A Breeder’s.... Veterinary Perspective Improving Reproductive Performance in Your Herd

Starting With Heifer Management

By Colin Palmer, DVM

A widely accepted management goal is to have heifers birthing their first calves by 24-months of age. Add a few months on to grow her first calf and you come up with at least 2½ years of development and maintenance costs that are now equivalent to the net revenue of 6 commercial calves. Recouping those costs points to longevity of the cow herd being a key variable in the profitability equation. There are almost no good financial reasons to keep an open cow as she will only have the additional burden of her current-year maintenance costs added to her costs of development and will not break-even. A nation-wide survey of American cow-calf producers conducted over 15 years ago pointed to non-pregnancy as the number 1 reason for culling with age/ bad teeth coming in a close second position. Together they accounted for 2/3rds of the culls. Cow productivity peaks at about 5 to 7 years of age and drops off more sharply after 10 years. Culling open cows is always a good thing to do but if more than 10 to 15% of your open culls are young cows then there is a need to consider what went wrong. Low level attrition in a cow herd can be due to a number of problems including difficult calvings; c-sections; prolapses; temperament; udder problems, cancer eye; poor-doing calves and other issues such as bad feet which when combined accounted for an additional 15% of the reasons for culling in that survey; often before these cows reach their prime.

Careful selection and development of replacement heifers is vital to improving herd productivity. The chances of establishing a pregnancy at a heifer’s first heat, or pubertal estrus, are about 20% less than that of her already cycling herd mates. Heifers need to be cycling regularly at the beginning of the breeding season which means that puberty should have occurred at least 2 months before turnout. By the 3rd or even 4th heat conception rates improve greatly. Pubertal status at the onset of the breeding season is one of the most important factors affecting conception rates. Several years of experience breeding heifers in timed artificial insemination (AI) programs has shown me that a proportion of heifers will not have reached puberty in time to be feasibly included. Although, timed AI protocols utilize treatments to encourage follicular recruitment, development and ovulation results show that pregnancy rates are much better in animals that are cycling and display estrus behaviour before breeding. Follicles produce estrogen that is responsible for causing estrus behaviour. If follicles are too small, they will produce very little estrogen, therefore, estrus behaviour is unlikely. Furthermore, small follicles may or may not respond to ovulation-inducing therapy and those that do have been associated with poor conception rates.

Several factors influence the onset of puberty in heifers. Heifer age and body weight are the most important, direct determinants. Breed, cow family and sire, including sire’s scrotal circumference are indirect factors that influence the age and weight when puberty will occur. Amongst the fertile, maternal breeds it is not uncommon to hear of heifer calves becoming pregnant while still nursing the dam. Within a herd these cases typically represent only a fraction of the heifer calves, and they are almost invariably the oldest and biggest. However, any underfed heifer including these precocious heifers will reach puberty later, at an older age, than her betterfed contemporaries despite her genetic makeup. Since body weight can have such a profound effect on age of puberty a good rule of thumb is that heifers should be developed to 2/3rds (about 65%) of her predicted mature weight at the time of breeding to ensure that they have reached puberty. Knowing what your mature cows weigh is very important. Frame sizes, hence mature weights, can vary by several hundred pounds within breed.

Selection of sires with larger scrotal circumferences translates to earlier puberty and increased lifetime fertility in female progeny. Earlier onset of puberty led to the idea that heifers could be grown to 50 - 55% of mature weight to save on feed costs. Initially, most research seemed to support this theory, but careful examination of the results suggests otherwise. In one study fewer heifers developed to 53% of mature weight were cycling at the beginning of a 45-day breeding season compared to their herd mates developed to 58%, but there was no difference in pregnancy rates. However, another study compared heifers with a more substantial, 10%, difference in is an Associate Professor of Theriogenology (Animal

Western College pre-breeding weights. Heifers developed to 55% and 65% of mature weight at breeding had no differences in pregnancy rates at the end of an 80-day breeding season, but it was noteworthy that significantly more heifers developed to 65% of mature weight were pregnant within the first 45 days of the breeding season and that the lighter weight heifers took longer to begin normal estrous cycles following their first calves. Not surprisingly in yet another study, substantially fewer heifers at 50% of mature weight established pregnancies in the first 30 days of the breeding season compared to herd mates developed to 55%. Knowing how big your mature cows are is an important consideration. Five percent of a 1400-pound mature weight is only 70 pounds – 65% of 1400 pounds is 910 pounds, whereas 55% is just 770 pounds. Targeting your heifer development program toward a 65% of mature weight at breeding will ensure that more heifers will get pregnant early in the breeding season and have a greater chance of remaining in the herd as productive females. Very young heifers that are not likely to make 50% of mature weight by breeding season are not viable replacements. Using this example, the nonviable heifers would weigh 700 pounds or less.

Reproductive tract scoring by transrectal palpation is a simple yet very worthwhile procedure; especially, when selecting heifers for timed AI programs. Ultrasonography may seem like the more modern and technologically advanced way to perform a reproductive exam, but uterine tone and texture can only be appreciated by touch. Assessment of the size and texture of the uterine horns and palpating the ovaries for follicles and corpora lutea (CLs) will reveal those heifers that are cycling. To ensure success, retain the cycling heifers for breeding and remove the others.

When is the best time to administer vaccines to heifers prior to breeding? For many, a convenient time is when heifers are being put through the chute for breeding programs - but is this safe? Recommendations tend to vary depending upon whether a modified live or a chemically altered, inactivated (killed) virus vaccine is being used and whether it is the first vaccination being administered to naïve animals, or a 2nd (booster) vaccination. Modified live virus vaccines have been shown to have a negative effect on corpus luteum development, the ovarian structure responsible for maintaining pregnancy, that may last for as much as 2 months. In a study which included non-vaccinated control heifers, heifers vaccinated with the same killed virus vaccine 36 and 8 days before breeding, heifers vaccinated 8 days before breeding with the killed virus vaccine and heifers vaccinated 8 days before breeding with the modified live vaccine it was evident that vaccination with a MLV vaccine just 8 days before breeding had a negative effect on pregnancy status. In a comparison of MLV and killed virus vaccine administered to heifers 30 days before a timed AI heifers vaccinated with the MLV had lower pregnancy success following AI than those vaccinated with killed virus vaccine or heifers not vaccinated at all. Chemically altered, inactivated virus vaccine has been known to impart a weaker immune response than MLV vaccines but have been proven to impart enough of a response to provide adequate protection from abortion during the subsequent pregnancy. Many studies comparing the effect of pregnancy status suffer from having too few animals to show a real difference. A very robust, welldesigned study where cows received a booster vaccination of either a MLV vaccine or a chemically/ altered inactivated virus vaccine administered between 27 to 89 days before AI showed that pregnancy results were significantly better in the killed virus vaccine group amongst animals vaccinated with either vaccine less than 6 weeks before breeding; however, the negative effect disappeared when the MLV vaccine was administered six weeks or more before AI. Therefore, sound advice is to complete your MLV virus vaccine protocol 6 weeks to two months before breeding. If this is not possible use of a chemically altered, inactivated vaccine is a very good way to prevent pregnancy loss in the herd.

Reference: Perry G. Improving reproductive management in cow herds. Clinical Theriogenology. 2022; 14: 174-192.