Feeding excess crude protein to gestating and lactating beef heifers: Impact on parturition, milk composition, ovarian function, reproductive efficiency and pre-weaning progeny growth

Angus-Simmental beef heifers confirmed pregnant to a single sire (n=78) were used to determine the effects of feeding excess CP during late gestation and early lactation on parturition, milk composition, reproduction, and pre-weaning progeny growth. At 192 d in gestation, heifers were allotted by BW within BCS to receive either a corn silage-based control diet balanced to meet or exceed NRC recommendations (2000) (CON), or a diet that considerably exceeded CP requirements consisting of corn stover and DDGS, where DDGS were fed as a primary energy source (HP). Diets were formulated to provide similar daily energy intake (2.09 Mcal/d NEg). Dietary treatments were stopped at 118±4 d postpartum (DPP) and cow/calf pairs were comingled. Calving parameters and pre-weaning calf performance were assessed. At 64±4 DPP, milk samples were collected to determine composition and fatty acid profile. At 32±4 DPP, a single follicular wave was mapped via ultrasonography in 12 anestrous heifers per treatment. Starting 34±4 DPP, blood samples were taken every 10 d until synchronization to determine resumption of estrous cyclicity. Gestation was longer (P=0.02) and incidence of dystocia was greater (P=0.003) for HP than CON, respectively. Birth BW was greater (P<0.001) in HP than CON progeny. However, milk fat, total solids and energy corrected milk production were greater (P≤0.03) in CON than HP, respectively. In contrast, milk urea N, MUFA, PUFA, and CLA content was greater (P<0.01) in HP than CON, respectively. During the anestrous period, dominant and secondary follicle diameter was greater (P≤0.05) and follicle wavelength tended (P=0.08) to be greater in HP than CON, respectively. Proportion of cyclic heifers at estrous synchronization did not differ between treatments, but HP tended (P=0.10) to resume estrous activity at fewer DPP than CON. Timed-AI and breeding season pregnancy rates did not differ (P≥0.30) between the HP and CON treatment. Calf BW was greater throughout the pre-weaning period (P<0.001) for HP than CON progeny. Results indicate that feeding DDGS as a primary energy source to first-parity heifers during late gestation and early lactation altered ovarian function and resulted in increased pre-weaning growth performance of progeny.