The breeding program for the Norwegian Red breed over the last 35 years has included fertility, health and functional traits in addition to milk yield and other production traits. Currently, the demands placed on individual cows are increasing particularly in regards to milk yield, dry matter intake, fertility, and longevity. The overall goal for the four studies included in this thesis was to assess the reproductive performance in modern Norwegian Red cows.
In the first field study, the 60 d non return rate, pregnancy incidence, calving rate and differences between non return rate and pregnancy incidence following first artificial insemination (AI) were investigated. None of the animals were treated with reproductive hormones prior to AI. The proportion of cows not returning to estrus was 72.5%, the pregnancy incidence 6 wk after AI was 63.7%, and the calving rate was 57.2%. The difference between non return rate and pregnancy incidence was higher for older cows compared with younger cows and heifers. Parity number did not affect the likelihood of pregnancy when heifers were excluded from the analysis, but heifers had higher pregnancy incidence than cows. The interval to first AI was not associated with pregnancy incidence, which could be due to a relatively long voluntary waiting period (85.3 d). Milk yield was also not associated with the pregnancy incidence. The moderate milk yield in Norwegian Red cows and inclusion of non return rate as a fertility trait in the Norwegian breeding program over the past 35 years are probably important reasons for such good reproductive performance as seen in this study.
Multiple ovarian cycles before first AI after calving have shown to reduce the number of AI per pregnancy. Hence, early reestablishment of ovarian activity and onset of luteal activity that usually occurs 4-5 d after first ovulation post partum are important factors for good reproductive performance in dairy cows. Consequently, in some countries recent attention has focused on breeding for early onset of luteal activity. Onset of luteal activity after calving and relationships with the occurrence of persistent corpus luteum (yellow body) and delayed ovulation were evaluated in clinical trials conducted in the dairy herd at the Norwegian University of Life Sciences. The onset of luteal activity after calving was investigated in cows selected for low milk yield, high milk yield, or the combination of high milk yield and fertility. Cows selected for high milk yield produced more, and had a longer interval to onset of luteal activity than cows selected for low milk yield. Whereas, time to
onset of luteal activity in cows that had been selected for both milk yield and fertility was between the two other selection lines. Consequently, the increase in the interval from calving to onset of luteal activity by selection for high milk yield can be reduced, at least partially, by inclusion of fertility in the breeding program. The energy balance after calving was negatively related to onset of luteal activity, but could not explain the whole difference between the selection lines. Hence, the impairment in reproductive efficiency following selection for high milk yield must be attributable also to factors other than negative energy balance caused by high milk production. Further research is necessary to investigate which factors other than negative energy balance affect the phenotypic difference in days to onset of luteal activity between cows selected for low or high milk production. The occurrence of a persistent corpus luteum and delayed ovulation after calving was lower in the study population of Norwegian Reds than reported for most other dairy populations. Cows experiencing a persistent corpus luteum after calving had shorter intervals from calving to onset of luteal activity. Hence, the inclusion of onset of luteal activity as the only fertility trait in a breeding program may be unwise because of the increased risk of the occurrence of a persistent corpus luteum, rather than continuous cyclic activity, in animals that resume luteal activity too quickly after calving. The occurrence of a persistent corpus luteum was not related to the pregnancy rate after first AI, but cows experiencing a persistent corpus luteum were more likely to be inseminated during the luteal phase. The likelihood of AI during the luteal phase was higher in the study population than in the general population of Norwegian Red. This may, at least partly, explain why the occurrence of persistent corpus luteum was associated with the likelihood of AI during the luteal phase.
When reproductive performance and udder health were compared in conventional and organic dairy farming systems, the interval from calving to first AI was shorter in conventional cows, although no differences were seen in the interval from calving to last AI or the calving interval. Conventionally managed cows were younger, milked more, and received more concentrates than organic cows. Organic cows had lower milk somatic cell count than conventional cows. This difference between the management systems persisted after adjustment for age and milk yield in the statistical models. Higher levels of concentrates are now fed in organic dairying than a decade ago. This is probably an important factor for the higher reproductive efficiency seen today in organic farms compared with previous studies. Consequently, the Norwegian Red is shown to be a sustainable breed that adapts well to both conventional and more extensive production systems, such as organic farming
