Examination of weekly mammary parenchymal area by ultrasound, mammary mass, and composition in Holstein heifers reared on 1 of 3 diets from birth to 2 months of age

Abstract

Monitoring in vivo growth of mammary parenchyma (PAR) has historically been difficult, necessitating slaughter studies to measure PAR quantity. Advances in ultrasound (US) technology warrant revisiting its use as a noninvasive tool to monitor PAR growth in vivo. The level of nutrient intake during the first 2mo of life may affect measures of mammary growth and composition. Objectives were to examine the utility of US as an in vivo tool to quantify PAR cross-sectional area in Holstein heifers reared on 1 of 3 diets from birth to 2mo of age, assessing potential dietary effects; assess the relationships between weekly US measurements, teat length, manual palpation of PAR scores, and PAR mass at 2mo of age; and examine mammary composition in experimental animals. Holstein heifers (n=24; 41±1kg of initial body weight) from a single farm were randomly assigned to 1 of 3 milk replacers that differed in source and amount of fat. Milk replacer was fed at 660g of dry matter/d until weaning at 42 d. Heifers had ad libitum access to a common calf starter (20% crude protein) and water for the duration of the 56-d trial. Teat length and palpation scores were obtained weekly. A real-time B-mode US with a 7.5-MHz convex probe was used to examine 2-dimensional PAR area in all 4 glands of heifers once weekly from 2 to 3 d of age to harvest at 56 d. The left front and left rear glands were also examined by US 24h postharvest to validate final US measurements, and then bisected to produce a sagittal plane view of PAR for comparison with US images. Mass and composition of mammary gland tissue were determined at 8 wk using standard methodology. Over the course of this 8-wk trial, average teat length increased from 11 to 17mm. The PAR area started small (6.6±3.2mm(2) per gland) and increased to 42.1±2.5mm(2) per gland by the end of the trial. As anticipated, based on measurements obtained at slaughter, US measurements were more related to amount of PAR (r=0.74) than either teat length (r=0.34) or palpation scoring (r=0.63). Importantly, US is quantitative, whereas palpation scoring is subjective. Diet did not affect mass or composition of PAR in young heifers; total udder PAR mass averaged 1.40±0.80g. In conclusion, we showed that in heifers younger than 2mo of age, obtaining weekly PAR measurements via ultrasound is an effective quantitative tool for measuring changes in PAR area in vivo. Future studies may incorporate and expand upon the methods developed here to determine what quantitative evaluation of PAR in young heifers can reveal about milk production capacity.

Feeding a higher plane of nutrition and providing exogenous estrogen increases mammary gland development in Holstein heifer calves

“…A dual-energy x-ray absorptiometry (DXA) machine was available for use during this trial. Due to increased interest in gauging mammary gland development noninvasively (Esselburn et al, 2015), we used a Lunar Prodigy Advance DXA machine (PA+130744, GE Healthcare, Little Chalfont, UK) to assess mammary tissue composition differences after the animals were killed. Scanning of each udder half lasted approximately 4 min.…”

“…In their recent report, Esselburn et al (2015) describe some of the history of the use of udder measurements and palpation to evaluate mammary development in heifer calves and demonstrate the utility of ultrasound for evaluating udder and mammary development in young heifers. We took advantage of available DXA scanning equipment to determine if this tool could be used to evaluate udder development in these heifers.…”

“…Consensus was that mammary parenchyma (PAR) growth was minimal during the preweaning period, until the onset of allometric mammary growth before puberty (Sinha and Tucker, 1969), but recent data have shown that allometric growth of the mammary PAR begins much earlier than once believed. Indeed, PAR mass can increase 60-fold from 30 d until approximately 90 d of life (Capuco and Akers, 2010;Esselburn et al, 2015).…”

Technical note: Mammary gland ultrasonography to evaluate mammary parenchymal composition in prepubertal heifers

“…Mammary glands consist of 2 major types of tissues that serve different functions. As summarized by Esselburn et al (2015), these are (1) epithelial tissue and surrounding stromal elements, collectively called mammary parenchyma (PAR), and (2) stromal tissue that lies adjacent to PAR and does not contain epithelial structures, and instead mainly consists of connective tissue structures such as adipose. For this reason, mammary stroma is often referred to as mammary fat pad.…”

“…Understanding of growth dynamics of these mammary tissues as well as the influence of nutrition on mammary gland development has been the subject of study over the years (Sejrsen et al, 1983;Brown et al, 2005;Meyer et al, 2006). Esselburn et al (2015) recently showed that in heifers younger than 2 mo of age, obtaining weekly PAR measurements via ultrasound was an effective quantitative tool for measuring changes in PAR area in vivo. Although estimation of PAR composition via ultrasonography was not an experimental objective of Esselburn et al (2015), those authors noted that such estimation is feasible.…”