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United States Department of Agriculture

Agricultural Research Service

Research Project: Alleviating Rate Limiting Factors that Compromise Beef Production Efficiency

Location: Livestock and Range Research Laboratory (LARRL)

Title: Impact of number of estrous cycles exhibited prior to start of breeding on reproductive performance in beef heifers

Authors
item ROBERTS, ANDREW
item Ketchum, J -
item Funston, R -
item GEARY, THOMAS

Submitted to: Western Section of Animal Science Proceedings
Publication Type: Proceedings
Publication Acceptance Date: May 15, 2013
Publication Date: June 20, 2013
Citation: Roberts, A.J., Ketchum, J., Funston, R.N., Geary, T.W. 2013. Impact of number of estrous cycles exhibited prior to start of breeding on reproductive performance in beef heifers. Western Section of Animal Science Proceedings. 64:254-257.

Interpretive Summary: Obtaining high pregnancy rates in replacement females is a common goal among beef cattle producers. One industry guideline recommends heifers should be managed so they achieve puberty in sufficient time to experience multiple estrous cycles before start of breeding. Research that led to this recommendation was conducted over 25 years ago, and substantial genetic change has occurred in the cattle population since that time. Objective of this research was to provide a thorough evaluation of whether number of estrous cycles exhibited before start of breeding affects pregnancy rates of replacement beef heifers. A total of 1,176 heifers were evaluated over a 9-yr period. Heifers were categorized into one of five groups (0, 1, 2, 3, or more than 3 estrous cycles) based on the number of estrous cycles they exhibited (21-d intervals) before start of breeding. Heifers in the 0 group were non-pubertal at start of breeding. These heifers were lighter and younger compared to heifers in the other groups. Heifers that were non-pubertal before start of breeding had lower pregnancy rates (84 ± 2%) than heifers exhibiting 1 (90 ± 2%) or more than 3 (94 ± 2 %) estrous cycles before start of breeding, but only tended to have lower rates than heifers having 2 (88 ± 2%) and 3 (89 ± 3%) estrous cycle before start of breeding. Heifers that were non-pubertal at start of breeding had 3- to 5-d longer interval (300 ± 1 d) from start of breeding to calving than other groups. Rebreeding pregnancy rates of 2-yr old cows (n = 781) were lowest for heifers from the 0 estrous cycle group (62 ± 3%), followed by heifers from the 1 (71 ± 4%) and 2 (73 ± 4 %) estrous cycle categories, which were less than heifers from the 3 (83 ± 5%) and more than 3 (86 ± 4%) estrous cycle categories. Attainment of puberty before start of breeding was associated with a modest increase in pregnancy rate, with little or no benefit from having more than one estrous cycle expressed before start of breeding. Thus, these results do not support the recommendation to manage heifers to allow 2 or more estrous cycles before start of breeding. In contrast, second season pregnancy rate was positively associated with number of estrous cycles expressed before start of first breeding season. These findings support the need for additional research to substantiate the potential impacts that timing of puberty before breeding may have on subsequent pregnancy rates the next year and investigate potential intervention strategies.

Technical Abstract: Objective of this research was to evaluate effect of number of estrous cycles exhibited prior to breeding on reproductive performance of replacement beef heifers. A total of 1,176 composite heifers (½ Red Angus, ¼ Charolais, ¼ Tarentaise) were evaluated over a 9-yr period. Circulating concentrations of progesterone measured in blood samples collected at 9- to 11- d intervals, beginning at an average age of 331 d, were used to estimate date of first estrus (estrus = 6 d prior to date of first sample > 1 ng/mL). Heifers were categorized into one of five groups by number of estrous cycles exhibited (21-d intervals) before start of breeding; 0 (non-pubertal; n = 395), 1 (n = 205), 2 (n = 211), 3 (n = 116), or > 3 (n = 249). Mean BW and age at start of breeding differed (P < 0.001) by categorization; non-pubertal heifers were 5 d younger (420 ± 1 d old) and 15 kg less BW (304 ± 2 kg) compared to other groups. Pregnancy rates differed (P = 0.007) by category; heifers that were non-pubertal before start of breeding had lower pregnancy rates (84 ± 2%; P < 0.05) than heifers exhibiting 1 (90 ± 2%) or > 3 (94 ± 2 %) estrous cycles before start of breeding, but only tended to differ (P < 0.15) from the 2 (88 ± 2%) and 3 (89 ± 3%) estrous cycle groups. Heifers that were non-pubertal at start of breeding had 3- to 5-d longer (P < 0.05) interval (300 ± 1 d) from start of breeding to calving than other groups. Rebreeding pregnancy rates of 2-yr old cows (n = 781) were lowest (P < 0.05) for heifers from the 0 estrous cycle group (62 ± 3%), followed by 1 (71 ± 4%) and 2 (73 ± 4 %) estrous cycle categories, which were less (P < 0.05) than 3 (83 ± 5%) and > 3 (86 ± 4%) estrous cycle categories. Attainment of puberty before start of breeding was associated with increased heifer pregnancy rate, with little or no benefit from having more than one estrous cycle prior to start of breeding. In contrast, second season pregnancy rate was positively associated with number of estrous cycles expressed prior to start of first breeding season.

Last Modified: 7/28/2014
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