Age at Puberty and Pregnancy Rate in Beef Heifer Genotypes Offered Contrasting Nutrition Levels

The objective was to determine the effect of nutrition and genotype on reproductive performance in beef heifers. A total of 309 spring-born early- (EM) and late-maturing (LM) sired heifers born to dams of dairy (D) or beef (B) origin were used over two successive years. Mean initial age (SD) and BW (SD) were 259 (14.0) days and 256 kg (52.0) kg, respectively. Heifers were blocked according to breed-type, age and weight and from within block, randomly allocated to one of two nutrition treatments; grass silage ad-libitum and a barley-based concentrate offered at 1.5 kg/head daily [M] or ad-libitum [H] for 141 d (Year 1) or 150 d (Year 2). Heifers were housed in a slatted floor shed in pens of 5 prior to turn out to pasture in mid-April where they rotationally grazed Lolium perenne dominant swards. A 12-week breeding programme commenced in late April, in both years, using semen from a single sire. Blood was sampled twice weekly for progesterone analysis. Pregnancy confirmation was carried out 30-40 days post-breeding. Data were analysed using SAS software in accordance with the 2 x 2 x 2 factorial design. Continuous and proportional data were subjected to the MIXED and GLIMMIX procedures, respectively. There were no interactions (P>0.05) between treatments. Compared to M heifers, H had higher ADG indoors (1.03 v. 0.51 kg, P<0.001), were younger at puberty (424 v. 437 d, P<0.001) and had a greater 6-week submission rate (91 v. 79%; P<0.01). However, there was no difference (P>0.05) in pregnancy rates between dietary groups after 6 (H=61%, M=56%) or 12 weeks (H=87%, M=91%) of breeding. Compared to D heifers, B had a lower ADG indoors (0.70 v. 0.84 kg, P<0.001), were of similar age at puberty (429 v. 432 days, P>0.05), tended to have a lower 6-week submission rate (82 v. 90%, P=0.08), had lower 6-week (52 v. 65%, P<0.05) and 12-week (83 v. 93%, P<0.01) pregnancy rates. Compared to EM heifers, LM had similar ADG indoors (0.76 v. 0.78 kg, P=0.19), were older at puberty (438 v. 423 days, P<0.001), had similar 6-week submission rate (83 v. 88%, P>0.05), but a lower 6-week (51 v. 66%, P<0.01). However, there was no effect of breed type (P>0.05) on 12-week, pregnancy rate. In conclusion, age at puberty was reduced by nutrition level and sire, but not dam breed, whereas pregnancy rate was increased by dam and sire breed but not nutrition.