318 The Effect of Developing Heifers to a Lower Percentage of Mature Body Weight at Breeding on Pregnancy Rates and Longevity Within the Herd

Abstract

Abstract Traditional recommendations suggest that a heifer should be at 65% of her mature body weight (BW) at the time of breeding to obtain optimal production efficiency and achieve the greatest pregnancy rates. The objective of this study was to determine how developing heifers at a decreased percentage of mature BW at breeding impacts heifer performance, stayability, and calf production. Data were collected from cows and heifers (n = 1,434) from the March- and May-calving herds at the University of Nebraska Gudmundsen Sandhills Laboratory (GSL) located near Whitman, NE from 2005 through 2019. Body weights were collected from heifers and cows, percentage of mature BW of heifers was calculated using heifer breeding weight and the average MBW of cows at 5, 6, and 7 years of age. A regression analysis was conducted on percentage of estimated mature BW at breeding in heifers from 50 to 70 % of mature BW. Heifers that were 60% of mature BW at the time of breeding or greater had increased (P < 0.01) pregnancy rates compared with lighter heifers. In contrast, a greater percentage (P = 0.05) of heifers at a mature BW of 60% or less at breeding calved during the first 21 d of the calving season. Calf BW at birth was influenced (P < 0.001) by percent of mature BW at breeding with calves from greater BW heifers being heavier. Subsequent pregnancy rate as a 2-yr old was influenced (P < 0.001) by percentage of mature BW at breeding as a heifer with greater percent mature BW at breeding having greater pregnancy rates than heifers with decreased percent mature BW. However, pregnancy rate as a 3-, 4-, and 5-yr-old were not influenced (P> 0.15) by percentage of mature BW at breeding as a heifer. Overall, greater percentage of mature BW at the time of breeding increased overall pregnancy rate; however, heifers at a decreased percentage of mature BW calved at a greater percentage during the first 21-d of the subsequent calving season. In addition, longevity in the cowherd was not influenced by heifer BW at the time of breeding.