 [plus or minus] 6 pounds when dams of twins were compared only with contemporary herd mates who remained in the herd equally long. Production in the lactation following the birth of twins was 21 [plus or minus] 11 pounds lower than in the preceding lactation.