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As NZ milk production is predominantly seasonal and reliant on pasture growth, a high pregnancy rate within a short time after planned start of mating is essential to match feed supply to production. A Holstein-Friesian strain trial was established in Hamilton, New Zealand, to investigate the physical and financial performance of three strains of dairy cattle under a range of pasture based systems that differ in stocking rate and supplementary feeding. The three strains of cows were: Overseas High genetic merit (OS), New Zealand High genetic merit (NZH) and New Zealand Low genetic merit (NZL). The objective of this study was to predict reproductive performance from information available prior to mating, as an aid to on-farm decisions.
Increasing genetic merit for production has been associated with a decline in dairy cow fertility. In order to sustain lactation it appears that appreciable amounts of body condition are being mobilised, which may impinge on fertility. Body condition score (BCS) of first lactation heifers is recorded by Holstein UK and Ireland (HUKI) as part of its national type classification scheme. BCS may be a useful selection criterion for improving fertility. Calving dates and hence calving interval (CI) are generally very reliably recorded, but the usefulness of CI as a selection criterion is hampered for a number of reasons, one being that only the most fertile cows have two consecutive calving dates. This is a serious issue that still needs to be addressed. Our aim here was to investigate if there is genetic covariation between BCS and CI after adjustment for milk yield and to investigate selection responses in all three traits when selection is for each trait in turn and how responses are affected by restrictions imposing no genetic change in one of the traits. Restricting a trait to no change when it is correlated to a trait under selection may be important in developing customised indexes to satisfy specific requirements.
In recent years there has been considerable genetic progress in milk production. Yet, increases in yield have been accompanied by an apparent lengthening of calving intervals, days open, days to first heat and a decline in conception rates, which appears to be both at the genetic and phenotypic level. Fertility has a high relative economic value compared to production traits such as protein, making it attractive to include in a breeding programme. To do this there needs to be genetic variance in fertility. Measures of fertility calculated from service dates have a small genetic compared to phenotypic variance, hence heritability estimates are small, typically less than 5%, although coefficients of genetic variance are comparable to those of production traits. Heritabilities of commencement of luteal activity determined using progesterone profiles are generally higher, and have been reported as being from 0.16 to 0.28, which could be because of a more precise quantification of genetic variance, as management influences such as delaying insemination and heat detection rates are excluded. However, it might not be the use of progesterone profiles alone, as days to first heat observed by farm staff has a heritability of 0.15. The most efficient way to breed for improved fertility is to construct a selection index using the genetic and phenotypic parameter estimates of all traits of interest in addition to their respective economic values. Index traits for fertility could include measures such as calving interval, days open, days to first service, or days to first heat but there may also be alternative measures. Examples include traits related to energy balance, such as live weight and condition score (change), both of which have higher heritabilities than fertility measures and have genetic correlations of sufficient magnitude to make genetic progress by using them feasible. To redress the balance between fertility and production, some countries already publish genetic evaluations of fertility including: Denmark, Finland, France, Germany, Israel, The Netherlands, Norway and Sweden.
Getting reliable genetic parameter estimates for dry matter intake is difficult because recording it is expensive, hence it is tempting to combine data from research herds. However, there are large differences in feeding and management systems, which causes differences in means across herds. Furthermore, variances or heritabilities may differ and genetic correlations may be less than one between herds. This is one of the reasons why it is important to investigate effects of genotype by environment interaction (GxE). Another reason is that it is important to understand how high genetic merit cows perform in different feeding systems. The objective of this study was to estimate the effect of GxE for three feeding systems at two research herds belonging to ID-Lelystad (ID) and to SAC/University of Edinburgh (Langhill).
Differences in banding scales for milk quality penalties, as determined by bulk tank somatic cell count (SCC), prevent the use of a single economic value for SCC in an overall economic-genetic selection index (Veerkamp et al., 1998) such as, Profitable Lifetime Index or £PLI. But SCC could be used as a predictor of mastitis as genetic correlation estimates between mastitis and SCC are medium to high (review of Mrode and Swanson, 1996). This suggests that, although deriving a direct single economic value (EV) for SCC based on bulk tank SCC is difficult, a single financial value could still be assigned to SCC based on its relationship with mastitis. Here we use a genetic regression method to calculate the EV of SCC (EVSCC) as a predictor of mastitis. However, the dependency of regression coefficients on mastitis incidence (p) could make such EVSCC variable. The main objective of this study was to evaluate the impact of such relationship on EVSCC and genetic selection in dairy cattle using predicted transmitting abilities of SCC (PTASCC).
Genetic improvement is permanent and cumulative. Improvements made in one generation are passed onto the next. In the UK two selection indexes are currently available to the dairy industry, they are PIN (Profit Index; production only) and £PLI (Profitable Lifetime Index; production plus lifespan). Much of the current and anticipated index research and development will be on broader breeding goals that include health and fertility traits. Economic responses expected for PIN and £PLI over a 20 year period were calculated in addition to a hypothetical index where it was assumed that PTAs (predicted transmitting abilities) for mastitis (M) and calving interval (CI) were available (£PLI+M+CI).
There is evidence from UK field data to suggest that selection for milk yield has led to a deterioration in the fertility of lactating dairy cows (Pryce et al., 1997). As cow fertility may be affected by metabolic load and lactation effects, it is important to investigate the fertility of non-lactating heifers. Since 1973 selection line (S) cows at the Langhill Dairy Cattle Research Centre have been bred to bulls with the highest predicted transmitting abilities for weights of fat plus protein (PTA F+P) available in the UK, while the control line (C) are of about UK average genetic merit for PTA F+P. Results using data from lactating Langhill cows show that S cows have calving intervals that are, on average, 12 days longer than C cows, which is a combination of poorer conception rates and a delay in first observed oestrus (Pryce et al., 1999). The objective of this study was to investigate the effect of selection for PTA F+P on the fertility of maiden heifers at Langhill.
The economic pressures on the dairy industry may force more farmers to consider reducing the amount of concentrates fed to cows in order to keep costs down. We have been testing whether the long-term performance of daughters of sires progeny tested in high concentrate systems maintain their advantage over cows of average genetic merit when managed in a lower input feeding system. This paper extends the scope of our initial report (Chalmers et al., 1997) and includes data on reproductive performance.
Data were from Holstein-Friesian cows managed at the Langhill Dairy Cattle Research Centre. Sires of the Selection (S) line are among the highest available in the UK for predicted transmitting abilities of weight of fat plus protein (PTA F+P). Sires of Control (C) line cows are about UK average for PTA F+P.
High producing dairy cows have been found to be more susceptible to disease (Jones et al., 1994; Göhn et al., 1995) raising concerns about the welfare of the modern dairy cow. Genotype and number of lactations may affect various health problems differently, and their relative importance may vary. The categorical nature and low incidence of health events necessitates large data-sets, but the use of data collected across herds may introduce unwanted variation. Analysis of a comprehensive data-set from a single herd was carried out to investigate the effects of genetic line and lactation number on the incidence of various health and reproductive problems.
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