Beta-hydroxybutyrate concentration influences timing of pregnancy in young beef cows

Timing of conception can influence productivity within the cow herd and can be negatively influenced by metabolic dysfunctions. Selection for maternal traits like milk production in beef cattle may decrease reproductive efficiency due to metabolic dysfunctions caused by an increased metabolic load of lactation. Therefore, our objective was to analyze the association of milk production, serum metabolites as an indicator of nutrient status, cow BW and BW change, and calf performance with timing of pregnancy in 183 spring-calving beef cows. Cows were classified by timing of pregnancy as cows that were diagnosed pregnant by timed artificial insemination (TAI; n=118) or natural breeding (NAT; n=65). In addition, cows were grouped by age to represent young (3-to 4-yr-old), mature (5- to 6-yr-old), and old (7- to 9-yrold) cows. Starting approximately d 30 postpartum, cow BW and BCS were recorded and blood samples were collected weekly through the end of breeding. Weekly serum samples were composited by cow within production periods: (1) pre-breeding; (2) timed- artificial insemination to end of natural breeding. Cow BW and BCS did not influence (P > 0.40) timing of pregnancy during the entire study. Calf, BW at birth and weaning were not different (P > 0.30) between timing of pregnancy groups. However, calf BW at weaning and value the subsequent year of the study were greater (P < 0.001) for TAI than NAT cows. An age group × treatment interaction (P < 0.01) occurred for serum β-hydroxybutyrate (BHB). Serum BHB concentrations for mature and old cows were similar regardless of timing of pregnancy. However, serum BHB concentrations for young NAT cows were greater than young TAI cows. In addition, serum NEFA exhibited (P = 0.04) a timing of pregnancy × sampling period interaction. Pre-breeding serum NEFA concentrations were greater for NAT cows than TAI cows. Contrarily, serum NEFA concentrations during the natural breeding season were similar regardless of timing of pregnancy. Results from this study indicate that only the young, postpartum beef cows during early lactation were susceptible to measured metabolic dysfunctions of elevated blood BHB concentrations, which may have caused a delay in timing of pregnancy.