Criteria for weaning Dry feed consumption Size – ~ 150 lb General health Age – usually 6-8 wk

Heifer-raising industry ~ __ million cows in US Average culling rate ___% Replacement heifers rank ___ in costs of milk production costs of the dairy enterprise

OPEN HEIFERS Open Heifer - Young female that has not been bred After weaning, calf should gain about 1.75 lb per day Reach puberty about 12 mos Heifers should be grouped by size and age Holstein heifers should weigh 800 – 850 lb by 15 mos of age

Normal Heart-Girth Measurement and Weight of Calves and Heifers During Growing Period Holstein Ayrshire Guernsey Jersey Age in months Inches Pounds Inches Pounds Inches Pounds Inches Pounds At Birth 31 96 29.5 72 29 66 24.5 56 1 33.5 118 32 98 31.5 90 29.5 72 3 40.25 213 38.75 179 38 164 35.25 138 5 47 335 45.5 291 44.25 265 41.5 228 7 52.5 455 51.25 408 49.75 362 47.25 325 9 57 559 55 485 53.75 448 51.75 409 11 60.5 658 58 563 56.75 521 55 481 13 63.25 740 60.75 630 59.25 587 57.25 540 15 65.25 805 63 703 61.75 640 59 585 17 67.25 874 65.25 758 63.5 696 60.5 635 19 69.25 946 66.5 813 65.5 752 62.5 687 21 71.5 1025 68.5 885 67.5 816 64 740

OPEN HEIFERS Housing Health Normally require protection from wind, wet weather, but adequate ventilation Health Vaccinations at 4 to 8 mos of age Stomach magnet Ionophores/Coccidiostats

HEIFERS Breeding Estrous cycle -- 18-22 (21) d in length Estrus -- 16 h in length Restless, more alert Red and swollen vulva Increased mobility, tries to mount other cows Stands to be mounted by other cows Ovulation occurs 12 - 18 h after end of estrus Pregnancy checked >35 d after breeding

Goals for heifer program Body weight after calving: 1200-1250 lb Age at first calving: 22-24 mo Height at calving: 55-57 in Body condition score at calving: ~ 3.5 Average gain for heifer: 1.8 - 2.0 lb/day

Development of the mammary gland 10 mo. 3 mo. The parenchymal tissue formed before puberty is the foundation for later growth. Milk production is partly dictated by the number of parenchymal cells at puberty.

Mammary glands Number and location Inguinal region--cows, goats, sheep, horses Abdominal – sows + correlation between # of glands, offspring

Lifetime of the mammary gland Periods of growth and regression related to reproductive cycle Mammogenesis Lactogenesis Lactation Involution

MG Structure - external Number of glands, teats Cow – four glands, with four teats Sheep, goats – two glands, with two teats Horse – four glands, with two teats Udder weight (cow) 30 – 80 lbs Skin (protection, little support) Teats – exit for secretion 25 – 50% cows have supernumerary teats

MG structure - internal Suspensory ligaments Blood flow Mammary tissue insensitive to heat or touch

Structures of the Mammary Gland

Lactation Success determined by ~ 305 d or longer in dairy cows Maintenance of cell #’s Secretory activity Milk removal ~ 305 d or longer in dairy cows

Milk production Increases to peak ~ 8 weeks after calving ~60% of milk produced by 2 rear quarters of dairy cow Increased by administration of growth hormone, or bST