A MODEL OF LITTER SIZE DISTRIBUTION IN CATTLE

Authors: Bennett, Gary; Echternkamp, Sherrill; Gregory, Keith

Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/17/1998
Publication Date: N/A
Citation: N/A

Interpretive Summary: One method of increasing the number of calves born in a herd is to increase the genetic level for twinning. Management strategies for raising twins have been developed. Along with genetic increases in twinning, the incidence of triplet births is expected to increase. However, an increase in triplet births is not as desirable because management strategies for triplets are not well developed. There are no estimates of the incidence of triplets in herds with genetic levels of 40% or greater twinning because there are no such herds. A prediction of the incidence of triplets was developed from data on ovulation rate in sheep and from data and hypotheses about embryo losses in cattle. Predictions were compared with seven years of data from a herd with an average twinning rate ranging from 14 to 34%. A total of 3880 single, 1229 twin, and 33 triplet births were observed. Based on the average litter size, 3877.8 single, 1231.6 twin, and 32.4 triplet births were predicted. Less than 5% of births are predicted to be triplets when 50% of births are twins or triplets. This prediction can be used as part of an evaluation to project changes in efficiency when genetic levels of twinning are increased.

Technical Abstract: Increased incidence of triplet and higher order births in cattle should occur as genetic levels of twinning increase. Estimates of triplets in populations with genetic levels of 40% or greater twinning do not exist because there are no such populations. However, predictions of triplets are needed to evaluate the potential impact of further genetic increases in ntwinning. Both empirical data and biological hypotheses were used to develop a prediction. Empirical data consisted of sheep ovulation rates and estimates of bovine losses of embryos in early and late phases of pregnancy. Ovulation rate was decreased from levels found in sheep to those of cattle. Biological hypotheses concerned the independent loss of embryos in early pregnancy and codependent failure of multiple embryos in late pregnancy. Pre-implantation losses were considered to be independent resulting in the loss of only the affected embryo. Post-implantation failure of an embryo was assumed to result in the loss of all embryos. Predictions were tested with data on cows calving from 1989 to 1995 in a herd selected for twinning rate. Totals of 3880 single, 1229 twin, and 33 triplet births were observed compared with predictions of 3877.8 single, 1231.6 twin, and 32.4 triplet births. Prediction of triplet births was accurate within the range of available test data. Predicted triplet births at 35, 45, and 55% multiple births are 1.4, 3.1, and 6.6%, respectively.