Monensin supplementation during late gestation of beef cows alters maternal plasma concentrations of insulin-like growth factors 1 and 2 and enhances offspring preweaning growth.

This study evaluated the effects of maternal prepartum supplementation of dried distillers grains (), with or without monensin addition, on maternal performance and physiology and offspring preweaning growth. On day 0 (approximately 197 ± 4 d prepartum), 150 multiparous, Brangus crossbred beef cows were ranked by their initial body weight (; 524 ± 51 kg) and body condition score (; 5.0 ± 0.63), and then randomly assigned into one of 15 bahiagrass () pastures (10 cows and 8.1 ha/pasture). Maternal treatments were randomly assigned to pastures (5 pastures/treatment) and consisted of no prepartum supplementation of DDG () or supplementation of DDG at 1 kg/cow/d (dry matter basis; ) added with 0 mg () or 200 mg/d of monensin () from days 0 to 77. Effects of maternal treatment and maternal treatment × day of the study were not detected ( ≥ 0.63) for any forage data. Cow BCS on day 35 and near calving (day 77) did not differ ( ≥ 0.19) between SUP and SUPMO cows but both groups had greater ( ≤ 0.001) BCS compared with NOSUP cows. Cow BCS at the start of the breeding season (day 142) and on day 168 were the greatest ( < 0.0001) for SUPMO cows, least for NOSUP cows, and intermediate ( ≤ 0.02) for SUP cows. Maternal plasma concentrations of glucose did not differ ( ≥ 0.25) among treatments. Plasma concentrations of insulin-like growth factor 1 () on day 77 were the least for NOSUP cows ( ≤ 0.05) and did not differ ( = 0.66) between SUP and SUPMO cows, whereas plasma concentrations of IGF-2 on days 35 and 77 were greatest ( ≤ 0.05) for SUPMO cows and did not differ ( ≥ 0.60) between NOSUP and SUP cows. Birth BW of first offspring did not differ ( = 0.77) between SUP and SUPMO calves but NOSUP calves were lighter at birth ( ≤ 0.05) compared with SUP and SUPMO calves. Percentage of cows pregnant with a second offspring did not differ ( = 0.72) between SUP and SUPMO cows and were the least for NOSUP cows ( ≤ 0.05). First offspring BW at weaning (day 325) was greatest ( ≤ 0.05) for SUPMO calves, least for NOSUP calves, and intermediate for SUP calves. Therefore, adding monensin into prepartum DDG supplements for -influenced beef cows did not increase cow prepartum BCS but led to greatest offspring preweaning growth, likely by modulating maternal plasma concentrations of IGF-1 and IGF-2 during gestation.