Improved fertility results enhance the options and flexibility to better manage both the farm and herd. There are several practices and factors that can have a major impact on herd reproductive performance.
Growing heifers well
To ensure a good return on investment, heifers must get in calf quickly, calve without difficulty and have a productive first lactation. Well-grown heifers reach puberty earlier, get in calf quicker, have fewer calving difficulties and have better fertility as first calvers. They also have higher milk production over their first three lactations, compete better with mature cows and have better survival.
Successfully rearing heifers and achieving desired growth milestones requires:
- Setting individual target weights.
- Feeding supplements to maintain growth when pasture availability or quality is low.
- Balancing diets to hit target growth rates.
- Calculating feed requirements of heifers.
- Ensuring availability of adequate mineral levels and water.
Transition cow management
Transition cow management has been one of the most significant advances in dairy nutrition in the past 20 years. It provides a significant opportunity to boost cow health, milk production and reproductive performance. It is estimated that 80 per cent of a lactating dairy cow's health problems occur during the transition period - the first four weeks after calving.
During the transition period it is vital to check that calcium supply is 0.4 to 0.6 per cent of dry mater intake or 40 to 60 g/day for a cow eating 10 kg DM/day. Phosphorus should be increased to between 0.25 per cent to 0.45 per cent of dry matter intake (25 to 45 g/day). In addition, magnesium should be increased to 0.4 per cent of dry matter intake (40 g/day). This is to minimise the risk of milk fever (hypocalcaemia) and grass tetany (hypomagnesaemia) after calving, and ensure cows have the best preparation for their upcoming lactation.
Another important mineral-related dietary factor, particularly around calving, is the dietary cation-anion difference (DCAD) value of the diet. Cows in the last four weeks before calving can benefit from having a diet with a DCAD value of around minus 50 (measured in milli-equivalents per kg DM). When this DCAD target is combined with the calcium, phosphorus and magnesium targets described previously, it minimises the risk of milk fever after calving.
To achieve a DCAD value of around minus 50 in the weeks before calving, cows need to be fed a diet low in potassium and sodium and high in chloride and sulphur. A low potassium/sodium diet generally requires cows to be removed from grazed pasture and fed a low potassium forage instead. Often cows also need to be supplemented with anionic salts which contain chloride and/or sulphur.
A feed analysis for potassium, sodium, chloride, and sulphur is recommended for forages that are to be used during the transition period. The repeated use of calving paddocks with a high accumulated effluent load on the pasture should be avoided due to the likely elevated potassium levels. Once cows have calved, the DCAD value of their diet needs to be 200-250 milli-equivalents (mEq) per kg DM to minimise risk of milk fever and other health related problems.
Artificial insemination (AI) practices can have a major impact on herd reproductive performance. Performance can be measured using the conception rate, estimated by dividing the number of diagnosed or identified pregnancies to artificial insemination by the number of semen straws used and multiplying by 100. AI practices should be reviewed if the conception rates are less than:
- 49 per cent in seasonal or split calving herds.
- 43 per cent in year-round herds.
- 55 per cent in well-grown heifers during the first mating period.
A low conception rate to artificial insemination generally results in lower in-calf rates. A decreased non-return rate can provide an early warning of a low conception rate to artificial insemination. If the non-return rate for a herd is less than 60 per cent or conception rate is as low as the percentages described above, further investigation is recommended.
Body condition scoring
The effective management of cow body condition and nutrition provides reproductive, milk production, feed conversion, cow health and welfare benefits. Regular measurement and early intervention are key factors in achieving effective body condition management. Body condition targets relate to the average herd body condition score and the proportion of cows that are too thin and too fat.
The following herd body condition score targets (based on the Australian 1-8 scoring system) are recommended:
- At calving: herd average body condition score between 4.5–5.5 (with less than 15 per cent of cows below score 4.5 and less than 15 per cent of cows above score 5.5).
- At mating: less than 0.6 decrease in average score of the herd since calving and less than 15 per cent of cows having lost more than one score since calving. Cows should maintain or gain body condition after commencement of mating.
- At drying-off: herd average body condition score between 4.5–5.5. Cows should maintain or gain body condition during the dry period.
It takes time for cows to change their body condition score. It is important to start modifying diets in the months leading up to calving for cows to be in their best shape for mating. Reliable, comprehensive, and accurate records of herd reproductive performance help farmers identify areas for improvement. Importantly, they also provide a means of monitoring progress and effectiveness when any change is implemented.