The Cow-Calf Manager

Livestock Update, January 2006

John B. Hall, Extension Animal Scientist, Beef, Virginia Tech

Precalving Period

The most critical period in the production year is the last 60 to 90 days before calving. Not only are dramatic changes occurring in the cow, but this period also sets the stage for reproductive success or failure in the months that follow. A sound understanding of this period is important to proper management of the cowherd.

Tremendous Change
Considerable tissue growth and change occur during the precalving period. As a result, nutrient needs of the cow are greatly increased. A majority of fetal (calf) growth occurs during these last 90 days. At the start of this period, the calf weighs less than 50% of its final weight. In addition to rapid fetal growth the uterus, placenta and surrounding fetal fluids must also increase in size during this time.

Mammary development in preparation for lactation is also occurring. Proper nutrition and hormonal support is essential during this period for adequate milk production after calving. Recently, we have seen some herds in which milk production was greatly decreased due to grazing highly endophyte infected fescue during the precalving period.

While increase in cow weight and mammary development are visible signs of physiological change during the precalving period, there are several unseen changes as well. Colostral (first milk) antibodies are produced, and restrictions in nutrition during this period will result in poor colostrum quality. Poor colostrum quality is associated with lower calf survival.

In addition, there are effects on the reproduction system that occur during the precalving period that impact the resumption of reproductive cycles and fertility after calving. Although not fully understood, nutrition during the precalving period affects reproductive hormone production from the brain as well as follicular (egg and associated structures) development on the ovary. It appears that these reproductive effects are a result of both nutritional reserves and types of nutrients available during the precalving period.

Impact of Body Condition Score at Calving
One of the best understood effects on precalving nutrition is on energy reserves at the time of calving. Cows in body condition score BCS 5 (1 = emaciated to 9 = obese) or better at calving have fewer days to first estrus and increased pregnancy rates (Rasby et al., 1981, Wettemann et al., 1981). Cows calving in BCS < 4 had a 9% to 29% lower pregnancy rate compared to cows calving at BCS > 5 (Makarechain and Arthur, 1990; Selk et al., 1988). Based on data from the literature, hypothetical pregnancy rates for cows of various body condition scores are illustrated in figure 1. Research from Oklahoma indicates that changes in BCS between 4 and 6 have a greater impact on pregnancy rate than changes in BCS above 6 or below 4 (Selk et al., 1988). In other words, little improvement in pregnancy rates is seen when cows calve in BCS above 6 while pregnancy rate does not get much worse below BCS 4.

In addition to the overall decrease in pregnancy rates, cows calving at BCS ² 4 that conceive become pregnant later in the breeding season (Table 1). As a result, these cows calve later in the calving season the next year. Late calving cows are more likely to fail to conceive during a controlled breeding season. Calves born late in the calving season will be lighter at weaning than calves born early in the calving season. At weaning, calves will be approximately 35 to 40 lbs. lighter for every 21-day delay in calving (Lesmeister et al., 1973).

Table 1. Effect of Body Condition Score at Calving on Cumulative Pregnancy Rates

		Day of the Breeding Season
	BCS	20 d	40 d	60 d
Mature Cows (Richards et al., 1986)		Cumulative % Pregnant
	<4	41	67	84
	> 5	51	79	91
First Calf heifers (Spitzer et al., 1995)		Cumulative % Pregnant
	4	27	43	56
	5	35	65	80
	6	47	90	96

First calf heifers are even more sensitive to the effects of BCS at calving on pregnancy rates. Dramatic decreases of 40% to 50% (Figure 2) occur as heifers drop from BCS 6 to BCS 4 (De Rouen et al., 1994; Spitzer et al., 1995). In contrast to mature cows, heifers exhibit a significant decrease of approximately 16% in pregnancy rate between BCS 6 and BCS 5. Therefore, the optimum BCS at calving is 6 or 7 in heifers.

Figure 2. Effect of Body Condition Score at Calving on Subsequent Pregnancy Rate in First Calf Heifers

Limited data indicates that cows that calve at BCS > 7 and heifers that calve in BCS > 8 may have impaired reproduction during the breeding season (Richards et al., 1986; Houghton et al., 1990). One should be cautious in drawing any conclusions about "fat" cows, as the numbers of cows with BCS > 7 in these studies were limited. In addition, it is not always clear if cows were in high BCS due to nutritional manipulation or physiological factors. Efforts should be made to keep cows in the BCS 5-7 range from an economic standpoint as well as a possible reproductive effect.

Management During the Precalving Period
The most important consideration during this period is nutrition. Cow energy requirements increase by 25% from immediately after weaning, and protein needs increase by 10%. So, better quality forage is needed during this period. Demands for calcium and phosphorus increase because of fetal growth. In addition, high levels of copper, selenium, zinc as well as vitamin A & D are needed for proper fetal body and immune system development. Therefore, a high quality mineral and vitamin supplement is recommended.

Cows will need to gain at least 100 lbs during the precalving period to accommodate this tissue growth without losing body condition. Without fail, cows should be body condition scored at least 90 days before calving. Cows that are thin (BCS ²4) or young cows should be fed separately from the mature cow herd.

Producers also need to avoid the old wives tale about restricting feed during the precalving period will reduce calving difficulty. Research at several universities including Virginia Tech demonstrated that pregnant heifers that are fed a low energy diet do not have less calving difficulty. However, the same research indicated that poorly fed heifers had more stillborn calves, weaker calves, lower calf survival, and poor colostrum. Heifer need to gain 2 to 2.5 lbs per day to continue to grow while the fetus is also growing.

The effects of severe winter weather should not be ignored. Extended (more than 5 days) cold, windy, or wet weather can increase cow energy requirements by 10 to 20%. In most cases, cows will increase hay consumption if hay quality is good. Otherwise, additional supplements should be fed during extreme weather. Normal feeding can resume when the weather breaks.

Hay needs to be tested for nutritional content especially this year as Extension agents and Specialist have emphasized all fall. Most first cutting hay from 2005 is inadequate in energy content for cows during the precalving period. Most hay will have to be supplemented. Feeding cows 3 to 4 lbs per cow per day of corn gluten feed, soyhulls or a corn-soybean mix should offset poor hay quality. To develop the most accurate and economical supplement for your herd, producers should contact their County Extension agent or nutritionist.

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