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The Relationships among Cracked Soybeans Fed, Barn Temperature and the Degree of Unsaturation of Milk Fat^1,2

Iowa State College, Ames

ABSTRACT

By randomization four similar lots of four cows each were subdivided into four outcome groups and fed similar rations, except for the protein supplements (linseed meal and cracked soybeans). Two groups of cows were fed rations by the double reversal method, while a control group was fed each of the experimental rations continuously throughout the experiment. Iodine, thiocyanogen and acid values of the milk fat and the pH of the cream and butter serum were determined. Barn temperatures were recorded daily.

In studying changes in fat composition, it seems advisable to utilize fat samples representative of one full day's milk yield, rather than samples obtained from individual milkings.

Highly significant correlations existed between the mean barn temperature (recorded the same day, 1 day before, and 2 days before the cream samples were taken) and the iodine and thiocyanogen values of the milk fat. The temperature recorded 1 day before the samples were taken had a closer correlation to changes in fat composition than that recorded the same day or 2 days before they were taken. The authors wish to point out that the correlation between the iodine and thiocyanogen values of the milk fat and temperature existed with cows which were from 1 to 2.5 mo. along in lactation when the experiment started (late November). It might be that had the animals freshened during November, February or April a different picture would have resulted (relative to apparent temperature effects) because of the lactation cycle.

The maximum effect of changing feeds on milk fat composition, as measured by the iodine and thiocyanogen values, appeared to have been reached in approximately 20 days after the first change of feeds. Apparently such factors as temperature changes and progress of the lactation and gestation periods interfered with fat metabolism so that it was impossible to determine when the full effect of the feeds had been reached following succeeding changes. Unless certain uncontrolled factors complicate the picture it appears from this study and a previous one (6) that 15 to 20 days may be required for a feed to exert its full effect on milk fat composition. This is in agreement with the work of Dean and Hilditch (5).

When the cows were fed either linseed meal or cracked soybeans as 11.1 per cent of the concentrate mixture over a long feeding period, they appeared to adjust themselves to the rations so that the iodine values of their milk fat were of about the same magnitude and their differences fairly consistent. On the other hand, when these feeds were fed for short feeding periods and then changed, the fat metabolism of the cows seemed to be disturbed so that the differences between the iodine values of the milk fat were rather inconsistent.

The changes in iodine values were largely dependent on the changes in the oleic acid content of the milk fat.

Non-significant differences were found between the iodine values of the milk fat from cows alternated from the linseed meal ration to the soybean ration and vice versa. However, the differences at certain periods were probably great enough to produce different effects on the quality of butter. A significant difference was found between the iodine values of the milk fats from cows fed linseed meal and cracked soybeans continuously at the rate of 11.1 per cent of the concentrate mixture. The iodine values of the milk fat from cows fed the linseed meal ration generally were higher throughout the experiment than were those of the milk fat from cows fed the cracked soybean ration. Although the iodine values of the milk fat from cows fed either of these two concentrates maybe practically the same (especially over a long feeding period) at certain times, they also may be quite different at various temperature levels or at different stages of lactation or gestation.

¹ The data contained in this paper are from a thesis presented by the senior author to the faculty of the Graduate School, Iowa State College, in partial fulfillment of the requirements for the degree of Doctor of Philosophy.

² Journal paper no. J1661 of the Iowa Agricultural Experiment Station. Ames. Project 692.

³ Present address: Louisiana State University, Baton Rouge.

⁴ Dairy Husbandry subsection.

⁵ Dairy Industry section.