Effects of diet during the first winter on replacement heifer weight gain and body condition score during the subsequent grazing season

摘要

Seasonal calving dairy production systems it is important at heifers attain puberty in an appropriate timeframe especially when they arc bred to calve at 2 years of age and systems that impose restricted breeding periods (Ferrell, Iw2). Achieving target weights at key time points is critical because, for example, the onset of puberty usually toccurs in dairy heifers at 30-40% of their expected mature BW(Heinrichs 1993) which corresponds to approximately 0-32O kg in Holstein heifers. Furthermore, heifers should be managed to achieve 55% to 60% of mature body/weight (BW) at mating start date (MSD; Patterson et al 1992). Archbold et al. (2012) has shown that both heifer BW and body condition score (BCS) at MSD are positively associated with calving date and potential milk fat plus milk protein yield when they enter the lactating herd. Heifer rearing is the second largest expense in the dairy system, accounting for approximately 20% of total costs (Gabler et al. 2000). Therefore, it is necessary to focus on reducing costs of production, particularly feed costs, as they account for approximately 80% of total variable costs (Shalloo et al., 2004). One of the methods of ' reducing feed costs in particular, is by sourcing lower cost feeds. Finncran et al. (2010) reported that kale grazed in-titu ranked as the cheapest alternative to grazed grass and was considerably cheaper than grass silage. Kale has higher crude protein (CP) content than grass silage (Kcogh etal. 2009) and may be suitable for inclusion in the diet of replacement dairy heifers. However, kale also tends to have a low neutral-detergent fibre (NDF) concentration (Kcogh et al. 2009) suggesting that feeds with a higher [NDF concentration (>500 g/kg DM), such as silage may need to be offered in order to avoid acidosis. The objectives of this study were to i) investigate five contrasting winter feeding regimes on heifer bodyweight (BW) gain and body condition score (BCS), ii) establish if similar BW gain is achieved from a kale only diet compared to a kale + grass silage diet and iii) determine if compensatory growth during the following grazing season exists in replacement heifer rearing systems.