Temperature
Temperature has been shown to have a significant impact on calf birth weight. Although using sires with low birth weight EPDs may reduce some calving problems, environmental factors are responsible for approximately 55 percent of calving difficulties. Calf birth weights can vary significantly from year to year even though the same genetics and management are used.
Several studies have shown that calves born in the fall weigh less than calves born in the winter and spring months. The increase in fetal weight during the cooler winter months is most likely because of increased nutrient intake from supplemental feeding by the cow. As the nutrient intake increases, nutrient flow to the fetus increases, which can result in increased growth rate.
A long-term study was conducted at the University of Nebraska to determine the effects of temperature on calf birth weight (Deutscher et al., 1999). The coldest winter during this trial resulted in an 11 degrees F difference in winter temperatures. This difference resulted in an 11 pound increase in calf birth weight. Their research concludes that calf birth weights will increase 1 pound, and calving difficulty will increase by 2.6 percentage points for each 1 degree F reduction in average winter temperatures.
Most fetal growth occurs three months prior to calving. Therefore, temperatures during this time period will have a greater effect on calf birth weight. When considering temperature, fall calving herds should have fewer calving problems and lower death loss than herds calving in the winter and spring months. Producers should avoid calving in the summer months. Research has shown that calves born from May through September have lower weaning weights than calves born in cooler months (Sprott, L-5381). High summer temperatures and humidity are stressful to young calves, which reduce their growth. In addition, forage quality declines throughout the summer and contributes to poor performance of calves born in the summer months.
Feeding
The effects of different feeding levels on dystocia and birth weight are summarized in Table 2. High feeding levels precalving had no significant impact on birth weight or dystocia. Reduced feeding levels, however, can actually reduce cow weight gain, decrease milk production, increase incidence of scours and, most importantly, decrease pregnancy rate (Table 3).
Developing heifers on a low nutrient diet has clearly demonstrated an increase in dystocia. This is primarily due to poor skeletal growth and therefore smaller pelvic areas. Research has demonstrated that heifers with a pelvic area of less than 140 cm2 have increased incidence of dystocia compared to their above-average contemporaries (Deutscher, 1990). Delaying the time of first calving until 3 years of age decreases but does not eliminate dystocia.
Table 2.Effects of precalving feed level and sex of calf on calving difficulty.
Precalvinga Feed Level
Number
AVG BW (lbs)
Calving Difficulty Percent
High (13.9 lbs TDN)
Male
16
64.2
50
Female
16
61.3
22
AVG
62.8
36
Low (7.5 lbs TDN)
Male
15
58.6
46
Female
15
58.6
34
AVG
58.6
40
a 90 days before calving.
Adapted from Bellows and Short, 1978.
Table 3. Effect of feed level on factors other than dystocia.
Feed Level
Cow Weight Change
Milk Production
% Treated for Scours
Pregnancy Rate
Low (9 lbs corn)
—
—
—
65%
High (15 lbs corn last 90 days
100 lbs
—
—
83%
Low (5 lbs corn)
(-142.2 lbs)
9.1 lbs
52
68%
High (12 lbs corn last 30 days)
(-21.8 lbs)
12 lbs
33.4
82%
Adapted from Bellows, 1978; Corah, 1978.
Overfeeding heifers causes internal fat deposition, which obstructs the pelvic canal. In a beef cattle operation, overfeeding is seldom a major contributing factor to dystocia. All managers, however, must balance between achieving maximum frame growth without allowing excessive fat deposits. Fat heifers will have high incidences of dystocia just as severely as underdeveloped heifers (Table 4). Feed heifers to calve with a body condition score of 5 to 6 (scale 1 to 9; 1 = emaciated and 9 = obese). These heifers will have a much better chance of producing a live calf with minimal difficulty and returning to estrus sooner than a cow in poor condition.
