Cow/Calf Influence of maternal protein restriction in primiparous heifers

Nutrient status in gestating beef cows has been shown to impact performance of the dam and offspring; however, most research has focused on energy or a total diet restriction and a single period of gestation. The objective of this study was to evaluate the effects of maternal metabolizable protein (MP) restriction in primiparous heifers during midand/or late gestation on dam and suckling calf performance through weaning. Two-year-old Angus × Simmental heifers (n = 108) were allocated to a randomized complete block design. Pens within each block were randomly assigned to either CON (slightly exceeding MP requirements) or RES (approximately 80% of MP requirements) treatments. Both diets were formulated to meet net energy requirements. Half of the pens on the CON treatment were reassigned to the RES treatment at the end of mid-gestation and vice versa, in a 2 MP level × 2 gestation period factorial structure. Heifer body weight (BW), body condition score (BCS), ultrasound body composition, milk production and composition, calving data, and calf weaning weights were measured. There was an interaction for mid-gestation treatment × time for changes in BW and BCS during mid-gestation, with heifers on the RES treatment losing BW while CON heifers maintained BW (P < 0.01). In a late gestation treatment × time interaction, restricted heifers gained approximately half as much BW and lost BCS compared with CON heifers (P < 0.05). There was a mid-gestation treatment × time interaction for longissimus muscle (LM) area, with restricted heifers losing more than twice as much LM area as CON heifers (P = 0.04). A midgestation × time interaction (P = 0.03) indicated a tendency (P < 0.10) for increased intramuscular fat loss in heifers on the RES treatment at the end of mid-gestation. In a late gestation treatment × time interaction, MP restriction in late gestation increased loss of LM area in RES vs. CON heifers (P = 0.03). There were no changes in 12th rib subcutaneous fat thickness (P > 0.05) across treatments or time. Dietary treatment did not affect calf birth or weaning weight, milk production, or subsequent cow reproductive performance (P > 0.05). Decreased available MP appeared to result in mobilization of maternal body reserves during the restriction; however, it did not impact subsequent cow reproductive performance, calf birth weight or calf growth to weaning. Introduction Nutrient status of gestating beef cows can have various long-term implications on growth, feed intake and efficiency, and performance of offspring (Funston et al., 2012). Research indicates developmental status of the fetus at the time of a maternal nutrient deficiency plays a role in postnatal responses of the offspring (Freetly et al., 2000; Morrison et al., 1999; Wiley et al., 1991). However, much of the available research has been limited to a single period of development (e.g. early or late gestation) and has evaluated the effects of an energy restriction or a reduction of total dietary dry matter intake. In many forage-based production systems, protein may be deficient in gestating beef cow diets due to increasing nutrient requirements, forage quality issues, inadequate supplementation, and/or environmental conditions (Caton