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 Age in months At Birth Holstein Inches Pounds 31 96 Ayrshire Inches Guernsey Pounds Inches Pounds Jersey Inches Pounds 29 5 72 29 66 24 5 56 98 31 5 90 29 5 72 1 33 5 118 32 3 40 25 213 38 75 179 38 164 35 25 138 5 47 335 45 5 291 44 25 41 5 228 7 52 5 455 51 25 408 49 75 26 5 362 47 25 325 9 57 559 55 485 53 75 51 75 409 11 60 5 658 58 563 56 75 44 8 521 55 481 13 63 25 740 60 75 630 59 25 587 57 25 540 15 65 25 805 63 703 585 67 25 874 65 25 758 64 0 696 59 17 61 7 5 63 5 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 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 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