When heifers calve very young, there is a greater risk of stillbirth and lower first-lactation milk production. When heifers are old at calving, their fertility may be negatively affected and it raises their culling risk. Plus, there is the cost of feeding them to that age before you get any return. So what is the sweet age for first calving to maximize average lifetime production? To answer that question, researchers at USDA analyzed production, reproduction, and lifetime data along with genetic (relationship) data from 13.9 million Holstein, 1.2 million Jersey, and 90,400 Brown Swiss cows. (Isn’t the national dairy database great? That’s just cows who first calved from 1997 through 2015!) Genomic data from aba Jersey cow andnewborn calfout 205,000 of those animals were also used.

One of the first interesting results of this study was documentation of the significant trend toward younger ages at first calving (see Table 1). It’s been most pronounced for Jerseys.

Table 1. Percentages falling into each age-at-first-calving (AFC) category in 1997 and 2012. (Data condensed from Hutchison et al. 2017.)

AFC (months) Holstein

1997           2012

Jersey

1997        2012

Brown Swiss

1997        2012

18–22     7.9   33.5   18.5   65.2     3.0   12.8
23–27   66.8   58.3   64.2   31.1   53.5   59.2
28–35   25.3     8.2   17.3     3.7   43.5   28.0

Age at first calving may serve as an indirect indicator of general productivity and survivability, as lower ages at first calving correlate with higher lifetime production and fertility. That is, heifers capable of getting pregnant at younger ages may just be more robust animals in general. In order to capitalize on those individuals, one shouldn’t start breeding too late. The data support a target age of 21-22 months for Holsteins and Brown Swiss to deliver their first calves and 20-21 months for Jerseys. However, breeding at ages younger than 11-13 months is not recommended because younger heifers are more likely to have stillborn calves. The authors of the study suggest that AFC be incorporated in bull selection indexes, which would enable population-level selection for an AFC that increases profitability.

the article: Hutchison et al. Genomic evaluation of age at first calving. Journal of Dairy Science. August 2017. 100:6853–6861.

two hornless calves on mowed grass
Hornless calves (named Buri and Spotigy) produced by gene editing. Photo from Carlson et al. 2016 Nature Biotechnology 34:479-481

We know that hornless cattle are safer for people, their herdmates, and themselves. Unfortunately, the combination of polledness and elite genes for other, more critical traits (like milk yield and productive life) don’t often appear in the same animals. We could spend several decades using polled sires to introgress the POLLED allele (allele = version of a gene) into the broader dairy population, but we would sacrifice gains in other traits, because along with the POLLED allele, the calves would get other stretches of less desirable DNA. (However, the nice thing about the POLLED allele is that it’s dominant, meaning that only one POLLED allele is required. At that same location on the other paired chromosome, there can be the horned allele, but we would still have a polled cow.)

You may have heard of gene editing, particularly with a system called CRISPRs. These CRISPR molecules can be introduced into target cells and are capable of recognizing a particular stretch of DNA and cutting at that location. If pieces of DNA containing the desired sequence for that location (e.g., the POLLED sequence) are made available to the cell at the same time, the cell’s DNA repair machinery will use that “new” DNA to repair the break in the chromosome. Voila! That repaired chromosome now contains the DNA sequence we want at that location, instead of the sequence that was originally there.

This type of gene editing has been successfully done in cattle embryos. In this case, the researchers used a more primitive version of CRISPRs called TALENs, but they do the same thing. In embryonic cells from horned cattle, the targeted section of DNA on chromosome 1 was replaced with the POLLED DNA sequence. In this proof-of-concept experiment, clones were created from these cells. And they grew no horns! The rest of the DNA in these animals remained the same as it was in the original genetic source, only the horned/polled location was altered. The two bulls produced (pictured above) will be used in breeding experiments to confirm that their offspring will also be polled.

This precise gene editing technique could be used to introduce polledness into elite dairy sires. In one generation, we could nearly eliminate the need to dehorn/disbud calves. That’s assuming there are no regulatory setbacks regarding the gene editing technology. (Ah, the potential sticking point.)

If you’d like some additional explanation accompanied by video of the polled bulls—currently residing at UC Davis—Science Friday has that here. The paper can be found here on page 479 (Carlson et al. 2016 Nature Biotechnology 34:479-481). I’ve glossed over some of the details, so if you’d like any additional explanation, please post a question via the “Leave a reply” link or email me.