Investigations into the nutritional efficiency of dairy heifers limit fed diets containing different levels of forage and concentrate

Limit feeding high concentrate or high digestibility diets for growing dairy heifers may offer an economical alternative to ad libitum consumption of high forage, low digestibility diets that are traditionally fed to dairy heifers. Literature pertaining to economic, physiological, and nutritional responses to alterations in feeding level and dietary concentrate level are reviewed. It is concluded that limit feeding higher concentrate diets do not affect growth or first lactation milk production compared to dairy heifers fed high forage diets when equivalent levels of gain are obtained. It is also concluded that limit feeding high concentrate diets does not need to be excluded as a management option and may offer an opportunity for heifer growers to reduce feed costs and environmental output under certain circumstances. The level of dietary crude protein resulting in maximum efficiency of nutrient utilization under a limit feeding, high concentrate management system and differences from limit-fed high forage feeding are not available from the literature. Therefore, the objective of the reported experiment was to evaluate efficiency of nutrient and N utilization of dairy heifers limit-fed a low forage/high concentrate and high forage diets at equal ME intakes and 4 levels of N intake.;The hypothesis of this experiment is that a low forage (LF) ration will be utilized with a greater efficiency than a high forage ration (HF) by dairy heifers and that the response will be affected by level of N intake. To test this hypothesis, 8 Holstein heifers (beginning at 362 +/- 7 kg and 12.3 +/- 0.4 mo) were fed eight rations according to a split-plot, 4 x 4 Latin square design. Treatments were formulated to contain 25% or 75% forage (corn silage and chopped wheat straw) and fed at 4 levels of N intake [0.94 (Low), 1.62 (MLow), 2.30 (MHigh), 2.96 (High) g N/kg BW0.75 per d]. Diets were limit-fed to maintain equal ME intake. Organic matter (OM) intake was greater for heifers fed HF, but, due to increased OM digestibility of LF (74.0 vs 67.6% +/- 0.9; P <0.01), digestible OMI was unaffected by forage level (P > 0.50). OM digestibility was affected by an interaction between forage level and N intake (P < 0.01); increasing to a plateau of 78.01% at 18.43 %CP for LF and 68.78% at 13.90 %CP for HF fed heifers. Apparent N digestibility was greater for heifers fed LF and increased from 47.7% to 80.8% between Low and High N intake. Less N appeared in the feces of heifers fed LF than HF (45.56 vs 52.60 g per d). Urea-N excretion was not different between forage levels, but increased linearly with N intake. Concentration of plasma urea N (PUN) was significantly higher for LF and with increasing N intake. Like urea-N excretion, daily urinary N excretion was affected only by N intake. Retained N responded linearly to increased levels of NI. As a result of a significant interaction between forage level and NI on fecal N excretion and numerical differences in urine N, retained N at maximum N intake was greater for LF than HF. In spite of this observation, the percent of N intake that was retained only tended to be affected by an interaction and was not significantly affected by forage level. It is concluded that increasing N intake increases the digestibility of OM, the magnitude of which depends on the level of dietary forage provided. Furthermore, differences in N utilization between LF and HF in this trial were small and were not evident until N intake increased to impractical levels.