Raising dairy heifers from birth to calving comprises the second largest expense of milk production on the dairy farm—while deriving no revenue until the onset of lactation (Heinrichs, 1993). Therefore, many of the experiments involving dairy heifers have focused on ways to minimize costs associated with the growth period or to hasten onset of the productive period.
Reducing the length of the growing period by decreasing the age at first calving below 22-24 months could overcome this lag between expenditure and revenue generation and reduce costs associated with the nonproductive period. This could be accomplished by increasing prepubertal average daily gain (ADG; Hoffman, 1997), which would subsequently result in a lower age at first breeding and presumably a lower age at first calving.
Although this strategy would ultimately accelerate the return on investment, high rates of prepubertal ADG have a negative impact on mammary development (Radcliff et al., 1997; Sejrsen et al., 1982) and first lactation milk yield (Lammers et al., 1999; Radcliff et al., 2000; Van Amburgh et al., 1998). A summary of recent literature on the association between prepubertal ADG and first lactation milk production showed total first lactation milk and protein yield were maximized when prepubertal ADG was around 800 g/d for Holstein heifers (Zanton and Heinrichs, 2005).
Many researchers, however, are looking for ways to allow greater ADG while maintaining optimal levels of mammary development and milk production. To date many of the approaches have shown little progress, conflicting results, or impracticable recommendations to enable a producer to overcome the detriment imparted by accelerated prepubertal growth.
Since accelerating prepubertal ADG necessitates nutritional alterations, most experiments investigating effects of prepubertal growth have also altered the nutritional status of heifers in one or several groups.
For instance, some studies altering prepubertal ADG have employed rations of vastly different composition for ad libitum consumption (i.e., forage or concentrate rations); others have fed an identical diet to each experimental group restrictively to obtain different ADG.
Minimally represented in the literature are reports of the effects on milk production of differing forage:concentrate ratios (F:C) when diets were fed to maintain a constant rate of growth. Serjsen and Foldager (1992) investigated this question using eight animals per treatment through 130 days of the first lactation. They found no differences in milk production between the groups fed high forage:concentrate (F:C) or low F:C to achieve equal ADG during rearing.
Since feed costs make the greatest contribution to heifer rearing expenses, comprising about 60% of total heifer costs (Gabler et al., 2000), it would follow that a reduction in feed expenses could significantly contribute to decreasing the overall monetary expenditure for raising dairy heifers. Since there is an optimum ADG for heifer growth, feed costs should be expressed in a manner that considers both the unit cost of feed and the amount that must be fed to obtain the optimal ADG.
Concentrated feedstuffs are usually more cost effective per unit of energy than forages. And if the energy requirement is fixed by the amount needed to obtain the optimal ADG, then feed costs can be reduced by replacing the more expensive forage energy with energy from concentrates.
Also, if it is true that there are no differences in milk production between diets differing in F:C, then expenditures to raise dairy heifers could be reduced without affecting revenue derived at the onset of lactation, leading to a more profitable heifer raising system. There is currently very little data in the literature concerning the effects of differing F:C, when delivered for the same level of growth, on responses obtained from dairy heifers. Reynolds et al. (1991a; 1991b) investigated the effects of differing F:C in growing beef heifers on energy metabolism at the level of the whole animal as well as for the portal-drained viscera (PDV) and the liver.
Reynolds et al. (1991b) found that when fed a constant level of metabolizable energy, heat production was lower for animals fed lower F:C (25:75 vs. 75:25) resulting in a significantly increased tissue energy accretion. The PDV accounted for proportionately less oxygen consumption for the low F:C; however, the total splanchnic tissue (TST) consumption of oxygen did not differ between diets.
Glucose release to the periphery was also significantly increased when feeding a low F:C, possibly due to decreased glucose metabolism by the PDV (Reynolds et al., 1991a). When nitrogen dynamics were considered, responses were difficult to resolve or to ascribe to a particular F:C due to differences in nitrogen intake between treatments. Notably, while nitrogen intake was greater for the high F:C ration, tissue retention of nitrogen was greatest for the low F:C ration.
Relative to intake, animals fed high F:C excreted more fecal dry matter, nitrogen, and energy and more urinary nitrogen than those fed low F:C. While it is unclear if improved nitrogen efficiencies were due to differences in nitrogen intake, the flow of some nitrogen-containing compounds (ammonia, α-amino nitrogen, and urea) across the PDV were not significantly affected by F:C, indicating that post-absorptive nitrogen efficiency may be improved by low F:C.
Huntington et al. (1996) fed iso-nitrogenous and iso-energetic diets to six multicatheterized beef steers to investigate nitrogen dynamics at varying F:C. In a comparison of diets containing 27 or 63% concentrate, more urea nitrogen and glucose were released by the TST to the periphery when fed 63% concentrate, while acetate release was reduced.
Amino acid release by the TST was numerically greater for the low F:C diet, however statistical significance was not attained. While urea release was affected by treatment, the results are difficult to interpret because of the inclusion of urea in the low F:C rations at up to 20% of dietary nitrogen. Feeding urea could have dramatically affected the efficiency of microbial capture of this very soluble nitrogen source before it was absorbed from the rumen and incorporated into urea by the liver.
Urinary excretion of urea was also greatest for the high concentrate diets, again possibly due to the solubility of the nitrogen source. While diet composition must be different when F:C is varied in order to provide an iso-nitrogenous treatment, the use of urea in this experiment may limit the ability to draw inferences about the effects of differing F:C per se on nitrogen dynamics.
It is therefore critical that data can be produced where these factors are not confounded, but controlled, so that nitrogen excretion for these diets can be more thoroughly understood in the growing dairy heifer. Furthermore, the combination of lower acetate with the possibility of increased amino acid release to the periphery on high concentrate diets could affect the composition of gain in heifers due to preferential use of acetate for lipogenesis in ruminants (Bergman, 1990) as well as increased availability of amino acids for protein synthesis (Owens et al., 1993).
A typical dairy heifer is fed a ration in which the majority of nutrients are derived from forages as opposed to concentrated feedstuffs. However, there is a large inefficiency associated with this method of feeding due to lower digestibility of most forages, greater metabolic protein and energy requirements associated with digesting forage, and higher feed costs per unit of energy as compared to concentrates.
The potential therefore exists to replace a significant proportion of the forage DM in a ration with concentrate DM, reducing the inefficiency associated with raising dairy heifers while maintaining similar ADG. To address this concept in raising dairy heifers, a series of experiments was recently conducted to evaluate growth characteristics of heifers fed high concentrate or high forage rations at restricted intakes to achieve a similar ADG.
The growth studies showed less DM was consumed by the heifers fed high concentrate rations than for the more traditional high forage rations at similar ADG, leading to significantly improved feed efficiency for the heifers receiving high concentrate levels. Daily gains of skeletal measurements were not different between treatments.
To further define differences between high forage and concentrate diets and their relationship to animal digestibility, we have conducted several digestion trials studying heifer diets with large differences in forage and concentrate constituents. These were completed in heifers ranging from 6 to 20 months of age, some with rumen cannulae to measure rumen dynamics. In addition, ammonia volatilization from manure was measured in several of these experiments.
In one study, the rations (75% concentrate vs. 75% forage; (corn silage, alfalfa and grass hay) were limit-fed in a total mixed ration (TMR) to heifers to provide 0.22 Mcal ME and 1.9 g N per kg EBW0.75. Identical ingredients in differing amounts comprised each ration. Organic matter intake was significantly lower for heifers fed high concentrate diets; however, due to improved organic matter digestibility the intake of digestible organic matter was not different between treatments.
Neutral detergent fiber digestibility was not significantly affected by dietary treatment. Nitrogen (N) intake was greater for heifers fed high concentrate diets; however, fecal N excretion tended to be lower because of improved apparent N digestibility. Urinary N excretion was not affected by treatment ration, leading to greater overall N retention for heifers fed high concentrate diets. The ammonia volatilized from manure, when adjusted to reflect the greater production of urine and feces by the high forage group, was significantly greater for heifers fed high forage diets.
A similar study was conducted with eight Holstein heifers limit-fed a TMR that consisted of 77% corn silage and 23% concentrates or 67% concentrates and 33% corn silage.
Dry matter digestibility was greater for high concentrate diets, and although nitrogen digestibility was not different between treatments, N retained was higher for high concentrate diets. From these studies, we conclude that high concentrate diets can improve efficiency of OM and N utilization when intake is controlled.
A key component of these studies was that intake was controlled within a range that would allow ADG of around 800 g/d. Rumen fermentation parameters have also been analyzed and minimal differences in pH and volatile fatty acid concentrations were found; with all levels within acceptable ranges for the age of animals studied. No detrimental effects, either short or long term were noted from this feeding management system.
Our studies have shown that feeding high concentrate rations to growing dairy heifers between 4 and 22 months of age in a restricted manner leads to similar growth performance with respect to weight gains and structural development. Feeding these diets can increase total dry matter and nitrogen utilization by growing heifers, which leads to reduced fecal nitrogen excretion and lower overall manure production. Reproduction and first lactation yield data are currently being collected on many of these animals. The ultimate potential of this research is to find diets that will be more efficient in terms of nutrient utilization and cost for the producer, with no detrimental effects on future productivity.
Authors: G.I. ZANTON, M.L. MOODY and A.J. HEINRICHS
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Dairy and Animal Science Department, The Pennsylvania State University, State College, Pennsylvania, USA