The Cow-Calf Manager

Livestock Update, February 2004

John Hall, Extension Animal Scientist, Beef, VA Tech

Post-Calving is the Most Difficult Period for Cows and Cattlemen

Management of the lactating beef cow prior to the breeding season has a major impact on the ability of cows to rebreed. As we discussed last month, cow body condition at calving is the primary affecter of rebreeding success. Cows that calve in body condition score 5 or greater have the best rebreeding percentages. However, positive or negative changes in body condition during the early lactation period affect rebreeding success. In addition, certain estrous synchronization products can enhance cyclicity as well.

Drastic Physiological change
Certainly calving results in dramatic changes in the cow. Lactation increases the nutritional needs of cows. Energy needs are 25% greater and protein requirements 40% greater than during late gestation. Diets for cows need to contain 58 to 60% TDN and 10 to 11% crude protein.

In addition to producing milk, the reproductive system of the cow undergoes great change. In about 20 days of calving, the cow's reproductive tract changes from an organ that held over 100 lbs of calf, placenta and fetal fluids to its normal size. Although ovarian follicles continued to grow and regress while the cow was pregnant, she has not ovulated for over 9 months. The hormones from the brain (LH and FSH) that control ovarian follicular development, egg maturation, and ovulation were suppressed during pregnancy. Now, the interplay of hormones that control the cow estrous cycle must be resumed. Normally, it takes about 45 to 60 days for these hormones to increase and cycles to resume. For first calf heifers and thin mature cows, the resumption of estrous cycles may not occur for 90 to 120 days after calving.

In addition to the physiological changes and increased nutritional demands on lactating beef cows, there are "built in mechanisms" that restrict the recurrence of estrous cycles and re-breeding. The cow is able to monitor two critical factors - presence of the calf/suckling and nutritional status. In order to resume cycles, the cow must "overcome" the negative effects of suckling and/or low nutrient intake.

Influence of the calf
For many years, researchers believed that cows nursing calves did not resume estrous cycles due to the increased nutritional stress of lactation. Nutritional stress is only part of the effect. When calves are removed from cows for only 48 hours, cows respond with an increase in the hormones LH and FSH which "jump start" cycles in non-cycling cows. This happens even before there is a significant change in body weight or nutritional status. It was also believed that stimulation of the udder caused feedback to the brain which prevented the release of LH and FSH. However, studies with mastectomized cows demonstrated that even though calves could not nurse just the presence of the calf could prevent resumption of cycles. So, it is the combination of nutritional demands from lactation, suckling stimulus, and presence of the calf that suppress the resumption of estrous cycles.

Influence of Body Condition Score Changes from Calving to Breeding
Although body condition score at calving has the greatest impact on cow reproduction, changes in body weight, and body condition score postpartum will also affect reproductive performance. Change in body condition score postpartum dramatically affect cows that calve at BCS £ 4. Low BCS cows that continue to lose weight and BCS after calving are unlikely to become pregnant during the breeding season. Thin cows that continue to lose BCS have a longer interval from calving to first heat (postpartum interval). This means a low percentage of these cows (0-40%) are cycling by the start of the breeding season. Often, it may take over 80 to100 days until these cows begin cycling. As a result of delayed cyclicity, thin cows losing BCS postpartum have low pregnancy rates, which are often 30 to 50 % lower than their well-fed counter parts.

Cows that calve in BCS > 5 are less sensitive to the effects of postpartum nutrition, but reproductive ability of cows losing weight after calving may be compromised. Interval from calving to heat is lengthened and pregnancy rate decreased in fleshy cows that lose weight postpartum. For example, researchers in Oklahoma reported an increase of 22 days in postpartum interval and a reduction in pregnancy rate of 14% in cows that calved at BCS 5.4 but lost 1 BCS before the start of the breeding season.

Producers often hope that feeding thin cows to increase BCS and body weight after calving will solve their reproductive problems. Unfortunately, once a cow has calved her metabolism has shifted to support milk production. Therefore, only a portion of the additional energy fed to postpartum cows is available to combat the effects of low BCS. Cows that calve at BCS £ 4 and are fed high energy diets postpartum usually have a 10 to 20 % reduction in cyclicity compared to moderate flesh cows that maintain their weight. A reduction in the percentage of cows cycling diminishes the chances high pregnancy rates. Occasionally, these refed cows have conception rates equal to cows maintained in better body condition, but this is rare.

First calf heifers are less responsive to attempts to feed them to gain weight after calving. First, these primiparous cows have a longer postpartum interval and are more sensitive to the negative effects of poor body condition on reproduction. Because they are growing in addition to lactating, enhancing dietary energy intake does not readily enhance reproductive performance unless gains are substantial. For example, heifers fed to gain 1 lb per day from calving to breeding had marginal reproductive rates even though they were gaining weight (Table 1). Heifers had to gain 2 lbs per day to have acceptable rebreeding rates. Most studies indicate that thin heifers that are refed during early lactation have lower pregnancy rates at the end of the breeding season compared to heifers that calve at BCS > 5 and maintain their body weight. Distribution of conception is also effected, as thin-refed heifers tend to breed later in the breeding season.

What does this mean to the beef producer?
Management of the postpartum cow from calving through rebreeding is crucial. Calving cows in good body condition (as we discussed last month) is the key. Calving in good body condition (BCS > 5 mature cows; BCS > 6 first calf cows ) provides a buffer to postpartum nutritional changes. However, cows in good condition must be managed nutritionally to maintain their body weight and condition whereas thin cows need to gain weight. Unless high quality second cutting hay or grazing is available, spring calving cows will require energy supplementation. Usually, four to six lbs of a feed such as corn gluten, soy hulls or dried brewers grains will meet the needs of lactating cows. However, producers should test their hay and work with their County Extension Agent or nutritionist to design an accurate and cost effective supplementation program.

Table 1. Effect of Postpartum Gain on Rebreeding of First Calf Heifers

		Percentage (%) pregnant by days of the breeding season
Rate of Gain	No. of heifers	20	40	60
Moderate (1 lb per day)	122	27	56	70
High (2 lbs. per day)	118	46	76	84

Adapted from Spitzer et al., 1995

First calf heifers need special nutritional treatment and should be fed separately from mature cows. Energy requirements for heifers are greater as they need to continue to gain weight and grow during early lactation and the breeding season. As discussed above, pregnancy rates of first calf heifers can be severely compromised if they do not gain 1.5 to 2.0 lbs per day. Depending on forage quality, supplements may need to be fed at up to 1.0 % of body weight or almost 10 lbs per heifer. To reduce supplementation costs, second cutting hay or round bale silage is highly recommended for first calf heifers.

Addition of fats, especially polyunsaturated fats, to postpartum cow diets may enhance reproduction beyond the increased energy they provide. We are currently researching the use of fats in diets for cows and heifers, and we will continue to keep you informed of the results. In the meantime, including fat up to a maximum of 5% of the diet certainly will not harm pregnancy rates and may enhance conception.

Removing calves for 48 hours at the beginning of the breeding season or estrous synchronization period is a good strategy for improving pregnancy rates in thin mature cows, first calf heifers, and late calving cows. Calf removal works best if calves are out of site of cows. Pregnancy rates and percentage of cows cycling can be increased by 5 to 15% with calf removal. However, considerable research (including research conducted at VT) has demonstrated only a limited advantage to calf removal in cows that calve in good body condition and maintain their body weight through rebreeding.

Another management option is to use a progestin such as MGA or a CIDR to "jump start" cows. Using these approved sources of progesterone either alone or as part of an estrous synchronization system will increase the number of cows cycling and conceiving at the beginning of the breeding season. For more information, on using these products contact your County Agent or Extension Beef Specialist.

Although we continue to increase the number of management options we have with postpartum beef cows, the old adage of "Ya gotta feed 'um to breed 'um" still rings true. With the poor quality hay crop in 2003, producers should test their hays and design their feeding programs based on sound science. Otherwise, breeding season 2004 may be a disappointment.

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