Subacute ruminal acidosis negatively affects conception rate in Holstein heifers

Abstract Text: Symptoms of subacute ruminal acidosis (SARA) have been studied at length, but its effects on reproductive performance are not fully understood. Our objective was, therefore, to determine if experimentally induced SARA reduces conception rates of Holstein heifers. One hundred and ten heifers were synchronized for artificial insemination by two injections of PGF_2α in a 13 d interval and assigned randomly to two treatments. The control heifers received a diet containing 32% (DM basis) barley-based concentrate, while the SARA challenge group received a diet containing 68% of this concentrate. The remainder of the diet consisted of corn silage, alfalfa hay, wheat straw, soybean meal, wheat bran and a vitamin-mineral supplement. Diets were fed ad libitum. The SARA challenge diet started 3 d after the 2^nd PGF_2α injection, and continued for seven days. Forty one heifers from the SARA group and 39 heifers from the control group showed visible signs of heat and were inseminated. Heifers in SARA group had higher DMI (10.4 vs. 9.0 kg d^-1, P < 0.01) and lower rumen pH (6.02 vs. 6.45, P < 0.01) and fecal pH (6.71 vs. 6.97, P < 0.01) at 6 h post feeding compared to control heifers. The SARA challenge increased rumen concentrations of ruminal lactate, propionate and valerate, but did not affect the concentrations of acetate, butyrate and isovalerate. The challenge did not affect glucose, urea nitrogen, aspartate aminotransferase, calcium and cortisol concentrations in blood, but it lowered blood beta-hydroxybutyrate (P < 0.01). Induction of SARA markedly reduced first service conception rate tested by ultrasonography 28 d after insemination (53.7 vs. 71.8 %, P < 0.05). Additionally, 100% of control heifers that were confirmed as pregnant in the 28 d test were also pregnant at 60 d test, whereas this ratio was only 73.9% (P< 0.01) in SARA-challenged heifers suggesting that SARA had a persistent effect on reproduction that lasted at least 60 d after insemination. Results suggest a negative effect of SARA on fertility of dairy heifers. Further studies are required to investigate the possible effects of lipopolysaccharide translocation and systemic immune response that is associated with SARA on embryo survivability to fully elucidate the mode of action of SARA on reproductive performance.

Keywords:  conception rate, fertility, subacute ruminal acidosis