Productive and reproductive efficiency of two Holstein Friesian lines of cows which differ genetically for live weight : a thesis presented in partial fulfilment of the requirements for the degree of Master of Applied Science in Animal Science, Massey University

Two lines of Holstein Friesian cows which differ genetically for live weight, the Light Line (LL) and the Heavy Line (HL), have been selected at the Dairy Cattle Research Unit (Massey University) since 1989. The aim of the current experiment was to compare the productive and reproductive performance of these two lines during early lactation. Measurements of milk production, liveweight (LW), and pasture intake were made in 1996, while reproductive data were analysed for 1992 to 1997. In experiment 1a, the milk production of the two lines was compared during the first 12 weeks of the lactation by the weekly measurement of the milk yield and the milk composition of 30 LL cows (average LW=412 kg) and 27 HL cows (average LW=445 kg), with the two groups of cows fed and managed identically. Pasture dry matter intake (DMI), calculated as pasture disappearance, was 13 to 15 kg DM a day during these 12 weeks. Although the HL produced slightly more milksolids (MS) than the LL, the difference was not significant (LL= 139 vs HL= 141 kg MS). However, the HL cows > 2 year old produced 7 kg MS more than the LL cows > 2 year old (P2 year old, the HL cows > 2 year old tended to calve and to conceive later in the calving and mating periods, respectively, because the HL cows had a lower conception rate at first service than the LL cows (LL= 70% vs HL= 58%, P<0.05). The ovaries of 10 cows from each line (LL= 405 vs kg HL= 481 kg) were scanned daily during a complete cycle before the start of mating. Cows from the HL had preovulatory follicles with larger diameter (LL= 12.7 vs HL= 15.7 mm, P<0.05) and corpus lutea with larger areas (LL= 690 vs HL= 859 mm2, P< 0.05) than the LL cows. No differences were detected in the diameter of the first and second dominant follicles. On average, the preovulatory follicles of the HL cows achieved their maximum diameter later in the cycle compared to the LL (LL= day 18th vs HL= day 20th). The results from the current experiment show that although the HL produced slightly more MS than the LL in the longer period, the two lines of cows achieved similar levels of MS yield during early lactation independently of their LW and size. Similarly, although the LL cows had a slightly higher MS conversion efficiency than the HL cows, the differences in energy and MS conversion efficiency between the two lines were not significant. The reproductive data analysed from 1992 to 1997 suggest that the LL cows achieved a better reproductive performance than the HL cows because of their higher conception rate at first service. However, more information is required from other stages of the lactation before any definite conclusion is reached about the feed conversion efficiency of the two lines. Similarly, considering the variation in the reproductive performance of the HL between the years, reproductive data from subsequent seasons must be collected in order to verify, or disprove, the current conclusions