Mammary development

The basics of mammary development during foetal and postnatal growth with the interrelationship of hormonal cues at puberty and gestation were considered as largely known in cattle. With recent findings contrasting the notion of postnatal mammary growth being largely isometric and showing that the allometric growth of the mammary gland also seems to depend on nutritional cues before weaning, new aspects not only for basic science but also for developing strategies for a targeted support of mammary development emerged. The most recent findings were summarised and provide an updated overview on mammary development in dairy calves (Geiger, Reference Geiger2019).

For a polytocous species, the pig, a comprehensive outline about mammary gland development is provided, starting with the very early embryonic stages when first primordia of the mammary glands appear over the following morphogenetic steps until birth and thereafter. In swine, postnatal mammary growth is isometric but turns allometric towards puberty. The importance of the hormonal cues related to puberty and later on in pregnancy is presented providing an in-depth insight in the respective functions of individual hormones, for example prolactin, until early lactation, when the secretory stage is reached (Hurley, Reference Hurley2019).

The importance of suckling in pigs is also reviewed starting from the cyclic changes of mammary development and involution during lactation and at weaning, towards the effects of teat use and duration of teat use for mammary development in the following lactation, and addressing also the importance of previous teat use and teat preference using intriguing experimental models (Farmer, Reference Farmer2019).

Regulation and manipulation of milk secretion at different stages of lactation

Most mammalian farm animal species are ungulates, and their newborn offspring depends on high-quality colostrum to ensure survival. New knowledge of colostrum formation is discussed by Quesnel and Farmer (Reference Quesnel and Farmer2019). During lactation, the function of the mammary gland in dairy ruminants is affected by several exogenous factors such as milking frequency and feed supply, and endogenous factors, for example, stage of lactation, or genetics. The cellular mechanisms mediating the reaction towards such factors comprise secretory activity, epigenetic marks and changes in cell numbers are reviewed contrasting also reversible versus irreversible alterations of milk production (Boutinaud et al., Reference Boutinaud, Hervé, Quesnel, Lollivier, Finot, Dessauge, Chanat, Lacasse, Charton and Guinard-Flament2019).

The manipulation of milk secretion goes new directions. Instead of pushing the mammary towards higher production by manipulating various endocrine systems, new approaches aim at a transient reduction of production in phases when the genetic drive for milk yield might be compromising animal welfare and health. The major stages of interest are the very first days of lactation to facilitate the adaptation to lactation after the dry period and also at dry off if cows produce still very high amounts of milk, and it is a special challenge to achieve a cessation of milk secretion. New results revising the role of prolactin for lactation in ruminants are reviewed focusing on the importance of prolactin inhibitors for transiently reducing milk yield (Lacasse et al., Reference Lacasse, Zhao, Vanacker and Boutinaud2019).

Alternatives to the 305-day standard lactation

The increase of milk yields achieved by genetic and, more recently, genomic selection, has also triggered considerable concerns about animal health in dairy cows. Besides the problems related to the negative energy balance in early lactation, high milk yields towards drying-off might impose compromised welfare and health. With this background, two new concepts emerged during the past decade for circumventing these problems: one takes variations in dry period length, ranging from complete omission of a dry period to differing lengths of the dry period, into consideration. The long-term consequences for metabolism and welfare are presented, and feeding strategies for dealing with the increased energy balance during the entire lactation period counteracting overconditioning are outlined. The individual reactions observed necessitate individualised dry periods depending on cows’ characteristics. Based on the literature available, such customised dry period lengths hold promise to reduce the metabolic stress and to improve health status and may also be beneficial for fertility (Kok et al., Reference Kok, Chen, Kemp and van Knegsel2019).

A second way, aiming to extend the duration of the lactation cycle, is based on postponing the time for insemination for inducing a new pregnancy well beyond the ‘normal’ time in order to achieve longer lactation periods. A comprehensive overview of the advantages and disadvantages of extended lactation periods is provided addressing also management and efficiency issues (Sehested et al., Reference Sehested, Gaillard, Lehmann, Maciel, Vestergaard, Weisbjerg, Mogensen, Larsen, Poulsen and Kristensen2019). Using model calculations, optimised lactation lengths depending on lactation number are presented. An individual selection of which cows are most suitable for extended lactation may further help implementing extended lactation strategies in praxis.

Old and new indicators in milk for health status and metabolic load

Milk samples are classically used for assessing udder health and diagnosing mastitis. Besides, the use of milk as sample matrix for also evaluating metabolic health gained increasing interest. Due to the ease of non-invasive sample collection, the possibility of implementing such analyses into routine control measures using mid-infrared as high throughput analyses is increasingly exploited. Focusing on ketosis as an important metabolic disease, the use of indicators, mainly ß-hydroxybutyrate in milk, is reviewed trading off the benefits and limitations of applying such measurements (Gross and Bruckmaier, Reference Gross and Bruckmaier2019).

With regard to mastitis, not only the advancement in proteomic but also metabolomics approaches in milk are compiled in a further review focusing on Streptococcus uberis infection as an experimental model. The identification of proteins and metabolites being up- or down-regulated in infected versus healthy animals is elucidating the defence reactions and extends our knowledge about factors that are relevant in this context (Eckersall, Reference Eckersall2019).

New developments in milking routines

Two review papers in the issue deal with milk removal in dairy cows (Odorcic et al., Reference Odorcic, Rasmussen, Paulrud and Bruckmaier2019), and in small dairy ruminants (Dzidic et al., Reference Dzidic, Rovai, Poulet, Leclerc and Marnet2019). A fast but gentle removal of milk by machine milking is aimed at in science and is increasingly implemented in milking routines on practical farm. For this, fast milking is combined with an early detachment of the teat cups in both conventional and robotic systems. This is a different approach compared to the past (partially still used) when the philosophy was a time-consuming complete emptying of the udder by the milking machine, often even followed by hand-stripping. This has been shown to cause a considerable mechanical load on the teat tissue, and leaving small amounts of milk in the udder seems to be advantageous compared to teat tissue damage and hyperkeratosis (Odorcic et al., Reference Odorcic, Rasmussen, Paulrud and Bruckmaier2019).

The Biology of Lactation in Farm Animals in all its complexity remains a central theme in animal and dairy science. The current collection of reviews provides the state-of-the-art knowledge as well as emerging concepts and is thus an indispensable update for both research and teaching.