Inventory practice: Improving estimates of cattle weights in New Zealand

Sadie S

Keywords: Expert review | animal weight | cattle | Oceania

Country context: Grassland-based animal husbandry makes major contributions to New Zealand’s economy. The country’s GHG inventory has used a Tier 2 approach for cattle, small ruminants and deer since the early 1990s. Since then, New Zealand’s Tier 2 livestock inventory has undergone three major stages of development (see Country Inventory Case Study: New Zealand). The inventory has maintained its current structure since 2009, and within that structure various improvements in emission estimates have been made (see Inventory practice: New Zealand’s Agriculture Inventory Advisory Panel).

What data needs were addressed? Improved estimates of live weights for ewes and beef cows.

Why was the data needed? New Zealand’s inventory is based methane emissions model that estimates emissions on the basis of estimated energy and feed intakes. Since most energy consumed by breeding animals is used for maintenance, animal live weight is closely related to energy and feed intake estimates. Feed intake is estimated on the basis of live weight, but estimation of live weight in the model is done using data on carcass weight and an assumed carcass ratio (i.e. dressing out percentage). A review of the national inventory model (Muir et al. 2008) suggested that the ewe and beef cow carcass or live weight estimates and carcass ratios used in the model were based on limited data and assumptions that might lead to significant errors in the inventory estimates.

Methods used: Expert review of available data, including slaughter weight data, and collection of primary data.

How was improved data derived? New Zealand has an advisory panel that meets annually to deliberate on and recommend improvements to the agricultural inventory (see Inventory practice: New Zealand Inventory Advisory Panel case study). Based on key information needs identified by the panel and the responsible ministry (the Ministry for Primary Industries), the ministry commissions reviews and other analysis to inform decisions about inventory improvements. In 2008, a review of the inventory model (Muir et al. 2008) suggested that the ewe and cow live weight estimates used in the model were based on limited data and assumptions that might lead to significant errors in the inventory estimates. The ministry commissioned a review of ewe and beef cow live weight estimates used in the model. The review assessed the appropriateness of the data and assumptions used in the inventory model by comparing the inventory live weight estimates with the best available published and unpublished data (including new data collected for the review) on both live weights and carcass ratios (i.e. killing out percentage). Because live weight data is typically collected either at mating time or at culling, the review also assessed the implications of the timing of data collection of providing an estimate of annual average live weight.

For beef cows live weight, the reviewers searched records in available journal publications, but most publications were found to be of limited use as they either reported results from feeding trials that are not representative of commercial production conditions, or reported on breeds that are not typical of the national herd. However, unpublished live weight data on 2100 cows was available from researchers. In addition, live weight of breeding cows was measured on 12 farms in 2009 and 2010 using breeds that are more representative of the national herd. Live weights were measured at weaning and pregnancy testing because this is when farmers identify animals for culling. By collecting data at this time, it was also possible to examine any differences in live weights between the culled animals and those that remained in the herd. For the culled animals, data on carcass weights was also collected to provide an estimate of the carcass ratio. The estimated carcass ratio was 42.6%, slightly lower than the 45% assumed in the inventory model.

Different sources of live weight estimates were compared. The most representative datasets were deemed to be the unpublished data from Landcorp (the state owned livestock enterprise), a research project previously funded by the ministry, and from the surveys conducted as part of the review. The first two data sources reported carcass weights, to which the carcass ratio estimated by the review survey was applied. The latter data source reported measured live weights. All these data sources reported heavier live weights that that used in the inventory model. The average across these data sources was taken as the basis for recommending that the inventory should use a figure of 547 kg for 2009/10.

Table 1: Live weight and carcass weight estimates for beef cows

DatasetHerd com LW (kg)Cull cow LW (kg)Carcass weight (kg)Carcass ration (%)
Landorp (2007/8)568*242
Ministry study (2007/8)537*229
Review survey (2008/9)51052722141.4
Review survey (2009/10)57355525243.9
Inventory model (2009/10)45145
Suggested revision54723642.6

* Estimated from measured carcass weight

Cow live weight at pregnancy testing or weaning (i.e. end of summer) is often the annual maximum live weight, and slaughter mostly takes place over the summer, when live weights tend to be greater. If the live weight at pregnancy testing or culling is used, this would tend to overestimate annual average live weight. Unpublished data from one researcher and one farm was available to describe the seasonal change in live weight and estimate the extent to which slaughter data would overestimate annual average live weight (Figure 1). This analysis suggested that using slaughter data would tend to overestimate annual average live weight by 5-10 kg.

Figure 1: Liveweights in mixed aged beef cows (n-492) in Northland farm

Source: Muir and Thomson (2011)

In addition to estimating live weight in 2009-10, live weight estimates for the inventory would have to be applied to the inventory time series going back to 1990. Data was available on carcass weight for almost 100,000 beef cows slaughtered by Landcorp between 1997/98 and 2008/09. Applying the carcass ratio estimated by the review, a live weight time series was constructed that suggested an annual average increase in live weight of 8.5 kg/year. Extrapolating this back to 1990 suggests that in 1990/1991 the average beef cow would have weighed 402.5 kg, which is slightly higher than the 378 kg assumed for that year in the inventory model.

A similar analysis was conducted for ewes, using published, unpublished and newly collected live weight data, assessing the representativeness of the breeds weighed and the implications of the timing of weight measurements for deriving annual average live weight estimates.

The revised estimates were then applied to New Zealand’s inventory from 2012 onwards.

Further Resources

Pickering A. 2010. MAF Policy Agricultural Inventory Panel Meeting 17 August 2010.

Muir PD, Thomson BC. 2011. Better estimation of national ewe and beef cow liveweights. Report prepared for the Ministry of Agriculture and Forestry.

IPCC Guidance (IPCC 2006 Vol 4 Ch 10 p.10.12).

Author: Andreas Wilkes, Values for development Ltd (2019)