Intensive Grazing/Seasonal Dairying: The Mahoning County Dairy Program

1987-1991

OARDC Research Bulletin 1190

Chapter 6: Reproduction

Lactating Cows

Management of the cow herd in a seasonal dairy is somewhat unique. As was mentioned in Chapter 1, duties on a seasonal dairy are mostly sequential rather than simultaneous. At the Mahoning Project, the cows freshened as a group during an eight-week period. In fact, most of them were clustered within four weeks as a result of high first-service conception rates, averaging 72% for years two through five of the project. The reproductive management was concentrated into a short time period. Subsequently, nearly all the problems associated with parturition and metabolic illnesses of early lactation were also limited to a brief period of time. Breeding problems were handled very efficiently with relatively few veterinarian calls. Heat checking was simplified by the mutual state of reproductive status of the herd. Usually, several cows were in or near estrus at the same time. This tends to enhance reproductive functions in all the cows.

Grazing management promotes successful reproduction as shown in Table 6.1. The cows were outside on good footing for estrous behavior which was assisted by their general vigor resulting from grazing. They were taken to and from pasture twice per day - a pattern leading to group interaction and displays of estrous behavior. The elevated levels of key vitamins such as A and E in high quality pasture may have enhanced reproductive health.

Spring breeding dune) probably is favored by photoperiod extension that improves endocrine gland activity related to ovulation. Yet, as this herd was bred during the months of June and July when heat stress was often severe, reproduction must have been compromised by high ambient temperatures and cow stress.

Pressure on the manager of a seasonal herd derives from the requirement for a 12-month calving interval. Without it, the herd cannot be kept in a seasonal pattern. Of course, low involuntary culling rates are the key to this accomplishment. The 12-month calving interval cannot be economically viable if it requires excessive involuntary culling. Involuntary culling rates for the project were 40, 23, 22, 3, and 12%, respectively, over the five years for an overall average of 20%. Culling during the first year was accelerated by the desire to advance the calving period by more than a month. Late pregnant cows were regarded as involuntary culls.

Tools used to assist estrous behavior observation (heat detection) for breeding decisions included veterinary support weekly through the breeding period, milk progesterone assays, and chalk marking on the rump. Treatment for cows found to have cystic ovaries was selected by the veterinarian according to milk progesterone levels. With problematic cows, milk progesterone levels were tracked to evaluate responsiveness to drug therapy and to assess pregnancy. These strategies recovered several cows that might ordinarily have been lost to infertility.

Reproductive performance for the first year of the project was the poorest overall. The seasonal timing was retarded a month by the difficulty of finding suitable heifers in a rapid initiation of the project with many preparations to be accomplished. The desire to move the freshening period forward led to heavier culling than would ordinarily have been needed. Also, nutritional adjustments were needed as part of the learning process about managing a seasonal, intensively grazed herd. In the first year, cows, all two-yearolds, were quite thin at breeding time. Subsequently, management improved to the advantage of reproduction. A different milk progesterone assay kit was used with improved confidence. Especially during the last two years, the nutritional program was changed with positive results. Cows experienced improved pregnancy rates, they were in heavier body condition as reflected by higher body condition scores, and the lactation curves had better persistence.

During the project, the driest year (1991) and an extremely cool and wet year (1989) occurred. There was also a nearly ideal year for grazing in 1990. Reproduction was best that year. Essentially, the program has worked under all probable conditions and extremes. The Jersey and Holstein cows appeared to be equal in reproductive performance. Since all normal heifer calves were successfully reared, excess replacements were available during the last three years of the project. Increased production culling was then possible.

The comparative advantage of reproduction in 1990 at an excellent 97% pregnancy rate with 84% firstservice conception causes the suspicion that the poorer statistics for 1989 and 1991 were related to inadequate nutrition. The weather adversity of those two years forced the feeding of hay along with poor pasture at breeding time. Yet, even then, the reproduction rates were admirable considering the short breeding period and the summertime season. There was no apparent relationship between production level and reproduction in this herd. Milk production steadily increased each year as the genetic base improved, the herd became more mature and the nutrition program was refined.

The average calving intervals shown were calculated on the Dairy Herd Improvement (DHI) report prior to the December culling of late-pregnant cows. Once those cows were removed, the average calving intervals fell to 11.6,11.9,12.0, and 12.1 months. Replacement heifers were bred to freshen during the third week of the calving period.

Average days open for 1988 through 1991 was 89, which is consistent with a high pregnancy rate on a 12month calving interval with low average numbers of services per conception (1.44). With these capabilities, it was not prudent to begin breeding cows before 60 days of lactation unless they were very late in the calving period. Consequently, the average daysin-milk (DIM) at first service was 72. This promotes greater production for a cow through delay of pregnancy and its retarding effect on lactation persistency (Bath et al., 1978; Erb et al., 1952; Schmidt et al., 1974).

The average DIM increased by 50 days from the first to the last years of the project, as the breeding period was pushed forward each year to arrive at the desired calving dates to make the most of the pastures. The herd was turned dry before Christmas as a convenience. Another twelve days of milking could easily have produced a normalized 305 day average lactation length.

Cows that were physically sound and worthy milk producers were bred as many as five times before being given up as beef cull candidates. When such cows became pregnant beyond the ideal eight-week period, they were marketed on the herd turn-dry day as dairy stock. On average, they brought $130 more salvage value than the beef culls.

All cattle, i. e., cows and yearlings, were artificially inseminated (AI). The semen was purchased from a single bull stud. Semen from the young sire group was used exclusively until the last year when the cattle would be leaving the project in pregnant condition.

Heifers

A slight difference in semen selection for heifers compared to the cows lay in the choice of proven calving ease Holstein bulls. Like the cows, all heifers were bred AI even though they were on pasture. This was facilitated by using ovulation synchronization with Norgestomet (Synchromate B)¹. Thus, pregnancies were tightly clustered in the desired breeding period and the freshening period was subsequently compacted within about four weeks. Typically, each heifer was serviced on signs of standing heat within four days of removing the synchronizing implant from the ear. Heifer reproduction data are presented in Table 6.2. The first service conception rate was 67% over the four years of breeding yearlings. Services per conception averaged 1.36. Over the four years, only one of 60 left the herd not pregnant within three services. Only two required three services for pregnancy. Later, these were sold as springers at a dairy auction because they were due to calve later than acceptable for this herd.

(1) Source: Sanofi Animal Health, Inc., 7101 College Blvd, Suite 610, Overland Park, KS 66210.