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Iowa Agricultural Experiment Station
Abstract
Calculation of the gestation length from known breeding date to known birth date using a "days of the year" table becomes rather cumbersome and tedious if many such calculations are to be made. A simple device that makes the chore less tedious and increases accuracy of computation is described here. It was devised for use with swine but the same principle can be applied to gestation periods of other classes of livestock and to plant data such as "days to flowering" etc.
Using paper ruled evenly in columns and lines, set down in the first column the daily dates of the breeding season, then skip a column and set down in the third column and opposite each date in the first column the date exactly 115 days later. The number, 115, is chosen because it is close to the mean gestation length for swine. Cut off a blank column of the paper and mark each space in numerical order, starting with number 100 in the case of swine. The numbers 100 to 130 are ample. Place this short numbered column in the empty column between the column of breeding dates and the column of farrowing dates. Set the base number 115 opposite the known breeding date. Then look in the right hand column for the known farrowing date. The figure on the sliding short numbered column opposite the farrowing date is the gestation length in days. When leap year is involved use 116 on the slide as the base number, rather than 115, in the case of animals bred before February 29 and farrowing after February 29.
1 Journal Paper No. J1613 of the Iowa Agricultural Experiment Station, Ames, Iowa, project No. 32.
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