Stage of lactation has a pronounced effect on composition of milk, especially in the bulk milk from compact-calved herd that graze on pasture. Countries such as New Zealand,
Ireland and some parts of UK where the climate is temperate and abundancy of grass in spring-summer-autumn, dairy herds are usually compact calved between late-winter and early spring. In temperate countries, where grass growth is sufficient from spring- autumn, grass is utilised as a feed to be cost-effective. The production of milk is from February to November or December, with increasing milk production until May-June and decreasing steadily thereafter until end of lactation (O’Brien & Guinee, 2011; O’Brien et al., 1999b). Stage of lactation can have a significant impact on the
composition of herd bulk milk obtained from compact calved herd containing predominantly either spring or autumn calved milk proportions. Thus large variations in milk composition are expected throughout the year (White & Davies, 1958). On the other hand, the herd management systems that utilise year-round calving, in countries such as Netherlands, USA and most parts of Europe, enable milk production throughout the year and the effect of stage of lactation is diminished. Lactational changes in milk composition occur as a result of physiological changes that occur in mammary glands of healthy cows fed on good quality diets, whereas, seasonal changes in milk are referred as to those arising due to lactation and the overlaid effects of other environmental factors such as climate. The effect of stage of lactation and/or season on milk composition and its physico-chemical and processing characteristics has been widely studied. These studies were conducted in different countries around the world such as Ireland (Kelly et al., 1982; Keogh et al., 1982; O’Keeffe et al., 1982; Phelan et al., 1982; O’Brien et al., 1999 a, b; Mehra et al., 1999), New Zealand (Auldist et al. 2000a),
United Kingdom (Chen et al., 2014), Netherlands (Heck et al., 2009), Sweden (Lindmark-Månsson et al., 2003) and Italy (Bernabucci et al., 2015).
1.3.2.1 Effect of stage of lactation or season on milk composition
Total protein content and casein concentration, which are important factors in determining the nutritional value of milk and in evaluating how milk is processed vary throughout lactation. For a spring or an autumn calved herd, total solids, protein, casein and fat concentrations increase while concentration of lactose decreases with lactation up to 280 days in milk; these changes are concurrent with the decrease in milk yield over lactation (O’Brien et al., 1999b; Auldist et al., 1998, Guinee et al., 2006, Hickey et al., 2006, O’Callaghan et al., 2016b). Increase in milk proteins and fat concentrations
during lactation can be related to the reduced milk volume in late-lactation and the concentrating effect of milk constituents in the udder (Auldist et al., 1998). In contrast, the reduced concentration of lactose in late-lactation coincides with the increase in the concentrations of sodium and chloride (Auldist et al., 1998), to maintain the osmotic pressure of milk similar to that of blood. Non-protein nitrogen (NPN), as a proportion of total N, and urea are also known in increase during late-lactation (Phelan et al. 1982; Mehra, et al., 1999; Auldist et al., 1998). NPN is particularly important as it does not partake in cheese-making process, thus may reducing the cheese-yielding efficiency. The effect of stage of lactation on major milk component (protein, fat, lactose) produced from spring-calved herd is shown in Fig. 1.4.
Figure 1.4: Effect of stage of lactation in milk protein (●), fat (○) and lactose (△) of Irish whole milk
from spring-calved herds during the period mid-March to early November 2015 (Adapted from Soodam & Guinee, 2018).
Comparatively little has been published on variations in serum nitrogen fractions, proportion of individual caseins and casein micelle characteristics (such as casein micelle size and casein hydration) due to lactation. In Irish manufacturing milk (predominantly from bulked spring-calved herd milk), it reported that the proportion of αs-casein decreased progressively throughout the year, while that of β-casein decreased from July onwards; the concentration of γ-caseins showed an opposite trend to that of β-
casein and the levels of -casein remained constant throughout the year (Donnelly & Barry 1983). A different trend was observed for milk from herds comprised of spring- and autumn- calving cows which had a higher proportion of αs1-casein than milk from
Spring, Summer or Winter, and a lower proportion of -casein than Spring milk (Lin et al., 2017). Alternatively, Heck et al. (2009) found that season had no effect on Dutch bovine casein proportions over the year.
Mineral concentration in milk have been found to be affected by the stage of lactation and season throughout the year. Elements such as Ca and P which are important nutritionally and functionally range from 111-147 and 55-112.3 mg/100g, respectively, over the lactation stage from spring-calved herd milk (Keogh et al., 1982; Auldist et al., 2004). Seasonal variation in other macro (e.g., Mg, Na) and trace- elements (e.g., Zn, Fe, Cu, Mo, Se, Mn) of bulk herd milk has also been reported (White & Davies, 1958; Moreno-Rojas et al. 1993; Moreno-Rojas et al. 1994; Pechova et al., 2008; Nantapo & Muchenje 2013).
Little or no lactational or seasonal effects have been observed in the milk for casein micelles size, casein hydration or ionic calcium by various studies (White and Davies, 1958; Keogh et al., 1982; Grimely et al., 2009; Glantz et al., 2010; Chen et al., 2014). In contrast, there was a pronounced seasonal trend in casein micelle size of milk
from Ayrshire cows, with smaller micelles in summer as compared to winter (Holt & Baird, 1978).