Twinning

[hillsdown]

Was thinking about this topic that has been discussed quite a bit lately and came across this study.

http://www.theriojournal.com/article/S0093-691X(04)00106-2/abstract

Spontaneous reduction of advanced twin embryos: its occurrence and clinical relevance in dairy cattle

Abstract
Twin pregnancies represent a management problem in dairy cattle since the risk of pregnancy loss increases, and the profitability of the herd diminishes drastically as the frequency of twin births increases. The aim of this study was to monitor the development of 211 twin pregnancies in high producing dairy cows in order to determine the best time for an embryo reduction approach. Pregnancy was diagnosed by transrectal ultrasonography between 36 and 42 days after insemination. Animals were then subjected to weekly ultrasound examination until Day 90 of gestation or until pregnancy loss. Viability was determined by monitoring the embryonic/fetal heartbeat until Day 50 of pregnancy, and then by heartbeat or fetal movement detection. Eighty-six cows (40.8%) bore bilateral and 125 (59.2%) unilateral twin pregnancies. Embryo death was registered in one of the two embryos in 35 cows (16.6%), 33 of them at pregnancy diagnosis. Pregnancy loss occurred in 22 of these cows between 1 and 4 weeks later. Thus, 13 (6.2% of the total animals) cows, carrying one dead of the two embryos, maintained gestation. Total pregnancy loss before Day 90 of pregnancy (mean 69 ± 14 days) was registered in 51 (24.2%) cows: 7 (8%) of bilateral pregnancies and 44 (35.2%) of unilateral pregnancies, and it was higher (P = 0.0001) for both right (32.4%, 24/74) and left (39.2%, 20/51) unilateral than for bilateral (8.1%, 7/86) twin pregnancies. The single embryo death rate was significantly (P = 0.02) lower for cows with bilateral twins (9.3%, 8/86) than for total cows with unilateral twins (21.6%, 27/125). By way of overall conclusion, embryo reduction can occur in dairy cattle, and the practical perspective remains that most embryonic mortality in twins (one of the two embryos) occurs around Days 35–40 of gestation, the period when pregnancy diagnosis is generally performed and when embryo reduction could be tried.

 

[regolith]

Raining here... (that's why I ain't outside drafting cattle)

So 1/7 of twin pregnancies are lost - 2/3 of those have no calf at all the remainder gestate a single.
If I read that right. 35 cows pregnancy death, 51 cows total pregnancy loss - I'm a bit lost there.

Was there a control group of cows carrying a single calf? Given that the stress if not the physical act of scanning may carry some slight risk of inducing abortion.

That's - interesting. I'm not sure what practical use though, unless cows are being scanned and identified as carrying twins at an appropriate stage.
Day 35 - 40 is almost borderline for detecting pregnancy, let alone twins, I would have thought. That's five - six weeks. Vets here don't like to scan at less than six weeks from last mating date.

 

[hillsdown]

Sorry, that was all the info the page had and the link is no longer viable. It looks like they just monitored twin pregnancies on the selected dairies, but have seen no data regarding single pregnancies. I did find this very interesting

1/7 of twin pregnancies are lost - 2/3 of those have no calf at all the remainder gestate a single

. and may explain why that one herd that a member posted about a few weeks ago had so many free martin heifers.

 

[regolith]

Yup, especially given the age of pregnancy at which the losses happen - I didn't know that, but it makes sense (most pregnancies that are lost are probably then or earlier).

I tried calculating back the age of those heifers, but was a year out from a severe summer/autumn drought in my area - his heifers would already have been born then. I wouldn't go past that as a reason, however - another poster has remarked on cows not cycling until it rained, and I've seen that effect in my herd as well following drought. Nutritional stress could be a factor in resorption.
Twinning rates? In my herd of 150 or so I was seeing no more than two or three sets a year, out of 280 cows I only recall one set (I could check). But I've heard of dairy farms getting up to 10% twin births - seems incredible. I don't know of any factors other than genetics/artificial hormones that can affect twinning rates.