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Effects of pregnancy on daily milk yield of Holstein–Friesian dairy cattle

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Abstract

First, second and third lactation curves of 21,885 first lactation dairy cows were allowed to vary depending on the months in milk at which the cow became pregnant. The curves were modelled using a 4th degree polynomial of days in milk. In the models for second and third lactation daily yield, days open in the previous lactation was included as a quadratic regression.

Allowing the curve coefficients to depend on the month in milk when pregnancy occurred significantly improved the goodness of fit of the model to the data. The linear and quadratic coefficients of second and third lactation daily milk yield on days open in first and second lactation, respectively, were also significant. We suggest that, in the evaluation of test day milk yield, the shape of lactation curves are allowed to vary depending on the months in milk when pregnancy occurs and that days open in the previous lactation, if appropriate, is included in the model.

Introduction

In genetic evaluation systems for dairy cattle production traits, a major benefit of models that directly utilize daily yields (test day models) rather than cumulative 305-day yield is that differences in the shape of the lactation curve can be accommodated (Jamrozik and Schaeffer, 1997). In a typical evaluation system based on daily yields, lactation curves will vary depending on lactation number, month or season of calving and age at calving.

Pregnancy also has an impact on a cow's production (Erb et al., 1952) as it causes mammary gland regression and competition for nutrients from the developing fetus, resulting in a reduction in yield. Researchers have shown that pregnancy not only has a significant effect on milk, fat and protein yields but the effect varies with gestation stage (Olori et al., 1997). In order to avoid penalising pregnant cows, most countries account for pregnancy by including days carried calf or previous days open in their genetic evaluation models (INTERBULL, 2003). Olori et al. (1997) suggested that, for genetic evaluations based on daily (or test day) yields, pregnancy and its interaction with stage of lactation should be factors in the model. The use of a constant adjustment ignores differences between the lactation curves of animals that are successfully inseminated early in lactation and those that become pregnant later in lactation, or not at all.

The objectives of this work were to investigate the benefits of including pregnancy in test day models and to suggest a possible method of allowing for pregnancy in these models.

Section snippets

Materials and method

Test day milk records were extracted for Holstein–Friesian heifers calving from July 1995 to June 1997. All heifers had at least seven tests and calved between 20 and 40 months of age. Observations beyond 400 days in milk were discarded. After validating dates of milk recording and removing small progeny and contemporary groups, the final data set comprised 224 840 test day milk records on 21 885 cows, progeny of 823 bulls. Second and third lactation test day records for these cows, if present

Results

The average proportion of North American genes in lactation 1 and 2 cows was 0.70, dropping slightly to 0.69 in third lactation.

Table 1 gives the percentage of cows in each ‘months in milk when pregnant’ group. Cows which are never pregnant (or always open) during the lactation are in group np, those becoming pregnant when less than 30 days in milk are in group p1, etc. Groups p10 and p11 for lactations 2 and 3 have been merged because of the small number of cows in each group.

Irrespective of

Discussion

In this analysis, animals first calved from July 1995 to June 1997 and many would have been in their fourth lactation when culling to prevent the spread of foot-and-mouth took place. The group of cows categorised as never pregnant during their third lactation is without doubt in error as some of these cows may have been pregnant but were culled before calving. The results for non-pregnant third lactation cows should therefore be treated with caution.

Researchers have demonstrated that high yield

Acknowledgements

We thank the Milk Development Council (MDC) who funded this study. We are grateful to National Milk Records and MDC Evaluations for providing the data.

References (11)

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