Effects of genetic selection for milk production on energy status and reproductive efficiency of high- and average-producing dairy cows
Two lines of Holstein cows resulting from a 20-year genetic selection project that used high and average sires chosen only for predicted differences in milk were used (high n = 10, average n = 10). Cows were on experiment from parturition (d 1) until d 75. Blood samples were obtained via jugular venipuncture daily for the first 2 wk, then 3 x per wk thereafter. Cows were milked and observed for estrous activity twice daily. Rectal palpation of the uterus and ovaries were conducted twice weekly, and cows were bred on any visual estrus after 50 d postpartum. Cows were fed a total mixed ration with feed intakes being determined daily and cows being weighed 3 x per wk. Biopsies of liver tissue were obtained on d 5, 15, and 30. Milk production on a 305-2X-M.E. basis was 10,814 kg and 6,912 kg for the high and average groups, respectively. No differences were observed in the interval from parturition to uterine involution, or first ovulation, or in the length of the first luteal phase between the two groups. Days to first visual estrus and number of ovulations before first visual estrus was greater (P <.05) for the high versus the average group (66 d vs. 43 d and 1.6 vs..7), respectively. Energy balance (Mcal/d) was less positive (P <.05) for the high versus the average group during wks 1, 2, 10, and 11. Feed intake as a percent of body weight was greater for the high versus the average group during wks 6, 7, and 10. Body weight loss from calving was greater (P <.05) for the high versus the average group during wks 3, 4, 5, and 11. Gross efficiency (4% fat corrected milk/dry matter intake) was greater for the high versus the average group throughout the study (P <.05). Glucose concentration was lowest during wk 2 for both groups, and increased throughout the remainder of the experiment. Non-esterified fatty acids and beta-hydroxybutyrate were greatest for both groups during wk 1 and 2, respectively, and decreased linearly thereafter. Liver triglyceride content was greatest and liver glycogen content was lowest at d 15 postpartum for all cows. Liver triglyceride content was not associated with any reproductive measurements. Thus, these data support the concept that high milk production is involved in suppression of estrous behavior. Energy status data suggests that the high group was more metabolically challenged during early lactation.