TRIENNIAL LACTATION SYMPOSIUM/BOLFA: Mammary growth during pregnancy and lactation and its relationship with milk yield1

Abstract

The number of secretory cells in the mammary gland is often cited as a major determinant of milk production. However, literature data for proxy measures of secretory cell number do not fully support such a claim. In particular, measurements of total mammary DNA in livestock explain only <25% of variation in milk yield, probably because of tissue heterogeneity for DNA concentration. Relative to BW, measurements of udder size in dairy cattle, as total DNA or udder weight, are approximately double those seen in most other livestock classes. Therefore, selection for dairy production, not surprisingly, has resulted in cows with greater secretory capacity. There is limited evidence that genetic selection is still increasing udder size in some cattle populations, but more recent data are needed. It is contended that the most important period of mammary growth for determination of milk yield is that occurring in pregnancy and early lactation. Mammary development is largely complete, at term, in sheep, goats, and cattle, but in pigs, the udder continues to grow during the first 3 wk of lactation, depending, in part, on litter size. Increased litter size in sheep and goats will enhance the extent of mammary development at the end of gestation (and milk yield) by 20 to 25%. However, twinning in dairy cattle does not affect milk production and, by inference only, is not likely to affect numbers of secretory cells at term. Milking frequency and suckling intensity in very early lactation will increase milk yield in cows and increase mammogenesis and milk yield in sheep, indicating that even at term, the ruminant gland retains some capacity for further development, if demand requires it. There is limited understanding of the hormonal signals in pregnancy that regulate mammary development relative to the number of young carried. Furthermore, the genetic differences between dairy and beef cattle that lead to substantially greater udder size in the dairy breeds have not been identified. During lactation, the drivers for secretory cell loss in relation to milking frequency and nutritional status are still not known. Measurement of mammary development and using this phenotype in genomewide association studies to identify key genetic variants for mammogenesis will provide knowledge that is fundamental to understanding the quantitative regulation of milk production.

Maternal genes and pathways affecting birth weight and weaning weight traits in sheep: A GWAS and pathway enrichment analysis

“…Most mammary growth takes place through pregnancy. Mammary gland cell proliferation and differentiation have a great impact on milk yield and lactation persistency [53]. As expected, must identified genes and GO terms have a role in milk production or mammary gland development which means feeding lambs by milk have the most impact on weight gain rather than other maternal effects.…”

Association between gestation length and lactation performance, lactation curve, calf birth weight and dystocia in Holstein dairy cows in Iran

“…However, Jenkins et al (2016) reported that cows with either short or long GL produced less milk, fat, and protein than those with an average GL. The association between GL and lactation performance can be partly explained by this fact that the greatest increase in the mass of parenchymal tissue occurs in late pregnancy (Davis, 2017); therefore, shorter the GL, less the mammary cells, and subsequently less the milk yield. In addition, it may be concluded that multiparous cows with a longer GL would have a longer dry period and more time for udder tissue recovery which can result in a higher partial and 305-d lactation performance (Norman et al, 2011;Atashi et al, 2013).…”

Invited review: Metabolic challenges and adaptation during different functional stages of the mammary gland in dairy cows: Perspectives for sustainable milk production

“…During lactation, no further mammogenesis that might modulate milk production occurs in bovine species (Akers, 2017a,b;Davis, 2017). Although rearing in early life significantly affects growth and later performance (Kaufhold et al, 2000), adult cows may adapt differentially to successive lactations, with a clear effect of the dry period between (Gabai, 2003;Chen et al, 2016a,b).…”