lecture 12

• milk yield is a reflex of: number of secretory cells and mean cell secretory rate (activity)
• factors affecting milk yield affect either the cell number or activity
• factors affecting milk yield: nutrition status, genetics, disease incidence, days in milk, milking frequency, breed, photoperiod, parity, dry period, and heat stress
• DNA in a cell is relatively constant (6.6 picograms/diploid cell); by quantifying total DNA, one can infer the number of cells present by dividing the total amount of DNA by DNA content per cell
• if approximate mammary cell number is the same, differences in milk yield would only be influenced by cell activity
• lactation curve decline is due to decrease in cell number or activity
• mammary cell number example: objective was to determine whether the decline in milk production during advancing lactation is due to a decline in cell number or cell activity in 20 lactating non-pregnant Holstein cows
• in the mammary cell number example, cows were euthanized at 14, 90, 120, and 240 days in lactation; data showed that milk production was greatest at 90 days, total DNA and epithelial DNA decreased over time
• mammary cell number example: showed decreased milk yield was associated with decreased mammary epithelial DNA
• mammary cell number example: primary factor to decreased milk yield in lactation is decreased mammary cell number because cell apoptosis occurs at a faster rate than cell proliferation so there is a small net loss as lactation progresses
• mammary cell number example: cows are not pregnant, progesterone presence
• mammary DNA increases with each parity
• mammary cell activity example: studied 26 multiparous Holstein cows; 13 dried off for 60 days, 13 continuously milked
• mammary cell activity example: mammary tissue samples were collected at -53, -35, -20, and -7 days relative to expected parturition; looked at total mammary DNA and 3H thymidine (mammary proliferation)
• mammary cell activity example: showed total mammary DNA did not differ between dry and lactating cows; the total number of cells did not differ between cows that had to those that did not have a dry period
• mammary cell activity example: 3H thymidine incorporation was greater in cows allowed to have a dry period, meaning more cell turnover; enhanced mammary epithelial cell turnover during the dry period may allow for greater cell activity and increased milk production
• mammary cell activity example: dry period allows old cells to be replaced with cells with higher activity rate
• the more frequently cows are milked, the more milk is produced; from 1 to 2 times (at least 40% more milk), from 2 to 3 times (10-20%), and 3 to 4 times (5-10%)
• increasing milking frequency from 3 to 4 times a day only increases milk production 5-10%, but it is not always profitable bc requires more labor costs (not enough to justify extra milking)
• speculation on physiological factors for milking frequency: decreased intramammary pressure generated with frequent milking allows for more milk production, increased stimulation of hormone activity favorable for milk production (galactopoietic hormones: growth hormone and T3), less negative feedback on the secretory cell synthetic machinery due to the buildup of milk components
• there is a negative relationship between pressure and milk secretion rate
• after 35 hours of no milking, no extra milk is produced
• milk yield remains elevated for a period after treatment; for 25 days after equal treatment, milk production still remained higher in cows previously milked 4x vs 2x a day
• increased milking frequency did not alter mammary cell population dynamics (proliferation and apoptosis)
• increased milking frequency: mitochondria count per cell was higher if milked more often
• increased milking frequency from 1x to 4x: increased milk production, increased mitochondria number in secretory epithelial cells, and did not affect programmed cell death
• photoperiod: amount of hours an animal is exposed to light in a 24hr period
• long-day photoperiod: 18 hours of light/day
• short-day photoperiod: 8 hours of light/day
• long day photoperiod during lactation increases milk production
• short day photoperiod during dry period increases milk production during lactation
• photoperiod allows milk production to be increased without changing diet or genetics
• milk yield increases always with light despite of location; may or may not increase dry matter intake
• long-day photoperiod: increases production by 2.5 kg per cow per day; milk composition is generally unaffected; DMI shown to increase in longer term studies; endocrine mechanisms not well understood
• photoperiod effects during lactation: increase in light decreases melatonin synthesis, increases IGF-1 and increases milk synthesis
• short-day photoperiod during 60 day dry period increases milk yield by 3.5 kg/day compared to long day photoperiod
• photoperiod during the dry period is easier to implement because it is only 60 days vs 305 days for lactation
• breed (genetics): describes why beef and dairy breeds produce different amounts of milk bc they are selected to do so
• breed study: compared mammary tissue in 10 Herford (beef) and 10 Holstein (dairy) heifers; animals were euthanized at 150, 180, and 260 days of gestation and at 49 days post calving
• breed: Holstein (dairy) cows had more mammary cells (DNA) per grams compared to Herford; dairy cows had more fully differentiated cells; beef cows had more poorly differentiated cells