Reproduction Abstracts

Introduction: This study investigates the effect of dietary crude protein on conception in range beef heifers and uses circulating progesterone as a measure of early embryonic loss known to be a major cause of reproductive inefficiency.

Methods: Primaparous 14 month old Bos indicus cross heifers (n=350) were selected from a range population. 60 days prior to artificial insemination they were randomly assigned to two equal groups and individually fed isocalorific high (14%) or low (7%) crude protein (CP) pellet diet. Heifers underwent an 8 day progesterone based synchronisation program and, on day 0, were artificially inseminated (AI) to one bull. At 23 days post-conception (dpc) the two groups were further split into high or low % CP creating four treatment groups: i) high/high (HH), ii) low/high (LH), iii) high/low (HL) and iv) low/low (LL). Pregnancy was confirmed by rectal ultrasound at 36dpc. Plasma progesterone was assessed prior to AI and at 23 and 36dpc.

Results: Overall pregnancy rate was 35.3%. Conception rates for the heifers who received high protein diet during the peri-conception period (−60 to 23dpc) was 8% higher than those that received the low protein diet (39.55% vs 31.30%). Heifers that received the high protein diet during the peri-conception period had significantly higher circulating progesterone levels at 23 and 36dpc compared to those that received the low protein diet (P<0.05). A reduction in circulating progesterone has been associated with increased levels of early embryonic loss. From the laboratory assay used, 7 ng/ml of progesterone in plasma was considered indicative of pregnancy. By this measure, 27 heifers lost the embryo between 23dpc and 36dpc, 19 of these (70%) had received low protein diet during the peri-conception period.

Conclusion: Protein supplementation during the peri-conception period in range heifers may increase pregnancy rates via increased circulating progesterone and decreased early embryonic loss.

Acknowledgement: We are indebted to S.Kidman and Co., Ridley Agriproducts and ARC for funding this research.