Energy balance during the mating period

InCalf's messages

The InCalf Book recommends seven quick checks to monitor herd nutrition during the mating period. In seasonal and split calving herds, this monitoring is particularly important from 2 weeks before mating starts to the end of mating. Where one or more of these checks indicate an abnormality, the diet should be assessed and appropriate adjustments made. Benefits and costs of changing nutritional management should always be considered. See pages 59 and 70-75 of The InCalf Book.

Whilst these recommendations are sound principles for maximising herd profitability, the effects of deviating from these recommendations on reproductive performance are unclear, precluding accurate economic analysis of various nutritional management changes.

What the current scientific literature says

Relationships between energy balance or nutritional changes during the mating period and reproductive performance are poorly defined, although large reductions in feed intake around the time of expected service can reduce conception rates. There is little published evidence to suggest that changing nutrition during the mating period will have beneficial effects on either submission or conception rates.

Effects of energy balance around the time of ovulation on reproductive performance have been studied using changes in liveweight, biochemical indicators of energy or nutrient balance, direct estimation of energy balance and nutritional intervention studies. Although the term ‘energy balance’ is commonly used, it is difficult or impossible to separate effects of energy balance from other nutritional changes in most studies.

Associations between liveweight or body condition change around time of service and conception rates have varied. Compared to cows losing liveweight in the 4 weeks around the time of service, much higher conception rates were observed in cows that gained liveweight (King 1968). Higher rates of liveweight gain during the breeding period (Armstrong et al 1990; Buckley et al 2003) or after first AI (Kennedy et al 2003) have also been associated with increased conception rates. In two Australian studies, liveweight change in the 4-7 weeks before insemination was positively associated with reproductive performance (McClure 1965; Fulkerson et al 2001). Youdan and King also reported that cows that conceived following service were losing liveweight more slowly than cows that failed to conceive to service (Youdan and King 1977). Cows losing body condition in the 28 days after insemination had lower conception rates than cows that were holding or gaining body condition (Santos et al 2001).

In contrast, Moller and Shannon reported that non-return rates were not associated with liveweight change in 3 weeks before insemination (Moller and Shannon 1972). Boyd also found no difference in conception rates (Boyd 1972) and Ducker et al actually observed reduced conception rates amongst cows gaining more than 0.45 kg per day in the 4 weeks before AI (Ducker et al 1985a). Liveweight gain during the mating period has also been associated with prolonged calving to conception intervals (Armstrong et al 1990). In dairy heifers, pregnancy rates appeared more closely associated with actual body condition than with rate of liveweight gain in the 12 weeks around mating (Baishya et al 1982).

Biochemical indicators of energy or nutrient balance measured around the time of ovulation have also been associated with changes in reproductive performance. In Australian research, Moss studied metabolic risk factors for conception to first service (Moss 2001). High serum concentrations of beta-hydroxy butyrate and low serum concentrations of cholesterol and albumin at the time of insemination were associated with reduced likelihood of conception. Elevated concentrations of beta-hydroxy butyrate at week 6 of lactation have also been associated with increased services per conception (Armstrong et al 1990). Elevated beta-hydroxy butyrate concentrations are associated with negative energy balance (Lean et al 1992) and liveweight loss (Ducker et al 1985b). Low cholesterol and albumin concentrations can occur in association with fatty infiltration of the liver following restricted feed intakes (Mostaghni and Askari 1996) and/or greater body condition loss (Reid 1980; Ruegg et al 1992a; Yamada et al 1994) after calving.

In a recent detailed study, energy balance was directly estimated from feed intake, milk yield and liveweight, and greater negative energy balance at time of service was associated with reduced chance of conception to that service (Reist et al 2003).

Indicators of nutrient balance measured around the time of ovulation have also been associated with expression and detection of oestrus. Westwood et al found that oestrous signs were more likely to be detected at first post partum ovulation amongst cows with higher dry matter intakes, plasma cholesterol concentrations and glucose to beta-hydroxy butyrate ratios (Westwood et al 2002). These risk factors are likely to be associated with energy balance (see discussion in Westwood et al 2002). In contrast, milk yield has not been consistently associated with the strength of oestrus signs (Harrison et al 1990; Van Eerdenburg et al 2002; Lopez et al 2004) or detection rates (Fonseca et al 1983; Westwood et al 2002).

Alternate hypotheses arise from these studies. The cow’s liveweight or body condition change and associated metabolic status around the time of ovulation may directly influence chances of insemination and conception. However, although these exposures were assessed around the time of ovulation or service, the reduced reproductive performance may actually be due to excessive and prolonged negative energy balance in early lactation, well before the time of service. From published studies, it is difficult to assess the relative importance of these alternate hypotheses because cholesterol and albumin concentrations are moderately repeatable up to 10 weeks after calving (Rowlands et al 1980) and these metabolic indicators are also associated with reduced subsequent reproductive performance when measured from soon after calving [elevated beta-hydroxy butyrate concentrations - (Whitaker et al 1993); low cholesterol concentrations - (Kappel et al 1984; Ruegg et al 1992b; Westwood et al 2002); low albumin concentrations (Rowlands et al 1980; Wilson et al 1985)].

Physiological studies suggest that subsequent poor reproductive performance is due, at least partly, to metabolic status in early lactation. Energy balance affects follicular and endocrinological patterns in early lactation, well before first service (Beam et al 1999; Butler 2000). Studies using metabolic indicators support this hypothesis. Metabolic indicators of energy balance 7 and 14 days after calving were associated with subsequent reproductive performance, whereas indicators at time of service were not (Whitaker et al 1993). High milk fat concentration within 28 days after calving was associated with chance of conception when cows were subsequently inseminated on average, 80-90 days after calving (Kristula et al 1995). High fat to protein ratio within 40 days after calving was associated with interval to conception where the mean interval was over 100 days (Heuer et al 1999).

Nutritional intervention studies could also help understand effects of nutritional status around the time of service independently of status soon after calving. However few suitable studies were identified and findings from these studies vary. Large, short-term reductions in feed intake around the time of oestrus synchrony alter follicular development and inhibit ovulation in cyclic beef heifers (Mackey et al 1999; Mackey et al 2000) and may affect follicular characteristics in cows (Comin et al 2002). Large reductions in feed intake for 10 days after AI reduce embryonic survival in beef heifers (Dunne et al 1999). Early Australian work involved a small number of cows fed forage oats. Cows supplemented with concentrates around time of mating had higher conception rates than unsupplemented cows (McClure 1970). In another very small trial, pasture-fed cows were fed 1.4 kg of concentrates for the first 3 weeks of the mating period. These cows had higher conception rates than unsupplemented cows (Wilson et al 1985). In herds producing around 8,000 L/cow/lactation, cows allocated to receive a high-energy supplement from 40 days after calving for around 5 weeks had increased conception rates to first service, relative to unsupplemented herd mates (Pehrson et al 1992).

Reproductive performance is not always increased following increased feed intakes around the time of service. Increasing ME intake by 12-15% for the 9 weeks around expected time of service did not increase reproductive performance (Ducker and Morant 1984). Cows with higher intakes lost less body condition over the 9-week treatment period and had higher milk yields but similar reproductive performance. In another study, Ducker et al allocated 48 first lactation heifers to either low or high feed intakes (125 and 145-160 MJ ME/day, respectively) around expected time of insemination (ie from weeks 6 to 18 of lactation)(Ducker et al 1985a). For much of the feeding period, lower intake animals had lower milk fat yields and were in slightly negative energy balance (-5 to 0 MJ ME/day). In contrast, higher intake cows were in positive energy balance (+10 to + 20MJ ME /day). Reproductive performance was substantially higher amongst heifers fed lower intakes. This effect was independent of nutritional status in early lactation; treatment groups had similar energy balance patterns from calving to start of the feeding treatment. Similar results have been reported in dairy yearlings (Ducker et al 1982). Although increased concentrate intake before and after service substantially increased growth rates, reproductive performance was not higher amongst high intake heifers. In later work, oestrus expression was no less vigourous amongst non-lactating heifers fed to induce negative energy balance relative to heifers in positive energy balance (Villa-Godoy et al 1990) and dietary restriction did not alter concentrations of reproductive hormone in cycling heifers (Ronchi et al 2001).

Compiled by Dr John Morton


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