Age at first calving (AFC) affects the profitability of dairy enterprises. This study examined the relationships between AFC and subsequent fertility, milk production and survival up to third calving in UK Holstein-Friesian heifers; Study 1 comprised 134 consecutively born heifers from the same herd (AFC 764 ± 8 days) and Study 2 contained 445 heifers born on 17 farms (AFC 796 ± 6 days). Growth rates and fertility during rearing were monitored and animals were subsequently divided into four AFC groups for analysis: <23 months, 23 - 25 months, 26 - 30 months and >30 months. The actual AFC reflected both growth rate and heifer fertility, with later calving heifers already lighter by 6 months of age. The proportion conceiving to first service was highest (93% and 84% for Studies 1 and 2, respectively) for the youngest AFC group. Fertility in the first lactation was best in cows calving at 23 - 25 months and worst in the oldest AFC group. Milk production parameters in the first and second lactations were broadly similar between AFC groups but significantly more heifers with an AFC < 26 months calved for a third time. These younger calving animals therefore achieved more days in milk over 5 years, with >44% of their days alive spent in milk production compared with only 18% - 40% in cows calving at ≥26 months. Hence cows with an AFC < 26 months produced the most milk in their first 5 years of life. These results indicate that an AFC of <26 months="" required="" both="" a="" growth="" rate="">0.75 kg/d up to 15 months and good heifer fertility and resulted in the best subsequent performance.
Rearing replacement heifers requires a large financial outlay, with no returns until the animals enter the milking herd; heifers do not normally become profitable until their second lactation. An earlier age at first calving (AFC) can reduce rearing costs due to decreased feed, labour and building costs. A previous economic analysis showed that reducing AFC from 25 to 24 or 21 months decreased replacement costs by 4.3% or 18% respectively [
Rearing heifers to join the herd at an age and body weight (BW) that will enable them to achieve their full lifetime potential, in terms of both yield and longevity, is also fundamental. Gestation length is fixed; therefore AFC is a function of the age at the commencement of first breeding, combined with the reproductive efficiency of the animal. The decision on when to start breeding is primarily a management one. It is usually based mainly on the age of the heifer, but is also influenced by nutrition and health. Poor growth during the rearing period due to underfeeding and/or disease has been associated with delayed first breeding and first calving [2,3]. Once animals have been bred, their fertility during the breeding period will also affect the age at which they calve. Poor fertility can lead to a large spread in AFC and in practice there may be large differences between the target and actual AFC achieved.
The majority of UK dairy producers aim to start breeding Holstein-Friesian heifers from around 14 - 15 months of age, and thus the target AFC is generally 24 months on UK farms. Most countries, however, report a mean AFC of more than 24 months (e.g. [4-6]) and only 14.6% of dairy operations surveyed in the US achieved an average AFC of ≤25 months [
A number of studies have shown that age and BW at calving can have a significant effect on both subsequent milk production [4,8] and survival in the herd [9,10] although results are not always consistent. Weight at calving is dependent on both age at conception and growth rate and some studies have shown that a higher BW at calving has a positive effect on subsequent milk production [9,11,12]. This may, however, be less economical due to higher rearing costs [
A significant number of heifers (14% - 19%) are culled during their first lactation, with poor fertility a major cause of such high losses of these relatively young animals [19,20]. In addition to premature involuntary culling of infertile animals, poor reproductive performance increases the number of services per cow, increases the calving interval and reduces the number of calves sold per year. The effect of AFC on subsequent reproductive performance remains unclear. Calving heifers at 25 - 26 months had a tendency to lower subsequent calving intervals compared to both younger (<24 months) and older age groups at first calving [
Current data on the influence of AFC on the subsequent performance of dairy cows are thus not always in agreement. These differences may in part be caused by variations in genetics and management between different studies and countries. There is also little information available on any influences of AFC on subsequent survival, although this trait is of increasing importance to the dairy industry [
Recruited farms provided a range of management practices representative of those commonly encountered on UK dairy farms; no aspect of herd management was changed for the duration of the study period. All farms had a target AFC for maiden heifers of approximately 24 months. Heifer calves were recruited at birth and monitored until either removal from the herd, or 5 years of age and/or third calving.
The first dataset (Study 1) comprised 134 consecutively born heifer calves from a single primarily research farm (herd size, 460) born between August and December 2001. The second dataset (Study 2) consisted of 445 heifer calves recruited from the same research farm and 16 commercial dairy farms all keeping Holstein-Friesian cows (median herd size, 228; range, 105 to 540 adult cows) born between August 2003 and October 2004. The recruitment period for each individual farm generally lasted 1 to 4 months during the main calving season, with the aim of obtaining a cohort of approximately 25 consecutively born live heifer calves per farm (mean cohort size 23, range 12 to 30).
Heifers in Study 1 were weighed at birth, 3, 6, 9 and 15 months of age. Heifers in Study 2 were weighed at approximately 1 month (30 ± 0.9 days), 6 months (185 ± 0.8 days), and 15 months (447 ± 3 days). BW was measured using a portable weigh platform with Tru-Test loadbars connected to an Eziweigh 2 indicator (Ritchey Tagg, Ripon, North Yorkshire, UK). Weigh equipment was calibrated before each sampling session. In Study 2 these data were used to calculate the average daily gain (ADG) in BW from 1 to 6 and from 6 to 15 months. Body condition score (BCS) for all heifers was measured at approximately 1 to 2 weeks before estimated calving date using an 11-point scale from 0 (very thin) to 5 (obese) with increments of 0.5.
All animals in Study 1 were served exclusively by artificial insemination (AI). Heifers were initially served at the first observed oestrus and then again at any subsequent oestrus from the start of the service period (25 November 2002 until the end of March 2003). Pregnancy was confirmed by transrectal palpation. If an animal was not pregnant at the end of this first insemination period she was turned out to pasture and served again from July 2003 onwards. Following calving, animals were artificially inseminated at observed oestrus after a voluntary wait period of 60 days, and pregnancy was confirmed by transrectal palpation.
Heifers in Study 2 were served either by AI (n = 218), an intact bull (n = 110), with embryo transfer (n = 12), or with a combination (n = 29); animals were bred according to the individual farm breeding protocol. A successful insemination, resulting in a conception, was validated by a pregnancy diagnosis (if performed) or by a subsequent calving date and gestation length of 282 ± 14 days.
Nulliparous fertility was recorded as age at first breeding (AFB), services per conception (S/C), age at conception, first service conception rate (CR), and AFC (
Milk records were collected for each lactation; peak milk yield, 305 day yield, number of days in milk (DIM), and total milk yield. Figures were also calculated over the first 5 years of life including total milk production, total DIM and the proportion of days alive spent in the milking herd (DIM/days alive × 100%). The days alive were capped at the cow’s fifth birthday (1825 days) unless the cow had already died or been culled. Milk yield per day of life until third calving was also calculated for animals in Study 2, as total yield over 2 lactations/(cull date or date of 3rd calving—birth date).
Heifers were grouped retrospectively based on their actual AFC; <700 days (<23 months), 701 - 760 days (23 - 25 months), 761 - 912 days (26 - 30 months) and >912 days (>30 months). This grouping strategy was based on the spread in the AFC observed in this study, and because calving at 700 to 750 days is usually considered economically optimum [9,10]. All data were initially analysed using Statistical Package for the Social Sciences (SPSS version 18.0; SPSS Inc., Chicago, IL, USA), and were tested for homogeneity of variance before analysis. Categorical data such as perinatal mortality at calving (0 = alive calf, 1 = dead calf) were compared between the four AFC groups using Chi-square analysis. Continuous data were analysed using one-way ANOVA for normally distributed data or non-parametric Kruskal-Wallis tests for data not normally distributed. For Study 2, traits at P < 0.1 were further investigated using a linear mixed effects model in SPSS, with posthoc LSD tests, including farm as a random effect and AFC as a fixed effect. Survival data were analysed using Kaplan Meier Survival Analysis, with CoxRegression used for further comparison between AFC groups. Data were considered statistically significant if P values were ≤0.05.
The data used in these analyses were observational, so the results are associations and cannot be used to imply causality. Of the 134 calves recruited at birth in Study 1, 17 (13%) failed to reach first calving. In Study 2, 51 of the 445 calves recruited (11%) failed to calve at all due to mortality or culling during the rearing period. In addition, six heifers in Study 2 aborted at 226 ± 12 days pregnant and started their first lactation at 698 ± 20 days. No further information on these animals is given here.
The mean AFC for all 117 heifers in Study 1 was 764 ± 8 days (range 662 to 1095), and for the 388 heifers in Study 2 it was 796 ± 6 days (range 636 to 1529). The mean AFC values were very similar between studies for the 23 month, 23 - 25 month and >30 month AFC groups (
In both studies, heifers in the youngest AFC group (<23 month) had the best fertility, with most heifers conceiving to first service (93% in Study 1, 84% in Study 2,
Values are mean ± SEM; within rows a > b > c > d using ANOVA for Study 1 and a mixed model for Study 2; †Analyzed using Chi square; +Includes animals served by AI only.
BW measurements during rearing for animals in both studies are given in
Data on fertility measures in the first lactation are summarized in
Values are mean ± SEM; within rows a > b using ANOVA for Study 1 and a mixed model for Study 2; NS = not significant.
Values are mean ± SEM; within rows a > b > c > d using ANOVA for Study 1 and a mixed model for Study 2; NS = not significant; #Not all recorded; †Analyzed using Chi square; +n = 2.
In Study 1 none of the fertility parameters measured in lactation 2 differed according to AFC group (
The overall relationship between the age at first calving and at subsequent calvings for all the animals in Study 2 is summarized in
The overall levels of perinatal mortality were similar between the two studies, 17% and 19%, for Study 1 and 2 respectively at first calving and 8% and 6% at second calving. Older calving cows in Study 1 produced more dead calves at first calving (7%, 16%, 12% and 55% per AFC group respectively, P = 0.011,
Values are mean ± SEM; within rows a > b > c using ANOVA for Study 1 and a mixed model for Study 2; NS = not significant; #Not all recorded; †Analyzed using Chi square.
Overall 59 cows of the 134 heifers recruited in Study 1 were still alive in the herd on their fifth birthday, representing 44% of those born, and 50% of those which reached first calving. In Study 2, 228 cows of the 439 recruited (excluding the 6 that aborted their first pregnancy) were still alive in the herd on their fifth birthday, representing 52% of those born, and 59% of those which reached first calving. Regardless of age, a total of 241 cows of the 439 heifers recruited in Study 2 calved for a third time (55%).
In Study 1, the number reaching third calving reduced from 64% of the <23 month AFC group to 0% of the >30 month AFC group (
Values are mean ± SEM; within rows a > b > c; NS = not significant; +Excluding short lactations <55 d; #DIM/days alive × 100%.
Of the 388 heifers calving for the first time in Study 2, 24 were not served during lactation 1 (culled after 219 ± 63 DIM, range 3 to 918 days), and 26 failed to conceive (7%, n = 26/364). Animals failing to conceive received a mean of 3.1 ± 0.5 inseminations (range 1 to 11). For animals which conceived (n = 338), the mean number of S/C was 2.5 ± 0.1 (range 1 to 18) but 22 of these failed to calve again. A total of 316 animals therefore calved for a second time. Of these, 30 animals were not subsequently served and 24 failed to conceive (8%, n = 24/286) despite receiving a mean of 4.6 ± 0.7 inseminations (range 1 to 13). For animals which did conceive (n = 262), the mean number of S/C was 2.4 ± 0.1 (range 1 to 17) but 21 failed to calve again. A total of 241 animals therefore calved for a third time. As in Study 1, as AFC increased beyond 25 months proportionately fewer animals in Study 2 achieved a third calving (
There were relatively few differences in milk production in the first 2 lactations according to AFC category. In Study 1 older calving cows performed worst, with the lowest peak milk yield (PMY) in the >30 month AFC group in the first lactation and only 1 cow completing a second lactation (
Values are mean ± SEM; within rows a > b > c using a mixed model analysis; NS = not significant; +Excluding short lactations <55 d; #DIM/days alive × 100%; †Calculated as total yield over 2 lactations/(cull date or calving 3 date—birth date).
When milk production was instead considered according to production over 5 years of life, then the benefit of a younger calving age became clearly apparent (Tables 6 and 7). In Study 1 cows with an AFC of <23 months spent 54% of their life to date in milk production with a total yield of 25,031 ± 1491 kg. In Study 2 all cows calving at <26 months had similar productivity, spending 45% - 46% of their lives in milk production with total yields of 21,072 ± 1400 kg and 22,477 ± 912 kg in the <23 months and 23 - 25 months AFC groups respectively. In contrast, animals calving at >30 months spent only 18% (Study 1,
The results of this study show that the animals which calved for the first time at 23 - 25 months outperformed their later calving compatriots in terms of fertility, milk production and survival over their first 5 years of life.
The mean AFC was 25.5 months in Study 1 and 26.5 months in Study 2 despite a target AFC of 24 months on all farms, confirming previous reports of delays in AFC in other countries. For example, AFC averaged 25.8 months in Ireland [
It is widely accepted that Holstein-Friesian cows have suffered reducing fertility over the past 40 years as genetic selection for milk yield has increased [
Growth rate is a major influence on heifer fertility. Heifers in Study 2 calving at >30 months were deemed too small to breed at 15 months, resulting from poor growth during the rearing period, and were not bred until on average 20 months of age. Growth rates vary considerably according to plane of nutrition. We previously reported differences in ADG between 1 - 6 months of 0.5 to 1.0 kg/d between farms [
Our results have confirmed previous reports that AFC affects fertility in the first lactation. Analysis of growth trajectories in dairy cows until the end of the third lactation show that animals continue to grow during this entire period, although growth rate slows once the animal reaches approximately 450 days of age [
Despite the variations in fertility, there were no significant differences in first lactation milk production between any of the AFC groups, other than peak milk yield in Study 1 which was lowest in the >30 month AFC group. This is an important observation because dairy producers often associate a younger AFC with lower first lactation milk yields. In the second lactation, the 26 - 30 months AFC group in Study 1 produced the lowest total yields and the youngest AFC group had the highest milk yields, while too few of the >30 months AFC group survived to make a valid comparison. In Study 2, in contrast, the 26 - 30 months AFC group produced numerically the highest total yields in both lactations but this difference was not significant when herd was considered. This suggests that genetic and/or management differences between herds were having a greater influence on yield than AFC. A number of previous studies have similarly found no adverse effects of lower AFCs on first lactation milk production [16-18]. On the other hand, others have reported that milk yields were higher in older animals at first calving [4,9]. A comparison of animals fed to calve at similar ages (24 - 25 months) but differing weights (540 or 620 kg) concluded that animals calving at lower weights produced less milk in the first but not the second and third lactations and were overall more economic to keep [
Previous studies have differed in their findings of calving difficulty experienced in relation to AFC. Fetopelvic disproportion is the main cause of dystocia in cows calving for the first time [
One of the key findings of this study was that the younger calving groups (<26 months) quite clearly produced the most milk per cow over 5 years of life associated with both more lactations per unit of time and a higher survival rate. Heifers do not become profitable until half way through their second lactation and we found that survival to lactation 2 was better for younger calving heifers (100% and 85% for Study 1 and 2, respectively). A number of previous reports also indicate that cows calving at a relatively young age have longer survival. For example, highest survival was achieved with an AFC of 25 - 26 months [
In conclusion, optimum performance in UK HolsteinFriesian cows over 5 years of life was achieved with an AFC of 23 - 25 months, as these animals performed well in terms of both production and fertility and so survived longer in the herd. They were also more likely to achieve 3 lactations; this is crucial for profitability, which increases as lactation number increases. Improving reproductive efficiency of heifers also increases profitability through lower rearing costs with no adverse effect on productivity after calving. Conversely, animals with poor heifer fertility also had reduced survival and productivity and are thus unlikely to repay their rearing costs.
The authors thank the farm staff and veterinarians who contributed to this project. The work was co-funded by DairyCo and Defra. Manuscript number VBS_00407.