Effect of rearing diet and early post-weaning pasture quality on the life-time growth, meat quality, carcass traits and environmental impact of dairy-beef cattle

Highlights

• Calves grew faster on high milk and pasture than low milk and concentrate diets.

• Improved pasture quality from 3 to 7 months increased liveweight to slaughter.

• High milk and pasture diets increased marbling of beef.

• Reduced time to slaughter may reduce nitrate leaching and methane production.

Abstract

The effects of divergent calf rearing diets and post-weaning pasture quality on growth, carcass traits, economics and the environment were investigated. Beef x dairy heifer calves were allocated to one of two rearing treatments, followed by a low (L)- or high (H) quality post-weaning pasture diet from 3 to 7 months old, in a 2 × 2 factorial design (n=50 per treatment). The R1 rearing treatment received 4 L milk/day for 5 weeks, then 2 L/day for two weeks plus concentrate pellets ad libitum for 7 weeks in group pens. These calves then grazed pasture from 7 weeks, with pellet-weaning at 12 weeks. The R2 rearing treatment were fed 8 L milk/day for 9 weeks, then 4 L for 2 weeks, weaning at 12 weeks, with ad libitum pasture outdoors after the first week. Cattle were managed as two groups, balanced for treatments, on a pasture-based diet from 7 months until slaughter at 19 to 25 months old. Calves offered the R1 diet grew slower than R2 calves with a 7 kg liveweight difference at 12 weeks (P<0.001) and an 11 kg difference at 7 months (P<0.001). Calves fed H pastures grew twice as fast as L fed calves from 3 to 7 months (P<0.001), providing a 31 kg liveweight advantage at 7 months. Treatment differences in liveweight were supported by differences in frame size, with liveweight differences largely maintained until slaughter. Treatments had minor effects on carcass traits and meat quality. Modelling of treatments from 12 weeks to slaughter showed H, and to a lesser extent R2 diets, may reduce nitrate leaching and methane emissions on beef farms compared to L and R1 diets.

Introduction

Artificially rearing surplus calves from the dairy industry reduces the need for beef breeding cows, presenting an opportunity to increase the conversion efficiency of feed into product on beef farms. Rearing systems for such calves may vary in the amount and type of milk and starter diet supplied, and the timing of weaning and introduction to pasture (Muir et al., 2000). Restricted milk feeding (approximately 10% of liveweight) was introduced to reduce the cost and labour requirements of rearing (Muir et al., 2000). Such diets hasten the onset of solid feed intake (de Passillé et al., 2011) and rumen development, enabling smoother, earlier weaning off milk compared to diets with ad libitum milk (Huuskonen and Khalili, 2008). On the contrary, calves reared on high milk allowances grow faster during rearing but have slower rumen development (Khan et al., 2011), requiring weaning at an older age.

In New Zealand, the majority of dairy cows calve in spring (July to early October), with the resulting beef-sired (typically Hereford) heifers and steers and Holstein Friesian bull calves largely reared by commercial calf rearers. Weaned calves are then sold to beef farmers, where they are finished on a pasture diet. The post-weaning, transition of calves onto a pasture diet coincides with the time when pasture quality is declining in New Zealand (Litherland et al., 2002) and other temperate regions, due to grass reproductive development. The resulting poor post-weaning growth rates of beef x dairy calves on pasture has been identified as a key weak point for this industry (Morris, 2019). Pasture intake is limited by rumen capacity (Waghorn, 2002), hence, to improve growth rates at this age, the quality of calves’ diets, or their ability to utilise dietary nutrients, needs to be improved.

Emerging evidence suggests that the feeding regimes of calves can have long-term consequences on growth, carcass traits and meat quality (Berge 1991; Greenwood et al., 2006). However, those studies were undertaken with suckler-beef calves or cattle finished in feedlot systems. Effects on life-time growth can in-turn affect time to slaughter, and hence economics, and the increasingly important environmental impacts. It has been reported that the best opportunity to reduce agricultural greenhouse gas (GHG) emissions per unit production is to increase the efficiency of feed conversion into product (Dynes et al., 2011). Similarly, nitrogen (N) leaching in pastoral systems is a function of dietary N, along with the associated deposition of urinary N and soil drainage (Cameron et al., 2007).

The effects of calf nutrition during and immediately after artificial rearing on the long-term performance of pasture-fed beef x dairy cattle are poorly understood. We hypothesise that nutrition during both of these periods can influence lifetime liveweight gain and meat quality. This study compared lifetime growth, carcass traits and meat quality as well as environmental impacts for pasture-based beef x dairy breed cattle under divergent rearing regimes with regards to milk, concentrate and pasture allocation and differing pasture quality from 3 to 7 months old.