Meeting minimum growth rates for heifers

Winter is when many producers experience the biggest gap between feed availability and the requirements of stock to maximise growth and/or reproduction. This may not necessarily affect herds of older cows, as these animals can utilise their fat stores and their progeny may experience compensatory growth once pasture becomes more abundant. However, for young replacement heifers, insufficient feed for growth from weaning to conceiving a second calf can cause major problems for their reproductive capacity and the structure of the herd in subsequent years.

Research conducted by the Beef CRC found young cattle that experienced nutritional stress eventually caught up to their peers in liveweight through compensatory growth of muscle and fat, but not skeletal growth. This resulted in heifers that did have a growth setback recording smaller pelvis sizes than their siblings that had no growth setback. From a breeding point of view, this may contribute to the risk of dystocia at their first and possibly future calvings.

This research highlights the importance of managing unjoined and joined heifers to avoid growth setbacks that could result in smaller pelvis size and ensure unjoined heifers reach minimum joining weights, particularly during periods of low pasture growth. The minimum recommended joining weight varies from herd to herd, depending on the average adult cow mature weight. As a general rule, heifers at their first joining should weigh approximately 60-65% of the herd average mature cow weight.

When managing heifers for growth and weight gain, a feed budget can be a useful tool to determine how much energy and protein are required to reach target growth rates and determine whether the pasture or supplements provided will meet these requirements.

The protein requirement for growing young stock is 13% of dry matter (DM) and does not vary; however, energy requirements - measured as MJ ME - will vary with weight and the growth rate. The table below shows the energy requirements of young stock at a range of liveweights and growth rates.

Table 1: Metabolisable energy requirement of young stock at different liveweights and for different target growth rates (Source: DEPI Drought feeding and management of beef cattle).

Liveweight (kg)	Target growth rate (kg/day)	Metabolisable energy requirement (MJ/day)
150	0	22
	0.5	37
	1.0	55
200	0	26
	0.5	44
	1.0	59
300	0	35
	0.5	57
	1.0	76
400	0	45
	0.5	71
	1.0	93
500	0	55
	0.5	82
	1.0	108

Undertaking a feed budget involves two key steps; determining the growth rate required and identifying the stock requirements.

To determine the required growth rate for heifers, calculate the minimum joining weight using the following steps:

• if mature cows average 500kg throughout the year, 60% of this is 300kg and this is the minimum joining weight for heifers at 15-months-old
• if heifers are weaned at 8-months-old, they have seven months or approximately 210 days to reach their minimum joining weight
• if the average weight of heifer calves at weaning is 200kg, they need to gain 100kg equating to a growth rate of 0.48kg/day
• allowing for a 10kg setback at weaning brings the required growth rate up to 0.5kg/day.

To identify stock requirements, consider the required growth rate:

• to achieve a growth rate of 0.5kg/day, an animal that weighs between 200kg and 300kg needs approximately 44-57MJ ME/day.

The ability of pastures to meet the energy requirements of cattle relies on availability of adequate feed of sufficient quality to the cattle. The following table, taken from Prograze®, shows the predicted growth rates of growing cattle when they graze pastures of varying yield (in DM/ha). It can be used as a rough guide for determining if pastures meet the energy requirements of growing animals.

Table 2: Expected growth rates (kg/day) of cattle of varying ages and liveweights grazing green pastures of different yields (Source: Prograze).

		Yield of green pasture
Age (months)	Liveweight (kg)	1300 kg DM/ha	1800 kg DM/ha	2600 kg DM/ha
8	200	0.7	0.9	1.1
10	250	0.6	0.8	1.0
13	300	0.4	0.7	0.9
16	350	0.4	0.6	0.8
20	400	0.3	0.6	0.8

A more precise tool for feed budgeting is the EverGraze Feed Budget and Rotation Planner. This tool allows producers to calculate whether pastures can supply enough energy for the required growth rate and, if there is a deficit, how much of varying types of supplementary feed are required to fill the deficit.

Another useful tool is the MLA Feed Demand Calculator (FDC).

Further information

• Claire Harris, Victorian Department of Economic Development, Jobs, Transport and Resources, Ph: 03 5663 9908