What are the impacts on the cow-calf system?
By Colby Redifer and Allison Meyer
Nutrient requirements increase dramatically in the late gestation beef female to allow for proper growth and function of the uteroplacenta, fetus, and mammary gland. Even in well-managed herds, it is possible for cows and heifers to be nutrient restricted during this time due to challenges of low forage quality or availability and environmental stress brought on by cold (spring-calving) or heat (fall-calving). For heifers, the added nutrient requirements needed for growth pose an even greater challenge during their first pregnancy.
A growing body of research demonstrates that nutrient restriction during pregnancy can have long-lasting effects on calves resulting from impaired development before and after birth. This is not surprising, as calves depend on their dams for all nutrients before birth, and the majority of nutrients pre-weaning.
Late gestational nutrient restriction research generally associates decreased nutrient delivery during gestation with programming fetal growth and development, resulting in poor postnatal outcomes. However, the effects gestational nutrient partitioning may have on the mammary gland and therefore postnatal nutrient delivery (milk production) is far less understood in beef cattle. It is also generally unknown how poor nutrition during one pregnancy impacts cows during their next pregnancy and lactation.
Experimental modelTo determine the effects of late gestational nutrient restriction in first-parity females, fall-calving Hereford-SimAngus heifers bred to a single sire were used in this study. Heifers received either 100% (control; n = 12) or 70% (nutrient restricted; n = 13) of metabolizable energy and metabolizable protein requirements for maintenance, pregnancy and growth from d 160 of gestation to calving.
Heifers were individually-fed using Calan gates and diets were based on low-quality chopped hay. Based on expected individual intakes, heifers were supplemented with whole corn, dried distillers grains, and soyhull pellets to meet targeted energy and protein.
After calving, treatments were terminated and all females were fed to meet energy and protein requirements for maintenance, lactation, and growth. Females continued to be fed individually in Calan gates to ensure individual nutrient intakes remained similar and to prevent calves from having access to feed other than milk.
At 5 months post-calving, cow-calf pairs were co-mingled and group-fed until calves were weaned at 8 months of age. Post-weaning, calves were backgrounded and then placed into the feedlot for finishing after a common number of days on feed.
OutcomesMajor effects of late gestational nutrient restriction on dam and offspring outcomes are shown in Figure 1. Nutrient restricted females lost body weight and condition and had less nutrients in maternal circulation during late pregnancy.
Post-calving, nutrient restricted dams weighed 141 lb less and were 2.0 lower body condition score. Nutrient restricted dams had lower maternal heart rates during pregnancy and altered placental growth, yet total uterine blood flow and placental weight were maintained and ensured calf birth weights similar to control dams. Fetal presentation at calving was normal for all control births, while 23.1% (3 of 13) of nutrient restricted dams had calf malpresentations.
It should be noted that when we conducted a similar study using the same experimental model the following year, calf birth weight was reduced by 15% and placental weight by 17%, but uterine blood flow remained unaffected. In combination, these results demonstrated that late gestational nutrient restriction is inconsistent in reducing fetal growth, which appears to be driven more by placental size than uterine blood flow.
While calf birth weight and gestation length were unaffected in this study, nutrient restricted dams had less vigorous calves that were slower to stand and 40% less colostrum yield. Colostrum of nutrient restricted females was more concentrated with immunoglobulin G, and calf 48-h serum IgG indicated that calves of both treatments had successful passive transfer of immunity. Colostrum of nutrient restricted females had less total lactose, but similar total protein and fat compared with control females. There was no neonatal calf death loss due to treatment; however, this was likely because females were monitored continuously during the calving season for research purposes.
Even when females were fed to meet their energy and protein requirements during lactation, late gestational nutrient restriction reduced milk yield by 15% and decreased milk protein concentration during the first 150 d of lactation. Lower milk production was partially explained by a 19% reduction in blood flow supplying the mammary gland during this time.
By d 21 of lactation, metabolic status of previously nutrient restricted dams had returned to be similar to control dams. During lactation, previously nutrient restricted females gained maternal weight faster and increased body condition. Still, previously nutrient restricted dams had not fully recovered by weaning, as they weighed 37 lb less and were 0.7 lower body condition score. Females in both treatments were similarly successful in rebreeding for a 2nd calf and comparable to industry averages for conception rate.
Less total lactose, protein, and fat provided by the milk resulted in calf body weight diverging by d 42 of age. Calves born to nutrient restricted dams remained smaller through weaning, when they weighed 59 lb less than control calves. Interestingly, calf metabolic status pre-weaning was not as severely altered as expected considering milk nutrient and calf growth differences.
At harvest, calves born to nutrient restricted dams had lower dressing percentage, but improved yield grade and backfat thickness. Final feedlot body weight, hot carcass weight, ribeye area, and quality grade were not affected by maternal treatment.
Outcomes in the second pregnancy and lactationFemales were managed together and followed through weaning of their 2nd calf to determine if there were carryover effects during the subsequent parity (Figure 2).
Previously nutrient restricted dams still weighed roughly 100 lb less and were 0.6 lower in body condition score when differences were averaged across multiple points during late gestation and lactation. Uterine blood flow, placental size, and calf birth weight were unaffected by nutrient restriction during the first pregnancy.
Colostrum yield, nutrient composition, and immunoglobulin G concentrations had recovered and were similar between treatments. Milk yield and nutrient composition also recovered; however, the reduction in milk protein concentration in previously nutrient restricted dams still remained.
Reduced mammary blood flow persisted in the 2nd lactation and was 18% less for previously nutrient restricted dams. During the second parity, differences in lactational performance were not severe enough to alter calf pre-weaning growth as weaning weight was similar between treatments.
ImplicationsAlthough late gestational nutrient restriction may not always reduce birth weight, our results illustrate that colostrum and milk production was impaired by nutrient restriction, resulting in decreased pre-weaning growth.
Calves born to nutrient restricted dams had lighter weaning weights, which would ultimately affect cow-calf profitability. While no calves died in the research setting, it is more likely that calf malpresentations at birth, reduced calf vigor, and altered colostrum composition could lead to decreased calf survival in nutrient restricted heifers in a production setting.
Additionally, persisting effects in the 2nd parity on maternal size, fleshing ability, and mammary blood flow demonstrate the possibility of long-term programming effects on the dam, which could have implications for cow longevity in the herd. These results reinforce the importance of providing adequate nutrients to pregnant beef cattle, especially first-calf heifers that are still growing.
This work was supported by USDA-AFRI grant 2017-67015-26587 and the Illinois Beef Association.
Above Dr. Eric Moore, director of technical services for Norbrook North America, discusses RX changes and Noromycin for spring challenges, and below we discuss the expansion of Tulieve to Norbrook's anti-infective portfolio.
Redifer is a PhD candidate and Meyer is an associate professor, both in the Animal Sciences Department, at the University of Missouri, Columbia, Missouri.
