Date Published: February 13, 2007
Publisher: BioMed Central
Author(s): Arne Ola Refsdal.
Declining reproductive performance is a serious breeding concern in many countries. To reveal the situation in Norwegian cattle, trends in reproductive performance were studied using insemination reports from 1985 to 2005 and data based on herd recording files from 1989 to 2005. The total number of first services was 469.765 in 1985 declining to 335.712 in 2005. The number of recorded herds and animals declined from 21.588 to 14.718 and 360.289 to 309.452 from 1989 to 2005, respectively. Sixty days non-return rate after single inseminations (NR60) increased from 68.1 in 1985 to 72.7% in 2005 (p < 0.001) and the number of services per inseminated animal (NIA) decreased from 1.8 to 1.6 (p < 0.001) from 1985 to 2005. However, return rates 0–3 days post insemination (RR0-3) increased from 6 to 12% in the same period (p < 0.001). NR60 was higher and the RR0-3 was lower in the summer season compared to the winter season during the whole period. A fertility index (FS), has been calculated from the herd recording files each year from 1989 to 2005. The average FS-index did not show a significant trend and the calving interval was also fairly constant between 12.4 and 12.6 months during this period. The average interval from calving to first and last insemination, respectively, increased from a low of 79 and 102 days in 1990 to a high of 86 and 108 days in 2005. Both intervals were consistently longer for cows in first lactation than for cows in later lactations. The percentage of inseminated animals reported culled because of poor fertility decreased from 6.0% in 1989 to 4.6% in 1996 and thereafter again increased to 6% in 2005. In conclusion, most fertility measures, mainly comprising the Norwegian Red (NRF) breed, show a relatively high level of reproductive performance with a positive or a relatively constant trend during the last two decades.
In many countries there has been a decline in reproductive performance in dairy cattle. Several studies show increasing number of days from calving to first service and decreasing pregnancy rates, e.g. [1-5]. As a result, the number of inseminations per inseminated cow, days from calving to conception and calving intervals have increased. To improve fertility and save labour, various pharmaceuticals to control the oestrous cycle and to treat reproductive disorders are extensively used in many herds, e.g. [6,7]. During the last decades the productivity of dairy cattle has increased considerably in many countries, not least because of progress due to genetic improvement. However, a serious breeding concern is that estimates from a number of studies present unfavourable genetic correlations, on average near 0.3, between various fertility measures and production . In contrast to many other countries, Norway has a long tradition of including fertility in the Total Merit Index (TMI). Viewed against this background, the primary objective of the present study was to describe the trends in some reproductive measures in Norway the last two decades. Seasonal variations in reproductive performance are also revealed.
The results obtained in the present study are based on insemination reports and herd recording files in Norway comprising 66.8% of the herds in 1985 increasing to 94.2% in 2005 . AI-technicians and veterinarians report all inseminations into the AI-database, and they are only paid when the inseminations are registered. From 1985 to 2005, the part of inseminations performed by veterinarians has increased from 45.8% to 59.7%. The rest of the inseminations was performed by technicians, but from 2002 also a small part by herdsmen, increasing from 0.3% to 0.7% in 2005. After attending an AI-course the herdsmen have to sign an agreement to report inseminations to the AI-database. Sixty days non return rates after single inseminations (NR60), return rates 0–3 days post insemination (RR0-3), average number of inseminations per animal inseminated (NIA) and seasonality are based on all inseminations performed in the country during the period, irrespective of membership in the milk recording system. Thus, these data are based on 469.765 number of first services in 1985  declining to 335.712 services in 2005 .
The number of first inseminations every 5th year from 1985 to 2005 is shown in figure 1. The major part of the inseminations is performed with semen from the Norwegian Red (NRF) breed, varying from a high of 97.8% in 1985 to a low of 92.3% in 2003. Semen from other breeds are various beef breeds (4.2% in 2005), mainly used on NRF cattle, and other dairy breeds (3.0% in 2005) . During the period of study, October – January represented the main breeding season with peaks in November and December. From February to September the monthly number of 1st services remained similar (Fig 2). The age of heifers at 1st insemination was at a low of 15.6 months in 1991 and increased to 16.2 months from 2001 to 2005. The average CFI interval has increased from a low of 79 days in 1990 to a high of 86 days in 2005 (Fig 3). The CFI interval for cows in first lactation was consistently longer than for cows in later lactations, increasing from 81 to 88 days and 78 to 84 days respectively, from 1990 to 2005. The average CLI interval has also increased during the period from a low of 102 days in 1990 to a high of 108 days in 2005 (Fig 4). The CLI interval for first lactation cows was also consistently longer than for cows in the second and later lactations, increasing from 106 to 113 days and 99 to 104 days respectively.
The aim of the present study was to describe some trends in reproductive measures in Norwegian cattle the last two decades. Since NRF has been by far the most dominant breed during this period, the data presented mainly reflects the reproductive performance of this breed. To describe the fertility trends, 60 days non return rates and number of services per inseminated animal are used among others. As a measure of fertility, non return rates have some disadvantages, as described by Salisbury et al. . Cows, once inseminated, may be culled, dead or bred naturally without recording, either on purpose or by accident, and appear in the records as non returns to the original insemination. On the other hand, cows that come in heat and are inseminated while pregnant will appear on the record as returns. This will also be misleading. Moreover, embryonic deaths or abortions cause some cows to return to later service even though they had conceived at an earlier one. However, when applied to large numbers of services like in this study, non return rates are considered to be very useful for studying fertility trends. The registration system in Norway is also considered to be very reliable as the inseminations are performed by technicians employed in one company, Geno (Norwegian breeding and AI-association) and by veterinarians, and both groups are paid by Geno when the inseminations are registered. Reports of inseminations being performed by herdsmen may be somewhat incomplete even though it should be done routinely according to an agreement. However, since inseminations were not performed by herdsmen before 2002 and represent a very small part of the inseminations since then, incomplete reports from this group would be of little significance for the study. Substantially, there have been no changes in the AI reporting routines during the last decades. Therefore, the positive trend in non return rate probably reflects a true fertility improvement. This trend is in accordance with Andersen-Ranberg et al., studying phenotypic and genotypic trends in heifers and first lactation cows . However, it is in contrast to a worldwide trend showing a decline in non return rates and pregnancy rates during the last decades, e.g. [2,5]. The non return rates in Norwegian cattle during the last decades probably also reflects a positive trend concerning pregnancy rates. Unfortunately, reliable data to confirm a close trend relationship between the two parameters have not been available so far. However, recently a Norwegian field study has indicated that the pregnancy rate is on average about 12 % lower than overall NR60 after single inseminations . In this study the overall pregnancy rate after single first inseminations in NRF was 60.7%, and the results for heifers, 1st lactation and >1 lactation cows were 68.8, 56.0 and 58.7% respectively. These results show that the pregnancy rates in NRF is relatively high when compared to studies from many other countries, e.g. [15-17].
In conclusion, most fertility measures in Norwegian cattle, mainly comprising the NRF breed, show a relatively high level of reproductive performance and a positive (NR60, NIA) or relatively constant trend (Calving interval, FS-index) during the last two decades. This is probably caused by a variety of reasons, one of them being the breeding strategy, which gives increasing weight to fertility and health traits. However, the interval from calving to first and last insemination, respectively, has slightly increased during the period and the RR0-3 has increased. The calving interval has been relatively constant in spite of increasing non return rates and lower number of services per animal inseminated, mainly because of a longer interval from calving to first insemination. This is also the main reason why the FS-index has been relatively constant. In contrast to many countries under subtropical and tropical conditions the reproductive performance in Norway is higher in the summer months compared to the winter season. This pattern has been similar over time and is probably caused by a variety of environmental factors.