REFERENCE : Selon Vieira-Neto A et al. Association among gestation lenght and health, production and reproduction in Holstein cows and implications for their offspring. J.Dairy Sci. 2017,100,3166-3181.

SOME PRECISIONS

In the USA, the length of gestation (DG) of heifers and cows is 277.8 ± 5.5 and 279.4 ± 5.7 days respectively (Norman et al. 2009).

This length of gestation depends on genetics (greater heritability from the sire than the dam, reduction of 9 days of DG by some sires), the sex of the calf (+ 1.1 days if male), twinning, the age of the dam and the season of calving (minus 2.8 days if calving in warm season; minus 3 to 4 days if thermal stress), the length of lactation (positive correlation).

The length of gestation has consequences on the risk of dystocia, milk production, neonatal mortality and puerperal metritis.

HOW DID THEY DO IT?

The study conducted in the USA (California) involved 3335 primiparous and 4909 pluriparous cows on two farms. All cows (milked 2 to 4 times a day and annual productions in 365 days of 12,050 to 13,100 kg of milk at 3.5% MG) were monitored for health and reproduction. Pathologies were recorded during the first 90 days postpartum and the risk of culling or death of the animal during the first 300 days.

Mean value of gestation length: 276 days ± 6 days (SD).

Three groups were formed

• Short duration (9.4%): (SGL) n = 762, avg = 266 d (256 to 269)
• Medium duration (76.4%): (AGL), n = 6,181, avg = 276 d, (270 to 282)
• Long duration (14.2%) : (LGL ) n= 1,152, avg = 285 d, (283 to 296)

SOME RESULTS

CHARACTERISTICS OF THE DURATION OF GESTATION

The DG of primiparous cows (274.1 ± 0.2 d) is significantly (P<0.01) lower than that of pluriparous cows (276.8 ± 0.2 d).

Hypothesis: the development and thus the lesser abdominal development of primiparous cows would induce a stress situation (Hypoxia) for the fetus which would trigger earlier parturition. The same would be true in case of environmental thermal stress or in case of gemellity.

In female calves (275.4 ± 0.1) the DG was significantly (P<0.01) lower than in male calves (276.7 ± 0.1). Similarly, in case of twin gestation (4.2% in total), there is a significant difference (P<0.05) between F/F (1.1% 274.3 ± 0.7d) or F/M (2.1% 273.6 ± 0.5d) than if M/M (1.0% 277.3 ± 0.6d).

Hypothesis: the longer gestation period in case of male calves would result from the longer time needed for the development of their adreno-hypothalamo-hypophyseal axis.

DURATION OF GESTATION AND FREQUENCY OF PATHOLOGIES (see Table 1)

In primiparous females, the % of neonatal mortality, placental retentions, puerperal metritis increases with decreasing gestation length (256 to 269). In pluriparous women, the % of dystocia, neonatal mortality, retained placenta, puerperal metritis increases with decreasing gestation length (256 to 269) or increases (283 to 296). The % of morbidity (manifestation of one and/or the other of the different pathologies identified – retention, dystocia, metritis, milk fever, displaced abomasum, respiratory pathology, lameness, mastitis) increases when the length of gestation decreases (256 to 269) or increases (283 to 296). The risk of culling during the first 300 days after calving increases as the length of gestation decreases.

Hypothesis: Any shortening of gestation length may be the result of a stressful situation for the fetus (hypoxia, hypoglycemia) inducing premature parturition. It is accompanied by pulmonary immaturity and a lower absorption of colostrum explaining the increase in neonatal mortality. The immune process of placental rejection and therefore expulsion would be inhibited in case of shortened gestation. This would result in an increase in the prevalence of retentions and therefore metritis. The negative effect of longer gestation is indirectly attributable to the increase in fetal size and therefore the risk of dystocia.

LENGTH OF GESTATION AND MILK PRODUCTION

In primiparous cows there is a decrease in daily milk production when the gestation period decreases (- 1.2 liters) or increases (-2.4 liters) compared to a normal gestation period (35.5 liters). In pluriparous cows, production decreased with the length of gestation (-2.8 liters) but increased with the length of gestation (+0.8 liters) compared to the average production of 41.6 liters).

Hypothesis: A reduction in the length of gestation is likely to modify the synthesis of prolactin and thus galactopoiesis, this synthesis being positively associated with that of estrone but negatively with that of progesterone. In addition, it is likely that the increase in the frequency of postpartum pathologies is not likely to favor milk production.

LENGTH OF GESTATION AND FERTILITY

No effect on the % of gestation at first insemination (37.1% to 40.7% for primiparous cows and 35.3% to 37% for pluriparous cows) despite the increase in pathologies. It should be noted, however, that cows with a reduced gestation period are less likely to be pregnant during the 300 days postpartum.

Hypothesis: In this case, these pathologies resulted in an increase in culling of cows before breeding.

LENGTH OF GESTATION AND FERTILITY OF DAUGHTERS

Compared to a normal gestation period of the dams (64.5%), the % of pregnancy of the daughters decreased significantly when the gestation period increased (60.3%) and increased when the gestation period decreased (69.3%). Furthermore, a change in the length of gestation results in a reduction in the % of heifers inseminated during the first 500 days after birth.

Hypothesis: these negative effects would be attributable to the increase in neonatal pathologies.