32 What determines placental size in beef cattle? The consideration of maternal and fetal factors

Abstract

Abstract Expelled placentas and calf size at birth were collected from crossbred beef cows during Spring 2017 (n = 27; age: 2-7 yr), Fall 2017 (n = 24; age: 3-6 yr), and Fall 2018 (n = 22; age: 2-4 yr) to determine the relationships of placental characteristics with cow parameters, calf sex, and fetal growth. Placentas deemed complete were dissected (cotyledonary vs. intercotyledonary) and dry weights determined. Mixed linear models were used to individually determine main effects of peripartum BCS category (< 5, =5, or ≥6), cow age category (2, 3-4, or 5-7 yr), and calf sex on placental measures. Study was included in all models. Correlations were determined for placental characteristics with pre-calving cow BW and calf size. Cow BCS and calf sex had no effect (P ≥ 0.25) on placental characteristics. Primiparous cows had less (P ≤ 0.02) total placental and average cotyledon weights compared with young cows. Cow BW and calf BW had moderate positive correlations (P < 0.001) with intercotyledonary and total placental weights and weak positive correlations (P ≤ 0.01) with cotyledonary weight. Cow BW had a weak negative correlation (P = 0.05) with placental efficiency [calf BW/placental weight]. Average cotyledon weight had a moderate positive correlation (P < 0.001) with cow BW and a weak positive correlation (P = 0.001) with calf BW. Shoulder to tailhead length had a weak positive correlation (P = 0.001) with placental efficiency. Heart girth had moderate positive correlations (P < 0.001) and abdominal girth had weak positive correlations (P ≤ 0.04) with cotyledonary, intercotyledonary, total placental, and average cotyledon weights. Ponderal index [calf BW/length3] had a weak positive correlation (P ≤ 0.06) with cotyledonary, intercotyledonary, and total placental weights. These data suggest that fetal and placental growth occur in concert; however, which imparts more influence on the other is unknown.