Range Beef Cow Symposium XX
December 11-13, 2007, Fort Collins, Colorado
Cow Condition and Reproductive Performance
Julie Walker and George Perry
South Dakota State University
Department of Animal and Range Sciences
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It has been understood for decades that reproductive performance is the most important factor affecting production efficiency of a cow-calf enterprise. To maintain a yearly calving interval (one calf every 365 days), a cow must re-breed in 80 to 85 days after calving. With the nutrient priority of beef cattle being body maintenance, growth, lactation, fetal growth, breeding, and body reserve according to Short et al. (1990) indicates that reproduction is low on the list. Body condition score at parturition has been implicated as the single most important factor affecting postpartum interval to estrus and pregnancy in multiparous cows.
BODY CONDITION SCORE
Body condition scoring (BCS) is an effective management tool to estimate the energy reserves of a cow. The most commonly used BCS system for beef cattle in the United States use scores from 1 to 9 (Table 1), with 1 being emaciated and 9 being obese (Whitman, 1975). Using BCS to evaluate cattle does not require any special equipment and can be conducted anytime during the year. Poor body condition is associated with reduced income per cow, increased postpartum interval, increased dystocia, and lower weaning weight.
Age of calf at weaning influences weaning weight more than any other factor. Therefore, producers have chosen to shorten the breeding season from 90 days to 60 days or even to 45 days. The length of time from parturition until the first estrus, referred to as the postpartum interval (PPI), is the main factor that determines if a cow will become pregnant during the breeding season (Wiltbank, 1970). In addition, fertility is decreased for the first 30 days after calving (Short et al., 1990) and the majority of cows experience a short estrous cycle (an estrous cycle of ≤10 days) following their first postpartum ovulation (Murphy et al., 1990). When short estrous cycles occur, the cow returns to heat before the body recognizes the presence of a fetus and pregnancy will not occur (Odde et al., 1980). This means that cows need to initiate estrous cycles prior to the start of the breeding season to become pregnant. Cow body condition is an excellent indicator of the potential of cows cycling.
IDEAL BCS FOR MATURE COWS
What is the optimum body condition score for mature beef cows? Lamond (1970) proposed the concept of a target BCS at calving. Numerous researchers have studied the minimum BCS for acceptable reproductive performance. Morrison et al. (1999) reported that pregnancy rates at 20, 40, or 60 days of the breeding season were not affected by prepartum BCS changes (BCS varied from less than 4 to greater than 7), but Dzulk and Bellows (1983), Richards et al. (1986), Houghton et al. (1990) and Morrison et al. (1999) reported that a BCS of 5 at calving seems to be the critical level affecting subsequent reproductive performance in mature beef cows.
Cow BCS at calving affected length of the PPI with thin cows (BCS < 5) exhibiting an extended PPI of over 80 days, which represents a postpartum anestrous interval 28 to 58 days longer than that exhibited by either moderately conditioned or fleshy cows (BCS > 5) (Table 2; Houghton et al. 1990). For optimum production (one calf per year per cow) cows need to maintain an acceptable PPI of 60 days or less.
Producers should also consider time of calving when they decide on a target body condition score at calving. Pruitt and Momont (1988) found that early calving cows can be slightly thinner than late calving cows simply because they have additional time to initiate estrous cycles prior to the breeding season (Table 3).
IDEAL BCS FOR PRIMIPAROUS COWS
The greatest single loss in potential calf crop is in the failure of cows to become pregnant during the breeding season (Wiltbank et al., 1961). Goehring et al. (1987) concluded that 2-year-old heifers needed to be at a BCS 6 at calving for a high probability of pregnancy during the following breeding season. Among primiparous beef cows, greater BCS at calving resulted in more cows in estrus and more cows pregnant by 40 and 60 days of the breeding season (Spitzer et al., 1995). Primiparous cows were assigned to one of two postpartum treatments: 1) moderate gain (0.98 lb/d) or 2) high gain (1.98 lb/d). Animals in the high treatment had a greater percent in estrus at 20, 40 and 60 days of the breeding season and their calves had heavier weaning weights compared to the moderate gain (Spitzer et al., 1995). Furthermore, Ciccioli et al. (2003) reported similar results with birth weights not affected by BCS at calving, but calves that suckled high treatment cows were heavier at the end of nutritional treatment, and the interval from calving to first estrus (normal luteal phase) was shorter for high than for moderate cows. Only 24% of moderate cows had ovulated and initiated a normal luteal phase before 80 days postpartum compared with 41% of high cows.
Research conducted over the past several years at New Mexico State University has looked at thin cows < 5 BCS that have maintained a 90% plus fall pregnancy rate within a 60 day or less breeding season. Typically their feed cost are less than $30 per year per cow but does not include cost of range forage. They are using glucogenic precursors to encourage nutrient repartitioning from lactation to synthesis of maternal tissues for maintenance, growth and reproduction by way of improved nutrient use.
Endecott et al. (2007) fed 2, 3 and 4 year-old cows for 65 days postpartum one of 3 treatments RUP0 no glucogenic potential, RUP80 80 g of propionate salt and RUP160 160 g of propionate salt. All treatments had similar levels of crude protein and ruminally undegradable protein. Supplementation ended at the start of the breeding season. Two-year-old cows fed RUP0 took longer to initiate estrous cycles than the other groups; however, as RUP0 cows age increased it took fewer days to return to estrus (Table 4). Increasing glucogenic precursor was beneficial on return to estrus for 2-yr-old cows. However, all treatment groups had above 95% pregnancy rates. Milk production showed a quadratic response to increasing supplemental glucogenic precursor; RUP80 produced the least amount of milk at 55 days postpartum (Table 5). Endecott et al, (2007) did not see any affect on weight loss or gain between groups.
What are the opportunities to change BCS to improve the probability of cows becoming pregnant? Houghton et al. (1990) found that thin cows gaining condition increased the probability of cows becoming pregnant, however, fleshy (fat) cows losing condition improved pregnancy rates (Table 6). The key to maintaining BCS for optimum reproductive performance is evaluating cows early. Wiltbank, (1982) illustrates the concept of weight gain necessary for cows of varying BCS prior to calving (Table 7).
Evaluating body condition at various stage of production may help to eliminate situations of high-energy density rations for pregnant cows. Blasi et al. suggest evaluating body condition at various stages of production and potential management strategies to ensure cows are in optimum BCS for reproduction (Table 8). Assessing BCS earlier allows for a slow rate of gain and potentially less expense. Changing a 1100 lb pregnant cow from BCS of 4 to 5 would require ADG of 0.62 lb/d over 120 days or 1.62 lb/d over 45 days (Buskirk et al., 1992). A BCS change from 3 to 5 would require 1.24, 1.63 or 3.31 lb/d over 120, 90 or 45 days, respectively.
In addition to getting cows bred within the desired breeding season. Research has shown that having cows calve early results in larger calves, more time to cycle and therefore more chances to breed during a defined breeding season. Pruitt and Momont, (1988) grouped cows as early calvers (first 21 day of calving season) or late calvers, the calves from early calving cows average 45 lbs heavier in September than the calves from the late calving cows (Table 9). Since most producers sell feeder calves in one lot on a given date, the calves born early in the calving season have the potential to be larger and generate more income. Let s say the price of 550 pound calves are $110/cwt and 500 pound calves are $115/cwt, you are looking at $606.10 for heavier (551 lbs) calves and $581.90 for lighter (501 lbs) calves. The price spread for heavier calves may change due to increasing amounts of corn going into the ethanol industry.
Kunkle et al. (1994) looked at the relationship of BCS, cow performance and income (Table 10). Lower BCS had lower pregnancy rate which translated into less income per cow exposed.
Body condition scores are an excellent indicator of reproductive performance. Evaluating cows/heifers early allows producers to change BCS as needed. Cows calving earlier in the calving season allows cows more time to cycle prior to breeding season, breed earlier and heavier calves at weaning. Glucogenic precursor in addition to protein supplements decreased the number of days to first estrus in 2-yr-old cows and may be a method to help cows in lower than optimum BCS.
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Murphy M.G., M.P. Boland, and J.F. Roche. 1990. Pattern of Follicular Growth and Resumption of Ovarian Activity in Post- partum Beef Suckler Cows. J. Reprod Fertil. 90: 523-533.
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