*Dr. Al Kertz is a board-certified, independent dairy nutrition consultant with ANDHIL LLC based out of St. Louis, Mo. His area of specialty is dairy calf and heifer nutrition and management. To expedite answers to questions concerning this article, please direct inquiries to Feedstuffs, Bottom Line of Nutrition, 7900 International Dr., Suite 650, Bloomington, Minn. 55425, or email [email protected].

PREVIOUS studies have shown that weight at first calving affects milk production (Keown and Everett, 1986), that heat stress during the dry period affects calf bodyweight and subsequent milk production (Thompson and Dahl, 2012) and that daily gain during the first two months of age affects subsequent milk production (Soberon et al., 2012).

However, there are other questions and relationships that include: the impact of the intrauterine environment, epigenetic effects during the embryonic stage as well as after birth, the parental genome effect, the impact of the dam's level of milk production during the calf's gestation and level of nutrients available during gestation partitioned among growth, fetal calf development and milk production of first-calf heifers.

With these factors in mind, Kamal et al. (2014) evaluated environmental and dam factors — including age at first calving and level of milk production during gestation — that might be associated with birth size in Holstein calves.

A retrospective cross-sectional design was used, with data collected from four small dairy herds: approximately 70 cows in Flanders, Belgium, and a herd of more than 2,000 cows in Rostock, Germany.

These herds were selected because of their long, collaborative history with the Ghent University Ambulatory Clinic in Belgium, they used an official milk-recording system and artificial insemination program and they were on a veterinary health program that closely monitored both health and productivity.

The Holstein cows in this study had an average 305-day milk production of more than 20,700 lb. Various other feeding and management practices and qualifications for calves included in the study were detailed in the journal paper.

Calf measurements were: birth weight, next-day heart girth, wither height, diagonal length and body condition index (BCI), which was calculated as birth weight / (wither height x diagonal length).

The database included 540 records of first calving, with an average age of 24.0 + 2.2 months. Age at first calving was divided into four classes by months of age: 20.3 to less than 22 (very young), 22 to less than 23.5 (young), 23.5 to less than 25.5 (standard) and 25.5 to 37.3 (old).

Gestation length was restricted to 265-295 days, and the following were calculated: days open, lactation length, dry period and calving interval in cows.

The season of calving was divided into June 21-Dec. 20 (summer and fall) and Dec. 21-June 20 (winter and spring).

Some parameters for first-calf heifers and their dams are given in the Table. The researchers noted several times in the text that many of these values were similar to a prior study in which I was involved (Kertz et al., 1997). The primary difference between these two databases was that both first-calf heifers and cows were taller by about 1 in. in the Kamal et al. study.

Heifer results. Calves born to first-calf heifers had lower values than dams in bodyweight, wither height, heart girth, diagonal length and BCI. Correlations to birth weight were greatest for heart girth (r = 0.75, P < 0.001) and BCI (r = 0.71, P < 0.001). The level of calving assistance increased with birth weight (P < 0.001) and was negatively correlated (r = -0.22, P < 0.001) with the sire's calving-ease breeding value.

For heifers, birth weight was affected (P < 0.001) by calf sex, season of calving, gestation length, heart girth, wither height, diagonal length and age at calving. The mean birth weight of calves born to heifers was 90.9 lb., with male calves weighing about 5.7 lb. more than female calves. Calves were 11 lb. lighter when born following a short (265-274 days) gestation length and 5 lb. lighter when born following a medium (276-285 days) gestation length compared (P < 0.001) to calves born following a long (286-295 day) gestation length.

The birth weight of calves was lower when heifers calved at an older age (P < 0.001), at a very young age and at a standard age compared to calves born to young heifers.

Cow results. Of the variables considered in the offspring birth weight model for cows, the calf's sex, season of calving, gestation length, parity dry period and cumulative milk production during gestation from conception to drying off were significant, but those factors explained only 26.2% of the variation in birth weight of the calves delivered by cows.

Male calves weighed 7.7 lb. more than female calves at birth. Calves born in summer and fall were 2.5 lb. lighter than calves born in winter and spring (P < 0.001). Calves born after short and medium gestation lengths were estimated to be 11 lb. and 5.5 lb. lighter than those born after a long gestation length (P < 0.001).

Calves born to second- and third-parity cows were 2.2 lb. heavier than calves born to older cows (P = 0.005).

The study found a number of other key observations:

* Since calf sex, season of calving, gestation length, parity and heart girth/wither height/diagonal length of the dam and dry period were significantly associated with calf birth size, this suggests that the uterine environment has a greater influence on size at birth than paternal genotype.

* Looking at the various factors affecting calf birth size, calves with a lower birth weight were also smaller in heart girth and BCI and were proportionally thinner.

* Male calves, which were larger than female calves, were also carried 1.3 days longer than female calves. Kamal et al. thought this was related to female fetuses being genetically more insulin resistant. The extra 1.3 days of gestation can account for a birth weight difference of 2.2 lb. This may be because first-calf heifers, which are smaller than cows, are still growing in their first lactation (12%, according to Kertz et al., 1997).

* Mature cows are more resistant to the anabolic effects of growth hormone and placental lactogen with lower insulin-like growth factor-1 and insulin concentrations. These factors can allow for more glucose availability, resulting in larger calves.

* Reduced dry matter intake during hotter spring and summer seasons, coupled with a longer photoperiod, leads to increased plasma prolactin and milk production, which can reduce nutrients for the fetus. Heat stress during late gestation is also associated with reduced uterine blood flow and reduced placental function.

* Heifers that are still growing at a younger age partition more energy not only to their developing fetus but also to their own growth. In this study, calf birth weight was curvilinear with the age of heifers at first calving. So, very young and older heifers have calves of smaller birth size than those that first calve at young and standard ages.

* A poor uterine environment associated with lower glucose, insulin and insulin-like growth factor-1 levels in high-producing persistent cows may lead to reduced glucose to the developing embryo and fetus, with negative effects on calf birth weight.

* The birth weight of calves was greater for cows with medium (45-54 days) and long (55-275 days) dry periods than for cows with a short (3-44 days) dry period.

The Bottom Line

Growth of the pregnant heifer can limit fetal calf growth and size in the first pregnancy. The level and persistency of milk production can also limit fetal calf size and development. Very young and older ages at first calving can limit calf birth size, as can short dry periods.

The level of nutrition can impair calf birth size, maybe even more for first-pregnancy heifers and first-lactation heifers since both must apportion nutrients for their own growth and that of their fetal calf.

References

Kamal, M.M., M. Van Eetvelde, E. Depreester, M. Hostens, L. Vandaele and G. Opsomer. 2014. Age at first calving in heifers and level of milk production during gestation in cows are associated with the birth size of Holstein calves. J. Dairy Sci. 97:5448-5458.

Keown, J.F., and R.W. Everett. 1986. Effect of days carried calf, days dry and weight of first calf heifers on yield. J. Dairy Sci. 69:1891-1896.

Kertz, A.F., L.F. Reutzel, B.A. Barton and R.L. Ely. 1997. Bodyweight, body condition score and wither height of prepartum Holstein cows and birth weight and sex of calves by parity. J. Dairy Sci. 80:525-529.

Soberon, F., E. Raffrenato, R.W. Everett and M.E. Van Amburgh. 2012. Preweaning milk replacer intake and effects on long term productivity of dairy calves. J. Dairy Sci. 95:783-793.

Thompson, I.M., and G.E. Dahl. 2012. Dry-period seasonal effects on the subsequent lactation. Prof. Anim. Scientist. 28:628-631.

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