Biggest may not be best
By Marianne Curtis
BIG may not necessarily be best when it comes to heifer size at calving.
Research shows heifers calving at 540kg lost less weight and body condition in early lactation than heavier animals, with positive implications for fertility.
Conducted at the Agricultural Research Institute of Northern Ireland, Hillsborough, and on 11 dairy herds in Ulster, the research compared heifers reared on four different diets to two target calving weights, says researcher Alistair Carson.
"We studied 113 heifers born in autumn 1996 with a PIN of £90. Calves were seven weeks old when they entered the programme."
Two groups received grass silage-based diets during two winter periods and grass-based diets in summer. The first group calved at 540kg and the second 620kg. Group three, which received straw/concentrate during winter and a grass-based diet in summer, calved at 620kg. Finally, group four received straw/concentrate during the winter periods and first summer period and grass during the second summer. This group also calved at 620kg.
Although heifers reared to 620kg at calving gave more milk in their first lactations – 7901-8020 litres, compared with 7222 litres for the 540kg group – this advantage had disappeared by the second lactation.
A separate study at the institute measuring intakes and weight loss – not possible in the on-farm studies – showed 620kg-calving heifers lost 0.3kg/day in weight over the first 10 weeks of lactation, whereas a group of 580kg heifers maintained their weight.
A similar trend is likely to have occurred with the 620kg and 540kg heifers in the on-farm studies and is reflected in fertility results, says Dr Carson.
"Calving interval was 394 days for first lactation group one heifers, compared with a range of 426-458 days for other treatments."
A similar pattern emerged during the second lactation with a 400 day calving interval for group one, ranging as high as 455 days for group three, he adds.
The costs of the three rearing systems also varied considerably. Taking into account feed, grazing, AI and vet costs, variable cost for group one was £345, group two, £474, group three, £477 and group four, £502 (see table). "Increasing weight at first calving increased milk fat plus protein yield by 6%, but it only partly offset higher rearing costs.
"The group one system, where heifers are reared to 540kg, appears to be the best for overall lifetime performance and has the lowest rearing cost. Returns are not there to justify growing heifers to heavier weights at calving." *
• Calve at 540kg.
• Less weight loss.
• Improved fertility.
Heifer rearing: Feed costs from seven weeks to 23 months
Group 1 Group 2 Group 3 Group 4
Silage amount (t) 5.8 5.5 – –
Silage cost (£) 116 110 – –
Concentrate amount (t) 0.27 1.26 1.91 2.18
Concentrate cost (£) 32.40 151 229 262
Grazing area (ha) 0.27 0.29 0.29 0.19
Grazing cost (£) 108 116 116 76
Straw amount (t) – – 0.95 1.58
Straw cost (£) – – 47 79
Total feed cost (£) 256.40 377 392 417
Source: Agricultural Research Institute of Northern Ireland, Hillsborough.