The author is in the Department of Animal Sciences at The Ohio State University, Wooster.
Rearing dairy heifers so that they become productive members of the herd is a goal that all dairy farmers strive to achieve. For more than a century, researchers have been investigating how nutritional management strategies can help farmers attain this goal.
In 1906, C. H. Eckles at the University of Missouri embarked upon what was to be an eight-year experiment centered on defining, if possible, practical ways for producing quality milk cows through heifer management strategies. The research he did was aimed at addressing the question of, "Does a good dairy cow or an inferior one derive her special dairy characteristics by inheritance, or is it a result of her treatment from birth to maturity?" Eckles' work was practical and focused on monitoring feed intake and overall body growth measures followed by milk production. The results that Eckles found hinted at the possibility that diet can impair milk production, but udders were not directly measured for effects on growth or tissue development.
Questions remained
It would take the passing of roughly 50 years before nutritional effects on udder tissue composition were considered as a possible explanation for the reduced milk yield observed in some instances. In the late 1970s/early 1980s, researchers designed multiple experiments that primarily used weaned heifers to examine the effects of nutrition on mammary development. Initial findings suggested that animals showed impaired mammary development when fed high-energy diets prior to the onset of puberty but not afterwards. Thus, researchers generally extended their conclusions to imply that the entire time from birth to puberty represented a time where the udder may be harmed by elevated daily body weight gains achieved through nutritional strategies. However, this is not necessarily true. It must be remembered that results should be interpreted within the context under which they were conducted. So, while initial conclusions likely were valid for weaned heifers, what about preweaned heifers? Might they respond differently to diet?
Going younger
About a decade ago, scientists revisited the study of nutritional effects on mammary development. This time they looked at preweaned heifers. The general consensus was that, at this stage of life, elevated nutrient intake actually may have positive effects on mammary development, much like what is seen with frame growth. As a result, the past decade has produced a great volume of experimental work focused on the nutritional management of preweaned heifers and measures of mammary growth and development, and, unfortunately to a lesser extent, subsequent milk production.
In the early 2000s, researchers at Michigan State University showed that higher energy and protein intakes commonly associated with "accelerated" calf growth programs added to growth of mammary parenchymal tissue (the epithelial portion of the gland that will one day secrete milk). If subsequent milk production had been measured in those animals, there may have been higher milk production due to enhanced development of this tissue, but we have no way of knowing this. Nonetheless, studies such as this have painted a slightly different picture of the effects of nutrition on mammary growth and development in preweaned heifers. This period of growth seems to be a time when the udder is not adversely affected by level of nutrition. In fact, many signs actually seem to point toward a positive effect.
Possible reasons
The biological reasons for this still are largely unknown. Many in the field ponder whether these newer findings simply are associated with the attainment of "normal growth" in calves that are not limit-fed. An alternative is that the various nutrients fed to preweaned heifers fundamentally alter the growth trajectory of the udder in some way that may have lifetime consequences. To that end, various research groups recently have observed that the two main types of tissue in the udder respond differentially to diet in preweaned heifers.
The parenchyma, reproductive tissue really, appears to be less impacted by diet than the mammary fat pad (the supporting tissue in the udder that does not play a direct role in milk secretion). It is the fat pad that directly responds to elevated nutrient supply by growing in size and fat content. This tells us that, despite not being fully functional until roughly 24 months of age, the mammary glands of heifer calves are far from inactive. These findings call for further study into the role of nutrition on early-life mammary growth and development.
Difficult research
From a production standpoint, it is undesirable to study mammary gland development and composition at select time points without having corresponding milk yield data. Likewise, having only milk yield data without an idea of the influences on mammary development and composition is undesirable from a mammary biology standpoint. These challenges have permeated 100 years of study in this area.
While it is easy to measure such things as initial body weight and final body weight, it is difficult, if not impossible, to measure initial parenchyma and mammary fat pad weights and compare them to final weights as measures of mammary growth in the same animal. Finding a solution to these challenges and many others while addressing experimental pursuits is daunting but not impossible.
As mentioned, the ultimate goal for those in this field is to find a way to rear heifers as efficiently as possible without negative consequences for their milk yield potential in the herd. Scientists and industry collaborators could benefit mutually from combined efforts to finally get to the bottom of diet-induced changes in mammary growth and development. Over 100 years of uncertainty is long enough!
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110225_133
Rearing dairy heifers so that they become productive members of the herd is a goal that all dairy farmers strive to achieve. For more than a century, researchers have been investigating how nutritional management strategies can help farmers attain this goal.
In 1906, C. H. Eckles at the University of Missouri embarked upon what was to be an eight-year experiment centered on defining, if possible, practical ways for producing quality milk cows through heifer management strategies. The research he did was aimed at addressing the question of, "Does a good dairy cow or an inferior one derive her special dairy characteristics by inheritance, or is it a result of her treatment from birth to maturity?" Eckles' work was practical and focused on monitoring feed intake and overall body growth measures followed by milk production. The results that Eckles found hinted at the possibility that diet can impair milk production, but udders were not directly measured for effects on growth or tissue development.
Questions remained
It would take the passing of roughly 50 years before nutritional effects on udder tissue composition were considered as a possible explanation for the reduced milk yield observed in some instances. In the late 1970s/early 1980s, researchers designed multiple experiments that primarily used weaned heifers to examine the effects of nutrition on mammary development. Initial findings suggested that animals showed impaired mammary development when fed high-energy diets prior to the onset of puberty but not afterwards. Thus, researchers generally extended their conclusions to imply that the entire time from birth to puberty represented a time where the udder may be harmed by elevated daily body weight gains achieved through nutritional strategies. However, this is not necessarily true. It must be remembered that results should be interpreted within the context under which they were conducted. So, while initial conclusions likely were valid for weaned heifers, what about preweaned heifers? Might they respond differently to diet?
Going younger
About a decade ago, scientists revisited the study of nutritional effects on mammary development. This time they looked at preweaned heifers. The general consensus was that, at this stage of life, elevated nutrient intake actually may have positive effects on mammary development, much like what is seen with frame growth. As a result, the past decade has produced a great volume of experimental work focused on the nutritional management of preweaned heifers and measures of mammary growth and development, and, unfortunately to a lesser extent, subsequent milk production.
In the early 2000s, researchers at Michigan State University showed that higher energy and protein intakes commonly associated with "accelerated" calf growth programs added to growth of mammary parenchymal tissue (the epithelial portion of the gland that will one day secrete milk). If subsequent milk production had been measured in those animals, there may have been higher milk production due to enhanced development of this tissue, but we have no way of knowing this. Nonetheless, studies such as this have painted a slightly different picture of the effects of nutrition on mammary growth and development in preweaned heifers. This period of growth seems to be a time when the udder is not adversely affected by level of nutrition. In fact, many signs actually seem to point toward a positive effect.
Possible reasons
The biological reasons for this still are largely unknown. Many in the field ponder whether these newer findings simply are associated with the attainment of "normal growth" in calves that are not limit-fed. An alternative is that the various nutrients fed to preweaned heifers fundamentally alter the growth trajectory of the udder in some way that may have lifetime consequences. To that end, various research groups recently have observed that the two main types of tissue in the udder respond differentially to diet in preweaned heifers.
The parenchyma, reproductive tissue really, appears to be less impacted by diet than the mammary fat pad (the supporting tissue in the udder that does not play a direct role in milk secretion). It is the fat pad that directly responds to elevated nutrient supply by growing in size and fat content. This tells us that, despite not being fully functional until roughly 24 months of age, the mammary glands of heifer calves are far from inactive. These findings call for further study into the role of nutrition on early-life mammary growth and development.
Difficult research
From a production standpoint, it is undesirable to study mammary gland development and composition at select time points without having corresponding milk yield data. Likewise, having only milk yield data without an idea of the influences on mammary development and composition is undesirable from a mammary biology standpoint. These challenges have permeated 100 years of study in this area.
While it is easy to measure such things as initial body weight and final body weight, it is difficult, if not impossible, to measure initial parenchyma and mammary fat pad weights and compare them to final weights as measures of mammary growth in the same animal. Finding a solution to these challenges and many others while addressing experimental pursuits is daunting but not impossible.
As mentioned, the ultimate goal for those in this field is to find a way to rear heifers as efficiently as possible without negative consequences for their milk yield potential in the herd. Scientists and industry collaborators could benefit mutually from combined efforts to finally get to the bottom of diet-induced changes in mammary growth and development. Over 100 years of uncertainty is long enough!
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