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 ( DDG ), 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 ( BW ; 524 ± 51 kg) and body condition score ( BCS ; 5.0 ± 0.63), and then randomly assigned into one of 15 bahiagrass ( Paspalum notatum ) 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 ( NOSUP ) or supplementation of DDG at 1 kg/cow/d (dry matter basis; DM ) added with 0 mg ( SUP ) or 200 mg/d of monensin ( SUPMO ) from days 0 to 77. Effects of maternal treatment and maternal treatment × day of the study were not detected ( P  ≥ 0.63) for any forage data. Cow BCS on day 35 and near calving (day 77) did not differ ( P  ≥ 0.19) between SUP and SUPMO cows but both groups had greater ( P  ≤ 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 ( P  < 0.0001) for SUPMO cows, least for NOSUP cows, and intermediate ( P  ≤ 0.02) for SUP cows. Maternal plasma concentrations of glucose did not differ ( P  ≥ 0.25) among treatments. Plasma concentrations of insulin-like growth factor 1 ( IGF-1 ) on day 77 were the least for NOSUP cows ( P  ≤ 0.05) and did not differ ( P  = 0.66) between SUP and SUPMO cows, whereas plasma concentrations of IGF-2 on days 35 and 77 were greatest ( P  ≤ 0.05) for SUPMO cows and did not differ ( P  ≥ 0.60) between NOSUP and SUP cows. Birth BW of first offspring did not differ ( P  = 0.77) between SUP and SUPMO calves but NOSUP calves were lighter at birth ( P  ≤ 0.05) compared with SUP and SUPMO calves. Percentage of cows pregnant with a second offspring did not differ ( P  = 0.72) between SUP and SUPMO cows and were the least for NOSUP cows ( P  ≤ 0.05). First offspring BW at weaning (day 325) was greatest ( P  ≤ 0.05) for SUPMO calves, least for NOSUP calves, and intermediate for SUP calves. Therefore, adding monensin into prepartum DDG supplements for Bos indicus -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.