Influence of corn supplementation to beef cows during mid- to late-gestation: Supplementation decreases placental microvascular surface density but does not alter uterine blood flow or neonatal performance1*

Gestating cows fed low-quality forage diets are often at risk of nutrient restriction, including energy, protein, or both. Thus, the objective of this study was to evaluate the effects of supplementing corn to gestating beef cows fed low-quality forage by tracking uterine and mammary hemodynamics and neonatal performance. We hypothesized that mid- to late-gestating beef cows receiving corn supplementation would have enhanced uterine and mammary hemodynamics and increased placental microvascularity. At d 100 of gestation, multiparous cows, predominantly of Angus breeding, and carrying male calves were assigned randomly to the following treatments: supplementation (SUP; n = 24) at the level of 0.2% (dry matter basis) of body weight (BW) or basal diet without supplement (CON; n = 23). All cows were fed the same basal diet (60% hay, 30% wheat straw, and 10% concentrated separator by-product). Intake was monitored individually with Insentec feeders from d 100 of gestation through calving. Every 28 days from the beginning of supplementation at d 100 until d 240 of pregnancy, uterine and mammary hemodynamics were monitored using Doppler ultrasonography, and jugular blood samples were obtained to determine hormone concentrations. At birth (CON: 277.6 +/- 3.1 days; SUP: 275.5 +/- 2.8 days; mean +/- stdev), cow and calf BW, colostrum samples, and placental tissues were collected. Representative cotyledons were excised from fetal membranes and histological image analysis of placental vascularity with hematoxylin and eosin staining was conducted. All measurements and data collected were analyzed with the MIXED procedure and means were separated using the LSMEANS statement in SAS. Hemodynamics, placental weight, and colostrum variables were not altered by supplementation (P >= 0.13). Placental microvascular surface density was suppressed (P < 0.01) by maternal corn supplementation. The differences in nutrient intake were not great enough to elicit differences in uterine or mammary hemodynamics, even though placental vascular surface density was decreased by corn supplementation in multiparous beef cows. Moreover, the few alterations in the maternal endocrine profiles due to corn supplementation did not translate into negative endocrine or early developmental changes in the offspring. Corn may be a good substitute for hay when costs and availability are economically advantageous to the producer.