The PSM-1 serv and PSM-Con intervals were the same between the two lines, but the HL cows tended to show longer PSM-Con intervals than the LL cows. This tendency shown by the HL is reflected in the lower percentage of HL cows which conceived in the first 2 1 days of mating compared to LL cows. The length of the PSM-1serv and PSM-Con intervals are significantly shorter than those reported by Grosshans et al ( 1 996), but similar to those reported by Macmillan et al (1995) and Xu et al (1995). However, Grosshans et al ( 1996) analysed the reproductive performance of 48534 Friesian cows, and in this trial, as in case of the other two experiments cited above, the number of observations involved were limited . A great variation in the PSM- 1ser and PSM-Con interval between the farms was indicated by the results reported by Macmillan et al (1990). For the two years old cows, the difference in PSC-Con interval between the two lines approached significance (P<0.06), and explained the significant effect of age on the analysis of this interval. In fact, the 2 years old cows showed a longer PSM-Con interval than the older cows. Conversely to these results, Grosshans et al (1996) did not find a major effect of age on the PSM-Con intervals, and the higher fertility of the younger cows has been reported elsewhere (Ziv, 1994; Xu and Burton, 1996).
Because the length of pregnancy was the same between the lines, the slightly different mating pattern between them was also reflected in the calving pattern. Although the PSC-C interval was similar for the two genetic lines, more LL cows calved in the first 21 days of calving, and fewer LL cows were induced. The differences were especially marked between the 3 years old cows of the two lines. The advantage of a compact calving in a seasonal system of milk production was indicated by earlier results at Ruakura ( Macmillan et al, 1 984). According to the data from the current experiment, late calving cows produced 3.7 kg MS less in the first 1 2 weeks of lactation for each 1 0 days of delay in calving date. However, the effect of the more concentrated calving pattern of the LL cows on the overall rnilksolid yield of a farm system can not be concluded from this experiment that was confined to the first 1 2 weeks of lactation . The slightly higher daily milksolids production showed by the HL cows could fully compensate for the extra days in milk by the LL cows.
Although the percentage of empty cows at the end of mating was the same for the two lines of cows, the LL of cows showed a higher conception rate (CR) at first service than the HL of cows. That explained the tendency of the HL cows to conceive later in the season. The different conception rate was more marked in 1994 and 1 995, and between the 3 years old cows of the two lines. The average CR in New Zealand is around 60%, with some farmers achieving 75% (Xu et al, 1995). Although the CR of the two lines are in the range of these values, the average ratio of the LL cows was consistently higher than that for the HL . In agreement with these results, after 30 years of divergent selection for body size in Minnesota, Hansen et al (1998) found that the large line of cows after their first parity required a significantly larger number services to conceive (2.08) compared to the small line cows (1 .79). In the second and third parity, although the conception rate of the small line of cows selected was higher than that of the large line, the differences were not significant. A genetic antagonism between body size and conception rate was also suggested by Badinga et al (1985), and Markusfeld and Ezra (1993). Batra et al (1986) found also a negative genetic correlation (-0.28) between LW of the cows at 1 1 2 days of lactation and conception rate at first service. However, negative genetic correlation were reported in Canada between calving interval, and size (-0.23) and stature (-0.25) (Dadati et al, 1 986), and the phenotypic correlations between reproductive parameters, and stature and size were reported to be close to 0 (Dadati et al, 1 986; Moore et al, 1992).
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The reasons for the antagonism between size and conception rate were unclear. The heavy cows had a greater incidence of periparturient diseases (Badinga et al, 1985) which significantly affect the conception rate of the cows (Peters, 1 996; Lewis, 1997). However, the incidence of periparturient diseases was similar for the two lines of cows developed at Massey (Garcia Mufiiz et al, 1 998). In general, conception rate at first heat is increased by a longer period between calving and first mating ( Butler and Smith, 1 989; Ferguson, 1996). Other authors suggested that the specific postpartum effect on conception rate only occurs within the first 40 days (Ferguson, 199 1 ; Zavy, 1 994). In the present analysis of the data, the mean Calving-First service interval was similar for the two lines and longer than 40 days ( LL=83 days vs HL=80 days). Macmillan and Clayton ( 1 980) showed the importance of the occurrence of one previous oestrus before the first mating, with only a minor effect from any additional pre-mating heat(>!). In the current experiment, the percentage of cows in anoestrus at the PSM was the same between the lines which means that the same number of LL cows and HL cows had at least one detected heat prior to the PSM. Furthermore, the average non return rates achieved by the HL and LL bulls used through artificial insemination throughout New Zealand were the same for both lines ( 69% ). Particularly for 1994 and 1995, the non return rate were also similar for the H and L bulls used ( 1 994: LL=71 % HL=70%, 1995: LL=68.5% HL=70%).
The larger and heavy cows lost more LW after calving (Markusfeld and Ezra, 1 986), and the energy balance of the cows was reported to affect the conception rate (Ferguson, 1996; Britt, 1992). Similar results were reported for Brahma and Angus cows selected by small and large frame (Olson, 1 994). However, in the current experiment for the 1 996 season, the two lines of cows started the lactation with similar BCS, and the two lines of cows had the same BCS at mating, and similar BCS and LW changes after calving. The effect of BCS at mating, BCS change between calving and mating, and LW change between calving and mating was not associated to the PSM-Con interval. But at the same time, the difference in conception rate were not marked in 1996, and no data about LW and BCS is available for the 1 994 and 1 995 seasons when the differences in fertility were larger.
The ovulatory follicles of the HL cows had a diameter 3 mm larger and ovulated later during the
cycle than those of the LL cows scanned. The former also showed larger area of CL on days 9, I 0 and
1 1 , which are the days of maximum development of luteal tissue ( Grygar et al, 1 997). Holstein cows
(USA) presented longer oestrus cycles, heavier CL, but lower progesterone in serum than Friesian cows (New Zealand) (Bilby, personal communication)and, as in the current experiment, both group of cows differed significantly for LW which may explain some of the differences detected in the follicular waves, CL weights and characteristics of the cycle. The conception rate of the cows was
reported to be affected by the follicular pattern and follicular diameter (Burke et al, 1996; Mackey et
al; 1 997) and the production of progesterone from the corpus luteum (Fonseca et al, 1983; Villa
Godoy et al, 1 988; Lucy et al, 1992). However, in case of the present experiment, the significance of
the differences detected in the size of the follicular and luteal structures are unclear, and to use them for explaining the difference in conception rate between the lines is very speculative.
Finally, although the reproductive data analysed from 1 992-1997 seem to suggest that the LL cows achieved a slightly better reproductive performance than the HL cows , this conclusion should be taken with caution because the reproductive results change significantly from year to year . Therefore, more information is required from subsequent years before any definite conclusion is reached about the reproductive performance of the two lines of cows.