Study of the Udder

Because the udder is so important, major attention was focused on that organ. Early in the study—even before the program of slaughtering cows was launched—regular examinations were made of the udders of cows in the Beltsville herd to detect individual differences and changes through lactating life in an effort to learn more about the qualities and characteristics of the udders of superior cows. With such a lifetime history as a background, we obtained more detailed information about the structure of the mammary glands at slaughter and afterwards.

When a cow was killed at Beltsville, her udder was removed and suspended from a frame in a natural position. It then was filled with a formalin solution by injection under pressure through the teats. The quantity of fluid held by the individual quarters was measured as a means of determining the capacity of the secretory system of the udder.

After the udder was filled and distended, it was placed in a low-temperature chamber and frozen. Then it was cut with a band saw into vertical cross-sections approximately an inch thick, care being taken to have one cut follow the long axis of the front teats and one the long axis of the rear teats. The sections were thawed, washed, and photographed. Various studies were started to determine the comparative physical characteristics of the tissues of individual udders to show variations and, if possible, the relation between such characteristics and production records of the cow during life. The results particularly with regard to the characteristics of mammary tissue—are not sufficiently complete yet to warrant conclusions. We have, however, determined the weights and capacities of udders of cows of different breeds, ages, stages of lactation, and levels of producing ability.

Individual variations were great. The udders of lactating Holsteins held an average of more than 60 pounds and the Jerseys more than 51 pounds of formalin solution. In both breeds the measured capacity was nearly one and a half times the weight of the udder.

The udder tissue is like a sponge. Blocks of tissue that have been treated with formalin and frozen can be subjected to pressures of 300 pounds for several minutes until they have yielded nearly all of the moisture they contained and become like thin wafers. When again placed in water or weak formalin they quickly become restored to their original thickness and appearance and pick up fluid to the extent of about tour times their pressed weight.

These facts have an important bearing on the results of experiments, begun at Beltsville in 1926, that proved to be revolutionary and provided a basis for much of the work since performed by research workers in the physiology of lactation. Before the experiments were made, it was taught that only a small amount of the milk obtained from a cow at a milking—perhaps a pint for each quarter—was present in the udder when milking began. People supposed that the stimulating effect of milking brought a sudden rush of blood to the udder and that the milk was manufactured rapidly during the brief time the cow was being milked. But it was found that a large part of the quantity regularly obtained at a definite milking time could be obtained from the same udder that had been amputated from a cow slaughtered just before her regular milking hour.

In 11 trials it was found that 70 percent of the average quantity obtained at regular milkings before slaughter was obtained from the amputated udders. In one case the post mortem yield was more than 100 percent of the average at corresponding milkings during the previous 10 days. When we cut open the udders, after we had removed as much milk as we could, we found that most of them contained a considerable quantity of milk—the milk was there, but the dead cow could not aid in its removal. We concluded that milk secretion is essentially a continuous process and that practically all of the milk obtained at a milking is present in the udder when milking is begun.

This emphasized the desirability of large udders, or udders with large storage capacity, and raised the question of how the udder of a cow producing 50 to 75 pounds of milk on two milkings or 100 pounds on three milkings daily could hold so much. This question has since been answered by the measurements of udder capacity. At the same time, it emphasized the importance of milking high-producing cows more than twice daily,

because secretion undoubtedly is slowed down and finally stops as pressure in the udder is built up by the accumulation of milk. The more frequent milking tends to keep the pressure down and permits unrestricted milk secretion at a maximum rate.

In a comparison of a highly specialized dairy cow and a highly specialized beef cow, body measurements were made, the internal organs were weighed and measured, the skeleton of each was completely mounted for purpose of comparison by measurements and photographs, and the udders were filled, frozen, and sectioned. Although the cows differed greatly in form, we found no differences in their internal organs big enough to indicate significant differences in function, and in most respects their skeletons were surprisingly similar. Aside from their external form, the most striking difference found was in the quantity of secretory tissue in their udders. The dairy cow's udder consisted almost entirely of glandular tissue, while that of the beef cow contained little except a mass of fat. The dairy cow was 19 years old, had been a regular breeder until she reached old age, and for nearly 5 years held the world's record for the Jersey breed in butterfat production. The beef cow had borne eight calves. She was said to have been a good milker for a beef cow, but in view of the almost complete lack of glandular tissue in her udder, it does not seem possible that she could have produced a large amount.

In another investigation of the differences between beef and milk cows, four Herefords were put in a dairy herd some 2 months before calving, so they could get used to being handled regularly. They were maintained through a lactation period in the same way a dairy herd is managed. Although they were milked up to the time that the milk yield was less than a pound a day, their lactation periods averaged only 211 days and production for the entire period averaged only 557.45 pounds of milk and 27.66 pounds of butterfat. One cow produced only 80.6 pounds of milk.

In skeletal structure, the Herefords were not greatly different—except for body size and weight—from the Holstein cows with which they were compared. The udder and some of the glands—particularly the thyroid and the pancreas—were comparatively small in the Herefords, both in actual weight and in relation to net body weight. The total quantity of mammary-gland tissue and the measured udder capacity of the Herefords were deficient. These results indicate that cows of the specialized beef breeds are relatively poor producers because they do not inherit a mammary development sufficient to enable them to be liberal milkers. The significance of the smallness of some of the endocrine glands is a point that needs further study.