Measurement of dry matter intake and residual feed intake under grazing conditions of lactating heifers during a complete lactation

Abstract

Dry Matter Intake (UMI) of 30 lactating heifers fed pasture grazed in situ was estimated with the n-alkane technique during weeks 3, 7, 11, 15, 18, 24, 29 and 34 of lactation, in the dairy farm of Lincoln University (Season 1997-1998). Dry matter digestibility (DMD) of pasture consumed by individual heifers was evaluated simultaneously using this technique. Metabolizable Energy Intake (MEl) of individual heifers was calculated from DMI and DMD.

Milk production, liveweight and live weight change were measured for the same time periods as was DMI. Based on these measurements the Metabolizable Energy Requirements (MER) of each heifer were calculated using the predictive equations of AFRC (1993). The individual heifer Residual Feed Intake (RFI = MEl - MER) was derived for each period of lactation.

The relationships between DMI, DMD and pasture availability highlight the strong animal x feed interaction presented in a seasonal pasture based dairy system.

Comparisons of DMI estimated with alkanes with intake predicted by five different equations showed that predictive equations underestimated DMI by 12-25% from the measured intake of lactating heifers under New Zealand grazing conditions.

Total MER estimated with two different maintenance costs (0.49 and 1.0 MJ of ME/kg 0.75) was calculated and RFI was derived using both values. Using a low maintenance cost the mean RFI calculated of 30 heifers was positively high for most part of the lactation. In contrast using the high maintenance cost, RFI results were near to zero. The conclusion drawn from these data was that DMI is underestimated under grazing conditions mainly as an effect of the underestimation of maintenance costs.

Predictive models for DMI, RFI and Gross Efficiency (GEf) were derived and analyzed. DMI and GEf showed that although there was variation between animals, they could be predicted under grazing conditions. In contrast across lactation RFI showed a large variation both between animals and within an animal, making its prediction a non-accurate process.